Improve Tail Latency

- TransactionStatement token aware routing
 - Mitigate convoy effect:
   - ExecuteTxnBacklog v1 can execute single key transaction backlogs and disseminate the result
     - Direct local execution possible for single key transactions
   - Use shardAppliedBefore instead of gcBefore to cleanup CFK faster
Also Improve:
 - Introduce distributed tracing
 - Combine RejectBefore and MaxConflicts
 - Self-addressed messages are delivered directly
 - Disable AccordSyncPropagator by default, as incompatible with large clusters
 - FastPathStrategy.SIMPLE should not modify fast path contents; FastPathStrategy.UP introduced to adopt this behaviour
 - Configurable queue prioritisation
 - ProtocolModifiers mostly final
 - Remove unnecessary condition check when serializing TxnUpdate
Also Fix:
 - SequentialExecutor ownership bug
 - BTree.XUpdater.reset()

patch by Benedict; reviewed by Alex Petrov and Ariel Weisberg for CASSANDRA-21361

CHANGES.txt

@@ -1,4 +1,7 @@
 6.0-alpha2
+ * Accord: Tail Latency Improvements (CASSANDRA-21361)
+ * Artificial Latency Injection (CASSANDRA-17024)
+ * Accord: Clean Shutdown/Restart, Rebootstrap, et al (CASSANDRA-21355)
  * Reduce memory allocations in SelectStatement.getQuery (CASSANDRA-21351)
  * Avoid allocation by getFunctions in SelectStatement.authorize (CASSANDRA-21347)
  * Avoid unit conversion in DatabaseDescriptor.getMaxValueSize() for every deserializing Cell (CASSANDRA-21295)

modules/accord

@@ -1 +1 @@
-Subproject commit d3c5935589082f784b10dbee4d89691612f680eb
+Subproject commit 01aff40402a55bada08f6e5753ee3a4010f407da

src/java/org/apache/cassandra/config/AccordConfig.java

@@ -22,23 +22,28 @@ import java.util.concurrent.TimeUnit;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
 
+import accord.api.ProtocolModifiers.CleanCfkBefore;
+import accord.api.ProtocolModifiers.CoordinatorBacklogExecution;
+import accord.api.ProtocolModifiers.FastExecution;
+import accord.api.ProtocolModifiers.ReplicaExecution;
+import accord.api.ProtocolModifiers.SendStableMessages;
+import accord.primitives.TxnId;
 import accord.utils.Invariants;
 
 import org.apache.cassandra.journal.Params;
 import org.apache.cassandra.service.accord.serializers.Version;
 import org.apache.cassandra.service.consensus.TransactionalMode;
 
-import static org.apache.cassandra.config.AccordSpec.CatchupMode.NORMAL;
-import static org.apache.cassandra.config.AccordSpec.QueueShardModel.THREAD_POOL_PER_SHARD;
-import static org.apache.cassandra.config.AccordSpec.QueueSubmissionModel.SYNC;
-import static org.apache.cassandra.config.AccordSpec.RangeIndexMode.in_memory;
+import static org.apache.cassandra.config.AccordConfig.CatchupMode.NORMAL;
+import static org.apache.cassandra.config.AccordConfig.QueuePriorityModel.HLC_FIFO;
+import static org.apache.cassandra.config.AccordConfig.QueueShardModel.THREAD_POOL_PER_SHARD;
+import static org.apache.cassandra.config.AccordConfig.QueueSubmissionModel.SYNC;
+import static org.apache.cassandra.config.AccordConfig.RangeIndexMode.in_memory;
 
-// TODO (expected): rename to AccordConf?
-public class AccordSpec
+public class AccordConfig
 {
     public volatile boolean enabled = false;
 
-    // TODO (expected): move to JournalSpec
     public volatile String journal_directory;
 
     /**
@@ -107,6 +112,37 @@ public class AccordSpec
         EXEC_ST
     }
 
+    public enum QueuePriorityModel
+    {
+        /**
+         * All work is queued on a first-come first-serve basis.
+         * Overload can lead to more rapid degradation, as later phases of the state machine are delayed
+         * by the arrival of new work.
+         */
+        FIFO,
+
+        /**
+         * If the work has an associated TxnId, prioritise by its HLC (and FIFO otherwise)
+         */
+        HLC_FIFO,
+
+        /**
+         * Prioritise Apply, Stable, Commit, Accept, and PreAccept messages in that order.
+         * Within a given message type, prioritise by HLC.
+         * Other messages will be mixed with PreAccept messages, but using the next counter rather than the HLC of the TxnId.
+         * Note: this can have some performance edge cases for contended keys, as we may process Stable messages for later commands before
+         *       we process earlier Accept/Commit, which may delay execution
+         */
+        PHASE_HLC_FIFO,
+
+        /**
+         * Prioritise Apply, Stable, Commit, Accept, and PreAccept messages from the original coordinator only, in that order.
+         * Within a given message type, prioritise by HLC.
+         * Other messages will be mixed with PreAccept messages, but using the next counter rather than the HLC of the TxnId.
+         */
+        ORIG_PHASE_HLC_FIFO
+    }
+
     public QueueShardModel queue_shard_model = THREAD_POOL_PER_SHARD;
     public QueueSubmissionModel queue_submission_model = SYNC;
 
@@ -115,6 +151,15 @@ public class AccordSpec
      */
     public volatile OptionaldPositiveInt queue_shard_count = OptionaldPositiveInt.UNDEFINED;
 
+    public QueuePriorityModel queue_priority_model = HLC_FIFO;
+
+    // yield to other executor threads after executing this many tasks in a row, if there are waiting threads and tasks
+    public int queue_yield_interval = 100;
+
+    /**
+     * If the HLC is older than this, queue by FIFO instead
+     */
+    public DurationSpec.IntMillisecondsBound queue_priority_age_to_fifo = new DurationSpec.IntMillisecondsBound(500);
     /**
      * The target number of command stores to create per topology shard.
      * This determines the amount of execution parallelism possible for a given table/shard on the host.
@@ -165,6 +210,10 @@ public class AccordSpec
     public volatile DurationSpec.IntSecondsBound shard_durability_cycle = new DurationSpec.IntSecondsBound(5, TimeUnit.MINUTES);
     public volatile DurationSpec.IntSecondsBound global_durability_cycle = new DurationSpec.IntSecondsBound(5, TimeUnit.MINUTES);
 
+    public volatile DurationSpec.IntSecondsBound topology_watermark_interval = new DurationSpec.IntSecondsBound(60);
+    public volatile boolean topology_sync_propagator_enabled_pre_start = false;
+    public volatile boolean topology_sync_propagator_enabled_post_startup = false;
+
     public enum TransactionalRangeMigration
     {
         auto, explicit
@@ -178,11 +227,6 @@ public class AccordSpec
      */
     public volatile TransactionalRangeMigration range_migration = TransactionalRangeMigration.auto;
 
-    public enum RebootstrapMode
-    {
-        full_repair, truncate_and_stream
-    }
-
     public enum CatchupMode
     {
         DISABLED,
@@ -196,15 +240,43 @@ public class AccordSpec
      */
     public TransactionalMode default_transactional_mode = TransactionalMode.off;
 
-    public boolean ephemeralReadEnabled = true;
+    // ******** PROTOCOL MODIFIERS ***********
+
+    public CoordinatorBacklogExecution coordinator_backlog_execution;
+    public Boolean permit_local_delivery;
+    public Boolean permit_coordinator_local_execution; // if disabled, the privileged coordinator optimisation will be counter-productive and should also be disabled
+    public TxnId.FastPath permit_fast_path;
+    public Boolean permit_track_stable_medium_path;
+    public Boolean permit_fast_quorum_medium_path;
+    public Boolean always_inform_durable_single_key;
+    public ReplicaExecution replica_execution;
+    public Float replica_execution_distributed_persist_chance;
+    public FastExecution fast_write_execution;
+    public FastExecution fast_read_execution;
+    public CleanCfkBefore clean_cfk_before;
+    public SendStableMessages send_stable;
+    /**
+     * include the least information expected to be necessary in messages -
+     * this is more efficient but may lead to some additional traffic and latency when earlier messages had not arrived
+     */
+    public Boolean send_minimal;
+    // note: simulator incompatible (for now)
+    public Boolean precise_micros;
+
+    public boolean ephemeral_reads = true;
     public boolean state_cache_listener_jfr_enabled = false;
 
     public float hard_reject_ratio = 0.5f;
-    public int min_soft_reject_count = 10;
-    public int max_soft_reject_count = 100;
+    public int min_soft_reject_count = 100;
+    public int max_soft_reject_count = 1000;
     public DurationSpec.LongMicrosecondsBound soft_reject_age = new DurationSpec.LongMicrosecondsBound("10s");
     public DurationSpec.LongMicrosecondsBound soft_reject_cumulative_age = new DurationSpec.LongMicrosecondsBound("60s");
 
+
+    public DurationSpec.IntSecondsBound commands_for_key_prune_delta = new DurationSpec.IntSecondsBound(1);
+    public int commands_for_key_prune_interval = 64;
+    public DurationSpec.IntSecondsBound max_conflicts_prune_delta = new DurationSpec.IntSecondsBound(1);
+
     public DurationSpec.IntSecondsBound catchup_on_start_success_latency = new DurationSpec.IntSecondsBound(60);
     public DurationSpec.IntSecondsBound catchup_on_start_fail_latency = new DurationSpec.IntSecondsBound(900);
     public int catchup_on_start_max_attempts = 5;
@@ -216,7 +288,7 @@ public class AccordSpec
     public enum RangeIndexMode { in_memory, journal_sai }
     public RangeIndexMode range_index_mode = in_memory;
 
-    public final JournalSpec journal = new JournalSpec();
+    public final JournalConfig journal = new JournalConfig();
 
     public enum MixedTimeSourceHandling
     {
@@ -225,7 +297,7 @@ public class AccordSpec
 
     public volatile MixedTimeSourceHandling mixedTimeSourceHandling = MixedTimeSourceHandling.reject;
 
-    public static class JournalSpec implements Params
+    public static class JournalConfig implements Params
     {
         public enum ReplayMode
         {
@@ -265,11 +337,17 @@ public class AccordSpec
              */
             EXIT,
 
+            /**
+             * @deprecated since alpha release, replaced by ALLOW_UNSAFE_STARTUP for consistency with FailurePolicy.ALLOW_UNSAFE_STARTUP
+             */
+            @Deprecated(since="6.0")
+            UNSAFE_STARTUP,
+
             /**
              * If the start marker exceeds the stop marker startup, assuming the consensus log has been determined complete externally.
              * Note this is VERY UNSAFE if you care about isolation guarantees.
              */
-            UNSAFE_STARTUP,
+            ALLOW_UNSAFE_STARTUP,
 
             REBOOTSTRAP
         }
@@ -290,7 +368,7 @@ public class AccordSpec
         public Version version = Version.DOWNGRADE_SAFE_VERSION;
         public boolean enable_compaction = true;
 
-        public JournalSpec setFlushPeriod(DurationSpec newFlushPeriod)
+        public JournalConfig setFlushPeriod(DurationSpec newFlushPeriod)
         {
             flushPeriod = newFlushPeriod;
             flushCombinedBlockPeriod = Long.MIN_VALUE;

src/java/org/apache/cassandra/config/CassandraRelevantProperties.java

@@ -44,6 +44,7 @@ public enum CassandraRelevantProperties
 {
     ACCORD_AGENT_CLASS("cassandra.test.accord.agent"),
     ACCORD_ALLOW_TEST_MODES("cassandra.test.accord.allow_test_modes", "false"),
+    ACCORD_DEBUG_EXECUTION("accord.debug_execution"),
     ACCORD_KEY_PARANOIA_COSTFACTOR(Invariants.KEY_PARANOIA_COSTFACTOR),
     ACCORD_KEY_PARANOIA_CPU(Invariants.KEY_PARANOIA_CPU),
     ACCORD_KEY_PARANOIA_MEMORY(Invariants.KEY_PARANOIA_MEMORY),

src/java/org/apache/cassandra/config/Config.java

@@ -165,8 +165,6 @@ public class Config
     @Replaces(oldName = "cas_contention_timeout_in_ms", converter = Converters.MILLIS_DURATION_LONG, deprecated = true)
     public volatile DurationSpec.LongMillisecondsBound cas_contention_timeout = new DurationSpec.LongMillisecondsBound("1800ms");
 
-    public volatile DurationSpec.LongMillisecondsBound accord_preaccept_timeout = new DurationSpec.LongMillisecondsBound("1s");
-
     @Replaces(oldName = "truncate_request_timeout_in_ms", converter = Converters.MILLIS_DURATION_LONG, deprecated = true)
     public volatile DurationSpec.LongMillisecondsBound truncate_request_timeout = new DurationSpec.LongMillisecondsBound("60000ms");
 
@@ -1238,7 +1236,7 @@ public class Config
      */
     public ParameterizedClass default_compaction = null;
 
-    public final AccordSpec accord = new AccordSpec();
+    public final AccordConfig accord = new AccordConfig();
 
     public static Supplier<Config> getOverrideLoadConfig()
     {

src/java/org/apache/cassandra/config/DatabaseDescriptor.java

@@ -126,6 +126,7 @@ import org.apache.cassandra.service.CacheService.CacheType;
 import org.apache.cassandra.service.FileSystemOwnershipCheck;
 import org.apache.cassandra.service.StartupChecks;
 import org.apache.cassandra.service.StorageService;
+import org.apache.cassandra.service.accord.AccordService;
 import org.apache.cassandra.service.accord.api.AccordWaitStrategies;
 import org.apache.cassandra.service.consensus.TransactionalMode;
 import org.apache.cassandra.service.paxos.Paxos;
@@ -574,7 +575,7 @@ public class DatabaseDescriptor
 
         applyGuardrails();
 
-        applyAccordProgressLog();
+        applyAccord();
 
         applyStartupChecks();
     }
@@ -1370,7 +1371,7 @@ public class DatabaseDescriptor
         }
     }
 
-    private static void applyAccordProgressLog()
+    private static void applyAccord()
     {
         try
         {
@@ -1382,6 +1383,7 @@ public class DatabaseDescriptor
         {
             throw new ConfigurationException("Invalid accord progress log configuration: " + e.getMessage(), e);
         }
+        AccordService.applyProtocolModifiers(getAccord());
     }
 
     public static StartupChecksConfiguration getStartupChecksConfiguration()
@@ -1663,12 +1665,6 @@ public class DatabaseDescriptor
             logInfo("truncate_request_timeout", conf.truncate_request_timeout, LOWEST_ACCEPTED_TIMEOUT);
             conf.truncate_request_timeout = LOWEST_ACCEPTED_TIMEOUT;
         }
-
-        if (conf.accord_preaccept_timeout.toMilliseconds() < LOWEST_ACCEPTED_TIMEOUT.toMilliseconds())
-        {
-            logInfo("accord_preaccept_timeout", conf.accord_preaccept_timeout, LOWEST_ACCEPTED_TIMEOUT);
-            conf.accord_preaccept_timeout = LOWEST_ACCEPTED_TIMEOUT;
-        }
     }
 
     private static void logInfo(String property, DurationSpec.LongMillisecondsBound actualValue, DurationSpec.LongMillisecondsBound lowestAcceptedValue)
@@ -5634,18 +5630,17 @@ public class DatabaseDescriptor
         }
     }
 
-
-    public static AccordSpec getAccord()
+    public static AccordConfig getAccord()
     {
         return conf.accord;
     }
 
-    public static AccordSpec.TransactionalRangeMigration getTransactionalRangeMigration()
+    public static AccordConfig.TransactionalRangeMigration getTransactionalRangeMigration()
     {
         return conf.accord.range_migration;
     }
 
-    public static void setTransactionalRangeMigration(AccordSpec.TransactionalRangeMigration val)
+    public static void setTransactionalRangeMigration(AccordConfig.TransactionalRangeMigration val)
     {
         conf.accord.range_migration = Preconditions.checkNotNull(val);
     }
@@ -5670,12 +5665,12 @@ public class DatabaseDescriptor
         conf.accord.enabled = b;
     }
 
-    public static AccordSpec.QueueShardModel getAccordQueueShardModel()
+    public static AccordConfig.QueueShardModel getAccordQueueShardModel()
     {
         return conf.accord.queue_shard_model;
     }
 
-    public static AccordSpec.QueueSubmissionModel getAccordQueueSubmissionModel()
+    public static AccordConfig.QueueSubmissionModel getAccordQueueSubmissionModel()
     {
         return conf.accord.queue_submission_model;
     }
@@ -6278,7 +6273,7 @@ public class DatabaseDescriptor
 
     public static boolean getAccordEphemeralReadEnabledEnabled()
     {
-        return conf.accord.ephemeralReadEnabled;
+        return conf.accord.ephemeral_reads;
     }
 
     public static AutoRepairConfig getAutoRepairConfig()

src/java/org/apache/cassandra/cql3/Attributes.java

@@ -153,9 +153,9 @@ public class Attributes
     public void collectMarkerSpecification(VariableSpecifications boundNames)
     {
         if (timestamp != null)
-            timestamp.collectMarkerSpecification(boundNames);
+            timestamp.collectMarkerSpecification(boundNames, this);
         if (timeToLive != null)
-            timeToLive.collectMarkerSpecification(boundNames);
+            timeToLive.collectMarkerSpecification(boundNames, this);
     }
 
     public static class Raw

src/java/org/apache/cassandra/cql3/ColumnsExpression.java

@@ -386,10 +386,10 @@ public final class ColumnsExpression
      * @param boundNames the variables specification where to collect the
      * bind variables of the map key/collection element in.
      */
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
         if (element != null)
-            element.collectMarkerSpecification(boundNames);
+            element.collectMarkerSpecification(boundNames, owner);
     }
 
     /**

src/java/org/apache/cassandra/cql3/ElementExpression.java

@@ -140,9 +140,9 @@ public final class ElementExpression
      * @param boundNames the variables specification where to collect the
      * bind variables of the map key/collection element in.
      */
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
-        keyOrIndex.collectMarkerSpecification(boundNames);
+        keyOrIndex.collectMarkerSpecification(boundNames, owner);
     }
 
     /**

src/java/org/apache/cassandra/cql3/Json.java

@@ -86,7 +86,7 @@ public final class Json
 
         public Prepared prepareAndCollectMarkers(TableMetadata metadata, Collection<ColumnMetadata> receivers, VariableSpecifications boundNames)
         {
-            boundNames.add(bindIndex, makeReceiver(metadata));
+            boundNames.add(bindIndex, makeReceiver(metadata), null);
             return new PreparedMarker(bindIndex, receivers);
         }
 
@@ -239,7 +239,7 @@ public final class Json
         }
 
         @Override
-        public void collectMarkerSpecification(VariableSpecifications boundNames)
+        public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
         {
             // We've already collected what we should (and in practice this method is never called).
         }

src/java/org/apache/cassandra/cql3/Operation.java

@@ -103,12 +103,13 @@ public abstract class Operation
      * Collects the column specification for the bind variables of this operation.
      *
      * @param boundNames the list of column specification where to collect the
-     * bind variables of this term in.
+     *                   bind variables of this term in.
+     * @param owner
      */
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
         if (t != null)
-            t.collectMarkerSpecification(boundNames);
+            t.collectMarkerSpecification(boundNames, owner);
     }
 
     /**

src/java/org/apache/cassandra/cql3/Ordering.java

@@ -134,14 +134,14 @@ public class Ordering
          * Resolves column identifiers against the table schema.
          * Binds markers (?) to columns.
          */
-        public Ordering bind(TableMetadata table, VariableSpecifications boundNames)
+        public Ordering bind(TableMetadata table, VariableSpecifications boundNames, Object owner)
         {
-            return new Ordering(expression.bind(table, boundNames), direction);
+            return new Ordering(expression.bind(table, boundNames, owner), direction);
         }
 
         public interface Expression
         {
-            Ordering.Expression bind(TableMetadata table, VariableSpecifications boundNames);
+            Ordering.Expression bind(TableMetadata table, VariableSpecifications boundNames, Object owner);
         }
 
         public static class SingleColumn implements Expression
@@ -154,7 +154,7 @@ public class Ordering
             }
 
             @Override
-            public Ordering.Expression bind(TableMetadata table, VariableSpecifications boundNames)
+            public Ordering.Expression bind(TableMetadata table, VariableSpecifications boundNames, Object owner)
             {
                 return new Ordering.SingleColumn(table.getExistingColumn(column), table);
             }
@@ -172,11 +172,11 @@ public class Ordering
             }
 
             @Override
-            public Ordering.Expression bind(TableMetadata table, VariableSpecifications boundNames)
+            public Ordering.Expression bind(TableMetadata table, VariableSpecifications boundNames, Object owner)
             {
                 ColumnMetadata column = table.getExistingColumn(columnId);
                 Term value = vectorValue.prepare(table.keyspace, column);
-                value.collectMarkerSpecification(boundNames);
+                value.collectMarkerSpecification(boundNames, owner);
                 return new Ordering.Ann(column, table, value);
             }
         }

src/java/org/apache/cassandra/cql3/Relation.java

@@ -191,7 +191,7 @@ public final class Relation
      * @return the <code>Restriction</code> corresponding to this <code>Relation</code>
      * @throws InvalidRequestException if this <code>Relation</code> is not valid
      */
-    public SingleRestriction toRestriction(TableMetadata table, VariableSpecifications boundNames, boolean allowFiltering)
+    public SingleRestriction toRestriction(TableMetadata table, VariableSpecifications boundNames, Object owner, boolean allowFiltering)
     {
         ColumnsExpression columnsExpression = rawExpressions.prepare(table);
 
@@ -210,7 +210,7 @@ public final class Relation
             if (column.isClusteringColumn() && baseType.isCollection() && !column.type.isMultiCell())
                 throw invalidRequest(FROZEN_MAP_ENTRY_PREDICATES_NOT_SUPPORTED, column.name);
 
-            columnsExpression.collectMarkerSpecification(boundNames);
+            columnsExpression.collectMarkerSpecification(boundNames, owner);
         }
 
         operator.validateFor(columnsExpression);
@@ -220,7 +220,7 @@ public final class Relation
             receiver = ((CollectionType<?>) receiver.type).makeCollectionReceiver(receiver, operator.appliesToMapKeys());
 
         Terms terms = rawTerms.prepare(table.keyspace, receiver);
-        terms.collectMarkerSpecification(boundNames);
+        terms.collectMarkerSpecification(boundNames, owner);
 
         // An IN restriction with only one element is the same as an EQ restriction
         if (operator.isIN() && terms.containsSingleTerm())

src/java/org/apache/cassandra/cql3/VariableSpecifications.java

@@ -21,6 +21,8 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
+import javax.annotation.Nullable;
+
 import com.google.common.collect.ImmutableList;
 
 import org.apache.cassandra.schema.ColumnMetadata;
@@ -32,6 +34,8 @@ public class VariableSpecifications
     private final List<ColumnSpecification> specs;
     private volatile ImmutableList<ColumnSpecification> immutableSpecs;
     private final ColumnMetadata[] targetColumns;
+    // TODO (desired): this is an ugly way to figure out which sub statement we're using as a key in transactions, but path of least resistance...
+    private @Nullable Object[] targetOwners;
 
     public VariableSpecifications(List<ColumnIdentifier> variableNames)
     {
@@ -40,6 +44,12 @@ public class VariableSpecifications
         this.targetColumns = new ColumnMetadata[variableNames.size()];
     }
 
+    public void setSaveTargetOwners(boolean saveTargetOwners)
+    {
+        this.targetOwners = saveTargetOwners ? new Object[variableNames.size()] : null;
+    }
+
+
     /**
      * Returns an empty instance of <code>VariableSpecifications</code>.
      * @return an empty instance of <code>VariableSpecifications</code>
@@ -77,14 +87,14 @@ public class VariableSpecifications
      *
      * Callers of this method should ensure that all statements operate on the same table.
      */
-    public short[] getPartitionKeyBindVariableIndexes(TableMetadata metadata)
+    public short[] getPartitionKeyBindVariableIndexes(TableMetadata metadata, Object targetOwner)
     {
         short[] partitionKeyPositions = new short[metadata.partitionKeyColumns().size()];
         boolean[] set = new boolean[partitionKeyPositions.length];
         for (int i = 0; i < targetColumns.length; i++)
         {
             ColumnMetadata targetColumn = targetColumns[i];
-            if (targetColumn != null && targetColumn.isPartitionKey())
+            if (targetColumn != null && targetColumn.isPartitionKey() && (targetOwners == null || (targetOwner != null && targetOwners[i] == targetOwner)))
             {
                 assert targetColumn.ksName.equals(metadata.keyspace) && targetColumn.cfName.equals(metadata.name);
                 partitionKeyPositions[targetColumn.position()] = (short) i;
@@ -99,12 +109,15 @@ public class VariableSpecifications
         return partitionKeyPositions;
     }
 
-    public void add(int bindIndex, ColumnSpecification spec)
+    public void add(int bindIndex, ColumnSpecification spec, Object owner)
     {
         assert immutableSpecs == null : "bind variable specs cannot be modified once we started to use them";
         if (spec instanceof ColumnMetadata)
             targetColumns[bindIndex] = (ColumnMetadata) spec;
 
+        if (targetOwners != null)
+            targetOwners[bindIndex] = owner;
+
         ColumnIdentifier bindMarkerName = variableNames.get(bindIndex);
         // Use the user name, if there is one
         if (bindMarkerName != null)

src/java/org/apache/cassandra/cql3/conditions/ColumnCondition.java

@@ -104,10 +104,10 @@ public final class ColumnCondition
      * @param boundNames the list of column specification where to collect the
      * bind variables of this term in.
      */
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
-        columnsExpression.collectMarkerSpecification(boundNames);
-        values.collectMarkerSpecification(boundNames);
+        columnsExpression.collectMarkerSpecification(boundNames, owner);
+        values.collectMarkerSpecification(boundNames, owner);
     }
 
     public ColumnCondition.Bound bind(QueryOptions options)

src/java/org/apache/cassandra/cql3/functions/FunctionCall.java

@@ -61,10 +61,10 @@ public class FunctionCall extends Term.NonTerminal
         fun.addFunctionsTo(functions);
     }
 
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
         for (Term t : terms)
-            t.collectMarkerSpecification(boundNames);
+            t.collectMarkerSpecification(boundNames, owner);
     }
 
     @Override

src/java/org/apache/cassandra/cql3/restrictions/CustomIndexExpression.java

@@ -45,11 +45,11 @@ public class CustomIndexExpression
         this.valueRaw = value;
     }
 
-    public void prepareValue(TableMetadata table, AbstractType<?> expressionType, VariableSpecifications boundNames)
+    public void prepareValue(TableMetadata table, AbstractType<?> expressionType, VariableSpecifications boundNames, Object owner)
     {
         ColumnSpecification spec = new ColumnSpecification(table.keyspace, table.keyspace, valueColId, expressionType);
         value = valueRaw.prepare(table.keyspace, spec);
-        value.collectMarkerSpecification(boundNames);
+        value.collectMarkerSpecification(boundNames, owner);
     }
 
     public void addToRowFilter(RowFilter filter, TableMetadata table, QueryOptions options)

src/java/org/apache/cassandra/cql3/restrictions/StatementRestrictions.java

@@ -175,11 +175,12 @@ public final class StatementRestrictions
                                  WhereClause whereClause,
                                  VariableSpecifications boundNames,
                                  List<Ordering> orderings,
+                                 Object owner,
                                  boolean selectsOnlyStaticColumns,
                                  boolean allowFiltering,
                                  boolean forView)
     {
-        this(state, type, table, indexHints, whereClause, boundNames, orderings, selectsOnlyStaticColumns, type.allowUseOfSecondaryIndices(), allowFiltering, forView);
+        this(state, type, table, indexHints, whereClause, boundNames, orderings, owner, selectsOnlyStaticColumns, type.allowUseOfSecondaryIndices(), allowFiltering, forView);
     }
 
     /*
@@ -193,6 +194,7 @@ public final class StatementRestrictions
                                  WhereClause whereClause,
                                  VariableSpecifications boundNames,
                                  List<Ordering> orderings,
+                                 Object owner,
                                  boolean selectsOnlyStaticColumns,
                                  boolean allowUseOfSecondaryIndices,
                                  boolean allowFiltering,
@@ -227,11 +229,11 @@ public final class StatementRestrictions
                 if (!forView)
                     throw new InvalidRequestException("Unsupported restriction: " + relation);
 
-                this.notNullColumns.addAll(relation.toRestriction(table, boundNames, allowFiltering).columns());
+                this.notNullColumns.addAll(relation.toRestriction(table, boundNames, owner, allowFiltering).columns());
             }
             else if (operator.requiresIndexing())
             {
-                Restriction restriction = relation.toRestriction(table, boundNames, allowFiltering);
+                Restriction restriction = relation.toRestriction(table, boundNames, owner, allowFiltering);
 
                 if (!type.allowUseOfSecondaryIndices() || !restriction.hasSupportingIndex(indexRegistry, indexHints))
                     throw invalidRequest("%s restriction is only supported on properly " +
@@ -241,7 +243,7 @@ public final class StatementRestrictions
             }
             else
             {
-                addRestriction(relation.toRestriction(table, boundNames, allowFiltering), indexRegistry, indexHints);
+                addRestriction(relation.toRestriction(table, boundNames, owner, allowFiltering), indexRegistry, indexHints);
             }
         }
 
@@ -756,7 +758,7 @@ public final class StatementRestrictions
             if (expressionType == null)
                 throw IndexRestrictions.customExpressionNotSupported(expression.targetIndex);
 
-            expression.prepareValue(table, expressionType, boundNames);
+            expression.prepareValue(table, expressionType, boundNames, table);
 
             filterRestrictions.add(expression);
         }

src/java/org/apache/cassandra/cql3/selection/Selectable.java

@@ -222,7 +222,7 @@ public interface Selectable extends AssignmentTestable
             // something a lot more helpful and in practice user can bind those markers by position or, even better,
             // use bind markers.
             Term term = rawTerm.prepare(table.keyspace, new ColumnSpecification(table.keyspace, table.name, bindMarkerNameInSelection, type));
-            term.collectMarkerSpecification(boundNames);
+            term.collectMarkerSpecification(boundNames, table);
             return TermSelector.newFactory(rawTerm.getText(), term, type);
         }
 
@@ -1481,7 +1481,7 @@ public interface Selectable extends AssignmentTestable
             ColumnSpecification boundSpec = specForElementOrSlice(selected, receiver, ((CollectionType) type).kind, "Element");
 
             Term elt = element.prepare(cfm.keyspace, boundSpec);
-            elt.collectMarkerSpecification(boundNames);
+            elt.collectMarkerSpecification(boundNames, cfm);
             return ElementsSelector.newElementFactory(toString(), factory, (CollectionType)type, elt);
         }
 
@@ -1571,8 +1571,8 @@ public interface Selectable extends AssignmentTestable
             // The equivalent of doing this when preparing values would be to use UNSET.
             Term f = from == null ? Constants.UNSET_VALUE : from.prepare(cfm.keyspace, boundSpec);
             Term t = to == null ? Constants.UNSET_VALUE : to.prepare(cfm.keyspace, boundSpec);
-            f.collectMarkerSpecification(boundNames);
-            t.collectMarkerSpecification(boundNames);
+            f.collectMarkerSpecification(boundNames, cfm);
+            t.collectMarkerSpecification(boundNames, cfm);
             return ElementsSelector.newSliceFactory(toString(), factory, (CollectionType)type, f, t);
         }
 

src/java/org/apache/cassandra/cql3/statements/BatchStatement.java

@@ -222,7 +222,7 @@ public class BatchStatement implements CQLStatement.CompositeCQLStatement
         // multiple tables, we won't send partition key bind indexes.
         return (affectsMultipleTables || statements.isEmpty())
              ? null
-             : bindVariables.getPartitionKeyBindVariableIndexes(statements.get(0).metadata());
+             : bindVariables.getPartitionKeyBindVariableIndexes(statements.get(0).metadata(), statements.get(0).attrs);
     }
 
     @Override

src/java/org/apache/cassandra/cql3/statements/DeleteStatement.java

@@ -176,11 +176,11 @@ public class DeleteStatement extends ModificationStatement
                 checkFalse(def.isPrimaryKeyColumn(), "Invalid identifier %s for deletion (should not be a PRIMARY KEY part)", def.name);
 
                 Operation op = deletion.prepare(metadata.keyspace, def, metadata);
-                op.collectMarkerSpecification(bindVariables);
+                op.collectMarkerSpecification(bindVariables, attrs);
                 operations.add(op, metadata);
             }
 
-            StatementRestrictions restrictions = newRestrictions(state, metadata, bindVariables, operations, whereClause, conditions);
+            StatementRestrictions restrictions = newRestrictions(state, metadata, bindVariables, operations, whereClause, conditions, attrs);
 
             DeleteStatement stmt = new DeleteStatement(bindVariables,
                                                        metadata,

src/java/org/apache/cassandra/cql3/statements/ModificationStatement.java

@@ -282,7 +282,7 @@ public abstract class ModificationStatement implements CQLStatement.SingleKeyspa
     @Override
     public short[] getPartitionKeyBindVariableIndexes()
     {
-        return bindVariables.getPartitionKeyBindVariableIndexes(metadata);
+        return bindVariables.getPartitionKeyBindVariableIndexes(metadata, attrs);
     }
 
     @Override
@@ -1328,7 +1328,7 @@ public abstract class ModificationStatement implements CQLStatement.SingleKeyspa
             for (ColumnCondition.Raw rawCondition : conditions)
             {
                 ColumnCondition condition = rawCondition.prepare(metadata);
-                condition.collectMarkerSpecification(bindVariables);
+                condition.collectMarkerSpecification(bindVariables, attrs);
 
                 builder.add(condition);
             }
@@ -1356,13 +1356,14 @@ public abstract class ModificationStatement implements CQLStatement.SingleKeyspa
                                                         VariableSpecifications boundNames,
                                                         Operations operations,
                                                         WhereClause where,
-                                                        Conditions conditions)
+                                                        Conditions conditions,
+                                                        Object owner)
         {
             if (where.containsCustomExpressions())
                 throw new InvalidRequestException(CUSTOM_EXPRESSIONS_NOT_ALLOWED);
 
             boolean applyOnlyToStaticColumns = appliesOnlyToStaticColumns(operations, conditions);
-            return new StatementRestrictions(state, type, metadata, IndexHints.NONE, where, boundNames, Collections.emptyList(), applyOnlyToStaticColumns, false, false);
+            return new StatementRestrictions(state, type, metadata, IndexHints.NONE, where, boundNames, Collections.emptyList(), owner, applyOnlyToStaticColumns, false, false);
         }
 
         public List<ColumnCondition.Raw> getConditions()

src/java/org/apache/cassandra/cql3/statements/SelectStatement.java

@@ -241,7 +241,7 @@ public class SelectStatement implements CQLStatement.SingleKeyspaceCqlStatement,
     @Override
     public short[] getPartitionKeyBindVariableIndexes()
     {
-        return bindVariables.getPartitionKeyBindVariableIndexes(table);
+        return bindVariables.getPartitionKeyBindVariableIndexes(table, table);
     }
 
     @Override
@@ -1468,7 +1468,7 @@ public class SelectStatement implements CQLStatement.SingleKeyspaceCqlStatement,
         private List<Ordering> getOrderings(TableMetadata table)
         {
             return parameters.orderings.stream()
-                                       .map(o -> o.bind(table, bindVariables))
+                                       .map(o -> o.bind(table, bindVariables, table))
                                        .collect(Collectors.toList());
         }
 
@@ -1498,6 +1498,7 @@ public class SelectStatement implements CQLStatement.SingleKeyspaceCqlStatement,
                                              whereClause,
                                              boundNames,
                                              orderings,
+                                             metadata,
                                              selectsOnlyStaticColumns,
                                              parameters.allowFiltering || !requiresAllowFilteringIfNotSpecified(metadata, true),
                                              forView);
@@ -1511,7 +1512,7 @@ public class SelectStatement implements CQLStatement.SingleKeyspaceCqlStatement,
                 return null;
 
             Term prepLimit = limit.prepare(keyspace, limitReceiver);
-            prepLimit.collectMarkerSpecification(boundNames);
+            prepLimit.collectMarkerSpecification(boundNames, null);
             return prepLimit;
         }
 

src/java/org/apache/cassandra/cql3/statements/TransactionStatement.java

@@ -204,6 +204,25 @@ public class TransactionStatement implements CQLStatement.CompositeCQLStatement,
         return bindVariables.getImmutableBindVariables();
     }
 
+    @Override
+    public short[] getPartitionKeyBindVariableIndexes()
+    {
+        if (returningSelect != null)
+        {
+            short[] result = returningSelect.select.getPartitionKeyBindVariableIndexes();
+            if (result != null)
+                return result;
+        }
+
+        for (ModificationStatement stmt : updates)
+        {
+            short[] result = stmt.getPartitionKeyBindVariableIndexes();
+            if (result != null)
+                return result;
+        }
+        return null;
+    }
+
     @Override
     public void authorize(ClientState state)
     {
@@ -733,6 +752,7 @@ public class TransactionStatement implements CQLStatement.CompositeCQLStatement,
             if (select != null || returning != null)
                 checkTrue(select != null ^ returning != null, "Cannot specify both a full SELECT and a SELECT w/ LET references.");
 
+            bindVariables.setSaveTargetOwners(true);
             List<NamedSelect> preparedAssignments = new ArrayList<>(assignments.size());
             Map<Integer, RowDataReference.ReferenceSource> refSources = new HashMap<>();
             Set<String> selectNames = new HashSet<>();

src/java/org/apache/cassandra/cql3/statements/UpdateStatement.java

@@ -220,7 +220,7 @@ public class UpdateStatement extends ModificationStatement
                 else
                 {
                     Operation operation = new Operation.SetValue(value).prepare(metadata, def, !conditions.isEmpty());
-                    operation.collectMarkerSpecification(bindVariables);
+                    operation.collectMarkerSpecification(bindVariables, attrs);
                     operations.add(operation, metadata);
                 }
             }
@@ -234,6 +234,7 @@ public class UpdateStatement extends ModificationStatement
                                                                            whereClause.build(),
                                                                            bindVariables,
                                                                            Collections.emptyList(),
+                                                                           attrs,
                                                                            applyOnlyToStaticColumns,
                                                                            false,
                                                                            false);
@@ -295,7 +296,7 @@ public class UpdateStatement extends ModificationStatement
                 else
                 {
                     Operation operation = new Operation.SetValue(raw).prepare(metadata, def, !conditions.isEmpty());
-                    operation.collectMarkerSpecification(bindVariables);
+                    operation.collectMarkerSpecification(bindVariables, attrs);
                     operations.add(operation, metadata);
                 }
             }
@@ -309,6 +310,7 @@ public class UpdateStatement extends ModificationStatement
                                                                            whereClause.build(),
                                                                            bindVariables,
                                                                            Collections.emptyList(),
+                                                                           attrs,
                                                                            applyOnlyToStaticColumns,
                                                                            false,
                                                                            false);
@@ -417,7 +419,7 @@ public class UpdateStatement extends ModificationStatement
                 ColumnMetadata def = metadata.getExistingColumn(entry.left);
                 checkFalse(def.isPrimaryKeyColumn(), UPDATING_PRIMARY_KEY_MESSAGE, def.name);
                 Operation operation = entry.right.prepare(metadata, def, !conditions.isEmpty() || isForTxn);
-                operation.collectMarkerSpecification(bindVariables);
+                operation.collectMarkerSpecification(bindVariables, attrs);
                 operations.add(operation, metadata);
             }
 
@@ -435,7 +437,8 @@ public class UpdateStatement extends ModificationStatement
                                                                  bindVariables,
                                                                  operations,
                                                                  whereClause,
-                                                                 conditions);
+                                                                 conditions,
+                                                                 attrs);
 
             return new UpdateStatement(type,
                                        bindVariables,

src/java/org/apache/cassandra/cql3/statements/schema/CreateViewStatement.java

@@ -310,6 +310,7 @@ public final class CreateViewStatement extends AlterSchemaStatement
                                       whereClause,
                                       VariableSpecifications.empty(),
                                       Collections.emptyList(),
+                                      null,
                                       false,
                                       false,
                                       true,

src/java/org/apache/cassandra/cql3/terms/InMarker.java

@@ -54,9 +54,9 @@ public final class InMarker extends Terms.NonTerminals
     public void addFunctionsTo(List<Function> functions) {}
 
     @Override
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
-        boundNames.add(bindIndex, receiver);
+        boundNames.add(bindIndex, receiver, owner);
     }
 
     @Override

src/java/org/apache/cassandra/cql3/terms/Lists.java

@@ -343,10 +343,10 @@ public abstract class Lists
         }
 
         @Override
-        public void collectMarkerSpecification(VariableSpecifications boundNames)
+        public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
         {
-            super.collectMarkerSpecification(boundNames);
-            idx.collectMarkerSpecification(boundNames);
+            super.collectMarkerSpecification(boundNames, owner);
+            idx.collectMarkerSpecification(boundNames, owner);
         }
 
         public void execute(DecoratedKey partitionKey, UpdateParameters params) throws InvalidRequestException

src/java/org/apache/cassandra/cql3/terms/Maps.java

@@ -276,10 +276,10 @@ public final class Maps
         }
 
         @Override
-        public void collectMarkerSpecification(VariableSpecifications boundNames)
+        public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
         {
-            super.collectMarkerSpecification(boundNames);
-            k.collectMarkerSpecification(boundNames);
+            super.collectMarkerSpecification(boundNames, owner);
+            k.collectMarkerSpecification(boundNames, owner);
         }
 
         public void execute(DecoratedKey partitionKey, UpdateParameters params) throws InvalidRequestException

src/java/org/apache/cassandra/cql3/terms/Marker.java

@@ -55,9 +55,9 @@ public final class Marker extends Term.NonTerminal
     }
 
     @Override
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
-        boundNames.add(bindIndex, receiver);
+        boundNames.add(bindIndex, receiver, owner);
     }
 
     @Override

src/java/org/apache/cassandra/cql3/terms/MultiElements.java

@@ -116,12 +116,12 @@ public final class MultiElements
         }
 
         @Override
-        public void collectMarkerSpecification(VariableSpecifications boundNames)
+        public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
         {
             if (type.supportsElementBindMarkers())
             {
                 for (int i = 0, m = elements.size(); i < m; i++)
-                    elements.get(i).collectMarkerSpecification(boundNames);
+                    elements.get(i).collectMarkerSpecification(boundNames, owner);
             }
         }
 

src/java/org/apache/cassandra/cql3/terms/Term.java

@@ -68,9 +68,10 @@ public interface Term
      * This is obviously a no-op if the term is Terminal.
      *
      * @param boundNames the variables specification where to collect the
-     * bind variables of this term in.
+     *                   bind variables of this term in.
+     * @param owner
      */
-    void collectMarkerSpecification(VariableSpecifications boundNames);
+    void collectMarkerSpecification(VariableSpecifications boundNames, Object owner);
 
     /**
      * Bind the values in this term to the values contained in the {@code options}.
@@ -246,7 +247,7 @@ public interface Term
     abstract class Terminal implements Term
     {
         @Override
-        public void collectMarkerSpecification(VariableSpecifications boundNames) {}
+        public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner) {}
 
         @Override
         public Terminal bind(QueryOptions options) { return this; }

src/java/org/apache/cassandra/cql3/terms/Terms.java

@@ -95,9 +95,10 @@ public interface Terms
      * This is obviously a no-op if the terms are Terminals.
      *
      * @param boundNames the variables specification where to collect the
-     * bind variables of the terms in.
+     *                   bind variables of the terms in.
+     * @param owner
      */
-    void collectMarkerSpecification(VariableSpecifications boundNames);
+    void collectMarkerSpecification(VariableSpecifications boundNames, Object owner);
 
     /**
      * Bind the values in these terms to the values contained in {@code options}.
@@ -453,7 +454,7 @@ public interface Terms
             }
         };
         @Override
-        public void collectMarkerSpecification(VariableSpecifications boundNames) {}
+        public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner) {}
 
         @Override
         public final Terminals bind(QueryOptions options)
@@ -621,9 +622,9 @@ public interface Terms
                 }
 
                 @Override
-                public void collectMarkerSpecification(VariableSpecifications boundNames)
+                public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
                 {
-                    term.collectMarkerSpecification(boundNames);
+                    term.collectMarkerSpecification(boundNames, owner);
                 }
 
                 @Override
@@ -687,12 +688,12 @@ public interface Terms
                 }
 
                 @Override
-                public void collectMarkerSpecification(VariableSpecifications boundNames)
+                public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
                 {
                     for (int i = 0, m = terms.size(); i < m; i++)
                     {
                         Term term = terms.get(i);
-                        term.collectMarkerSpecification(boundNames);
+                        term.collectMarkerSpecification(boundNames, owner);
                     }
                 }
 

src/java/org/apache/cassandra/cql3/transactions/ConditionStatement.java

@@ -93,7 +93,7 @@ public class ConditionStatement
             {
                 // In the IS NULL/IS NOT NULL case, the reference will always be on the LHS
                 RowDataReference reference = ((RowDataReference.Raw) lhs).prepareAsReceiver();
-                reference.collectMarkerSpecification(bindVariables);
+                reference.collectMarkerSpecification(bindVariables, null);
                 return new ConditionStatement(reference, kind, null, false);
             }
                 
@@ -124,8 +124,8 @@ public class ConditionStatement
                 throw new IllegalStateException("Either the left-hand or right-hand side must be a reference!");
             }
 
-            reference.collectMarkerSpecification(bindVariables);
-            value.collectMarkerSpecification(bindVariables);
+            reference.collectMarkerSpecification(bindVariables, null);
+            value.collectMarkerSpecification(bindVariables, null);
             return new ConditionStatement(reference, kind, value, reversed);
         }
     }

src/java/org/apache/cassandra/cql3/transactions/RowDataReference.java

@@ -77,10 +77,10 @@ public class RowDataReference extends Term.NonTerminal
     }
 
     @Override
-    public void collectMarkerSpecification(VariableSpecifications boundNames)
+    public void collectMarkerSpecification(VariableSpecifications boundNames, Object owner)
     {
         if (elementPath != null)
-            elementPath.collectMarkerSpecification(boundNames);
+            elementPath.collectMarkerSpecification(boundNames, owner);
     }
 
     @Override

src/java/org/apache/cassandra/db/virtual/AccordDebugKeyspace.java

@@ -74,7 +74,6 @@ import accord.local.DurableBefore;
 import accord.local.MaxConflicts;
 import accord.local.Node;
 import accord.local.PreLoadContext;
-import accord.local.RejectBefore;
 import accord.local.SafeCommand;
 import accord.local.SafeCommandStore;
 import accord.local.StoreParticipants;
@@ -107,7 +106,7 @@ import accord.utils.async.AsyncChains;
 import accord.utils.async.AsyncResult;
 import accord.utils.async.AsyncResults;
 
-import org.apache.cassandra.config.AccordSpec.JournalSpec.ReplayMode;
+import org.apache.cassandra.config.AccordConfig.JournalConfig.ReplayMode;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.cql3.statements.schema.CreateTableStatement;
 import org.apache.cassandra.db.ColumnFamilyStore;
@@ -211,7 +210,6 @@ public class AccordDebugKeyspace extends VirtualKeyspace
     public static final String NODE_OPS           = "node_ops";
     public static final String PROGRESS_LOG       = "progress_log";
     public static final String REDUNDANT_BEFORE   = "redundant_before";
-    public static final String REJECT_BEFORE      = "reject_before";
     public static final String TXN                = "txn";
     public static final String TXN_CACHE          = "txn_cache";
     public static final String TXN_BLOCKED_BY     = "txn_blocked_by";
@@ -252,7 +250,6 @@ public class AccordDebugKeyspace extends VirtualKeyspace
             new NodeOpsTable(),
             new ProgressLogTable(),
             new RedundantBeforeTable(),
-            new RejectBeforeTable(),
             new TxnBlockedByTable(),
             new TxnTable(),
             new TxnCacheTable(),
@@ -819,7 +816,9 @@ public class AccordDebugKeyspace extends VirtualKeyspace
                         "  token_start 'TokenUtf8Type',\n" +
                         "  table_id text,\n" +
                         "  token_end 'TokenUtf8Type',\n" +
-                        "  timestamp text,\n" +
+                        "  any text,\n" +
+                        "  write text,\n" +
+                        "  reject text,\n" +
                         "  PRIMARY KEY (command_store_id, token_start)" +
                         ')', Int32Type.instance), FAIL, ASC);
         }
@@ -843,7 +842,9 @@ public class AccordDebugKeyspace extends VirtualKeyspace
                                 .lazyCollect(columns -> {
                                     columns.add("token_end", entry.end(), AccordDebugKeyspace::printToken)
                                            .add("table_id", tableIdStr)
-                                           .add("timestamp", entry, TO_STRING);
+                                           .add("any", entry.any, TO_STRING)
+                                           .add("write", entry.write, TO_STRING)
+                                           .add("reject", entry.reject, TO_STRING);
                                 });
                              return rows;
                         }, rows
@@ -1084,47 +1085,6 @@ public class AccordDebugKeyspace extends VirtualKeyspace
         }
     }
 
-    public static final class RejectBeforeTable extends AbstractLazyVirtualTable
-    {
-        private RejectBeforeTable()
-        {
-            super(parse(VIRTUAL_ACCORD_DEBUG, REJECT_BEFORE,
-                        "Accord per-CommandStore RejectBefore State",
-                        "CREATE TABLE %s (\n" +
-                        "  command_store_id int,\n" +
-                        "  token_start 'TokenUtf8Type',\n" +
-                        "  table_id text,\n" +
-                        "  token_end 'TokenUtf8Type',\n" +
-                        "  timestamp text,\n" +
-                        "  PRIMARY KEY (command_store_id, token_start)" +
-                        ')', UTF8Type.instance), FAIL, ASC);
-        }
-
-        @Override
-        protected void collect(PartitionsCollector collector)
-        {
-            CommandStores commandStores = AccordService.unsafeInstance().node().commandStores();
-            for (CommandStore commandStore : commandStores.all())
-            {
-                RejectBefore rejectBefore = commandStore.unsafeGetRejectBefore();
-                if (rejectBefore == null)
-                    continue;
-
-                collector.partition(commandStore.id()).collect(rows -> {
-                    TableId tableId = ((AccordCommandStore)commandStore).tableId();
-                    String tableIdStr = tableId.toString();
-                    rejectBefore.foldlWithBounds((timestamp, rs, start, end) -> {
-                        rs.add(printToken(start))
-                          .lazyCollect(columns -> columns.add("table_id", tableIdStr)
-                                                         .add("token_end", end, AccordDebugKeyspace::printToken)
-                                                         .add("timestamp", timestamp, AccordDebugKeyspace::toStringOrNull));
-                        return rs;
-                    }, rows, ignore -> false);
-                });
-            }
-        }
-    }
-
     /**
      * Usage:
      * collect N events (may be more than N messages)
@@ -1259,6 +1219,7 @@ public class AccordDebugKeyspace extends VirtualKeyspace
                         "  event text,\n" +
                         "  at_micros bigint,\n" +
                         "  command_store_id int,\n" +
+                        "  remote_node_id int,\n" +
                         "  message text,\n" +
                         "  PRIMARY KEY (txn_id, id_micros, event, at_micros)" +
                         ')', TxnIdUtf8Type.instance), FAIL, UNSORTED, UNSORTED);
@@ -1302,6 +1263,7 @@ public class AccordDebugKeyspace extends VirtualKeyspace
         @Override
         public void collect(PartitionsCollector collector)
         {
+            int nodeId = AccordService.unsafeInstance().nodeId().id;
             tracing().forEach(id -> true, (txnId, events) -> {
                 events.forEach(e -> {
                     if (e.messages().isEmpty())
@@ -1318,6 +1280,8 @@ public class AccordDebugKeyspace extends VirtualKeyspace
                             .eagerCollect(columns -> {
                                 columns.add("command_store_id", m.commandStoreId)
                                        .add("message", m.message);
+                                if (m.nodeId >= 0)
+                                    columns.add("remote_node_id", m.nodeId);
                             });
                         });
                     }

src/java/org/apache/cassandra/journal/Segment.java

@@ -18,6 +18,7 @@
 package org.apache.cassandra.journal;
 
 import java.nio.ByteBuffer;
+import java.util.concurrent.RejectedExecutionException;
 
 import accord.utils.Invariants;
 
@@ -44,8 +45,12 @@ public abstract class Segment<K, V> implements SelfRefCounted<Segment<K, V>>, Co
 
         public final void tidy()
         {
-            if (executor != null) executor.execute(this);
-            else onUnreferenced();
+            if (executor != null)
+            {
+                try { executor.execute(this); return; }
+                catch (RejectedExecutionException rje) { /* fallthrough and call directly */ }
+            }
+            onUnreferenced();
         }
     }
 

src/java/org/apache/cassandra/metrics/AccordCoordinatorMetrics.java

@@ -34,6 +34,7 @@ import accord.local.Node;
 import accord.primitives.Ballot;
 import accord.primitives.Deps;
 import accord.primitives.TxnId;
+import accord.utils.UnhandledEnum;
 
 import org.apache.cassandra.service.accord.api.AccordTimeService;
 import org.apache.cassandra.tracing.Tracing;
@@ -248,10 +249,14 @@ public class AccordCoordinatorMetrics
                 {
                     switch (path)
                     {
+                        default: throw new UnhandledEnum(path);
                         case EPHEMERAL: metrics.ephemeral.mark(); break;
                         case FAST: metrics.fastPaths.mark(); break;
                         case MEDIUM: metrics.mediumPaths.mark(); break;
                         case SLOW: metrics.slowPaths.mark(); break;
+                        case BACKLOG:
+                        case RECOVER:
+                            break;
                     }
                 }
             }

src/java/org/apache/cassandra/metrics/AccordExecutorMetrics.java

@@ -39,6 +39,7 @@ public class AccordExecutorMetrics
     public final ShardedDecayingHistogram elapsedPreparingToRun = HISTOGRAMS.newHistogram(TimeUnit.SECONDS.toNanos(1L));
     public final ShardedDecayingHistogram elapsedWaitingToRun = HISTOGRAMS.newHistogram(TimeUnit.SECONDS.toNanos(1L));
     public final ShardedDecayingHistogram elapsedRunning = HISTOGRAMS.newHistogram(TimeUnit.SECONDS.toNanos(1L));
+    public final ShardedDecayingHistogram elapsed = HISTOGRAMS.newHistogram(TimeUnit.SECONDS.toNanos(1L));
 
     // number of keys involved
     public final ShardedDecayingHistogram keys = HISTOGRAMS.newHistogram(1 << 12);

src/java/org/apache/cassandra/metrics/LogLinearDecayingHistograms.java

@@ -167,6 +167,13 @@ public class LogLinearDecayingHistograms
             Invariants.require(buffer.histograms == LogLinearDecayingHistograms.this);
             buffer.add(histogramIndex, value);
         }
+
+        public void clear()
+        {
+            flush();
+            totalCount = 0;
+            Arrays.fill(buckets, 0);
+        }
     }
 
     private static final long ANTI_DECAY_REFRESH_RATE = TimeUnit.SECONDS.toNanos(1L);

src/java/org/apache/cassandra/metrics/LogLinearHistogram.java

@@ -317,4 +317,10 @@ public class LogLinearHistogram
         snapshot.totalCount += totalCount;
         snapshot.cumulative = null;
     }
+
+    public void clear()
+    {
+        totalCount = 0;
+        Arrays.fill(buckets, 0);
+    }
 }

src/java/org/apache/cassandra/metrics/ShardedDecayingHistograms.java

@@ -76,6 +76,41 @@ public class ShardedDecayingHistograms
             return shard.histograms.get(histogramIndex);
         }
 
+        public LogLinearSnapshot refresh()
+        {
+            synchronized (ShardedDecayingHistograms.this)
+            {
+                long now = Clock.Global.currentTimeMillis();
+                List<LogLinearSnapshot> snapshot = new ArrayList<>(ShardedDecayingHistograms.this.snapshot);
+                if (snapshot.size() <= histogramIndex)
+                    return ShardedDecayingHistograms.this.refresh(now).get(histogramIndex);
+
+                LogLinearSnapshot result = LogLinearSnapshot.emptyForMax(initialMaxValue);
+                snapshot.set(histogramIndex, result);
+                for (DecayingHistogramsShard shard : shards)
+                    shard.updateSnapshot(histogramIndex, result, now);
+                ShardedDecayingHistograms.this.snapshot = snapshot;
+                // don't update snapshotAt, since we have only refreshed one histogram
+                return result;
+            }
+        }
+
+        public void clear()
+        {
+            for (DecayingHistogramsShard shard : shards)
+            {
+                shard.lock.lock();
+                try
+                {
+                    shard.histograms.get(histogramIndex).clear();
+                }
+                finally
+                {
+                    shard.lock.unlock();
+                }
+            }
+        }
+
         @Override
         public boolean isCumulative()
         {
@@ -108,6 +143,19 @@ public class ShardedDecayingHistograms
             }
         }
 
+        private void updateSnapshot(int histogramIndex, LogLinearSnapshot update, long at)
+        {
+            lock.lock();
+            try
+            {
+                histograms.get(histogramIndex).updateSnapshot(update, at);
+            }
+            finally
+            {
+                lock.unlock();
+            }
+        }
+
         public LogLinearDecayingHistograms unsafeGetInternal()
         {
             return histograms;

src/java/org/apache/cassandra/metrics/ShardedHistogram.java

@@ -139,4 +139,20 @@ public class ShardedHistogram extends OverrideHistogram
     {
         return isCumulative;
     }
+
+    public void clear()
+    {
+        for (HistogramShard shard : shards)
+        {
+            shard.lock.lock();
+            try
+            {
+                shard.histogram.clear();
+            }
+            finally
+            {
+                shard.lock.unlock();
+            }
+        }
+    }
 }

src/java/org/apache/cassandra/net/ArtificialLatency.java

@@ -33,6 +33,8 @@ import java.util.stream.Collectors;
 
 import org.agrona.collections.Object2LongHashMap;
 
+import accord.api.Tracing;
+
 import org.apache.cassandra.concurrent.ExecutorLocals;
 import org.apache.cassandra.concurrent.Interruptible;
 import org.apache.cassandra.config.CassandraRelevantProperties;
@@ -258,6 +260,9 @@ public class ArtificialLatency extends ExecutorLocals.Impl
                     long now = nanoTime();
                     while (null != (delayed = out.peek()) && delayed.deadline <= now)
                     {
+                        Tracing tracing = (Tracing)delayed.message.params().get(ParamType.ACCORD_TRACING);
+                        if (tracing != null)
+                            tracing.trace(null, (delayed.message.verb().isResponse() ? "Reply" : "Request") + " delayed to " + ClusterMetadata.current().directory.peerId(delayed.to));
                         delayed.sink.accept(delayed.message, delayed.to, delayed.type);
                         out.poll();
                     }

src/java/org/apache/cassandra/net/InboundMessageHandler.java

@@ -35,6 +35,7 @@ import org.apache.cassandra.net.FrameDecoder.CorruptFrame;
 import org.apache.cassandra.net.FrameDecoder.IntactFrame;
 import org.apache.cassandra.net.Message.Header;
 import org.apache.cassandra.net.ResourceLimits.Limit;
+import org.apache.cassandra.service.accord.debug.AccordRemoteTracing;
 import org.apache.cassandra.tcm.ClusterMetadataService;
 import org.apache.cassandra.tracing.TraceState;
 import org.apache.cassandra.tracing.Tracing;
@@ -422,6 +423,7 @@ public class InboundMessageHandler extends AbstractMessageHandler
 
         TraceState state = Tracing.instance.initializeFromMessage(header);
         if (state != null) state.trace("{} message received from {}", header.verb, header.from);
+        AccordRemoteTracing.traceOffWire(header);
 
         callbacks.onDispatched(task.size(), header);
         header.verb.stage.execute(ExecutorLocals.create(state), task);

src/java/org/apache/cassandra/net/Message.java

@@ -61,7 +61,6 @@ import static org.apache.cassandra.net.MessageFlag.ARTIFICIAL_LATENCY;
 import static org.apache.cassandra.net.MessagingService.VERSION_40;
 import static org.apache.cassandra.net.MessagingService.VERSION_50;
 import static org.apache.cassandra.net.MessagingService.VERSION_60;
-import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 import static org.apache.cassandra.utils.FBUtilities.getBroadcastAddressAndPort;
 import static org.apache.cassandra.utils.MonotonicClock.Global.approxTime;
 import static org.apache.cassandra.utils.vint.VIntCoding.computeUnsignedVIntSize;
@@ -154,6 +153,12 @@ public class Message<T> implements ResponseContext
         return header.hasFlag(flag);
     }
 
+    @Override
+    public Map<ParamType, Object> params()
+    {
+        return header.params();
+    }
+
     /** For how long the message has lived. */
     public long elapsedSinceCreated(TimeUnit units)
     {
@@ -223,8 +228,6 @@ public class Message<T> implements ResponseContext
      */
     public static <T> Message<T> out(Verb verb, T payload)
     {
-        assert !verb.isResponse() : verb;
-
         return outWithParam(nextId(), verb, payload, null, null);
     }
 
@@ -240,7 +243,6 @@ public class Message<T> implements ResponseContext
 
     public static <T> Message<T> out(Verb verb, T payload, boolean isUrgent)
     {
-        assert !verb.isResponse();
         if (isUrgent)
             return outWithFlag(verb, payload,  MessageFlag.URGENT);
         else
@@ -249,7 +251,6 @@ public class Message<T> implements ResponseContext
 
     public static <T> Message<T> outWithFlag(Verb verb, T payload, MessageFlag flag)
     {
-        assert !verb.isResponse();
         return outWithParam(nextId(), verb, 0, payload, flag.addTo(0), null, null);
     }
 
@@ -260,24 +261,7 @@ public class Message<T> implements ResponseContext
 
     public static <T> Message<T> outWithFlags(Verb verb, T payload, Dispatcher.RequestTime requestTime, int encodedFlags)
     {
-        assert !verb.isResponse();
-        if (ArtificialLatency.isEligibleForArtificialLatency())
-            encodedFlags = ARTIFICIAL_LATENCY.addTo(encodedFlags);
-
-        return newWithFlags(nextId(), verb, payload, requestTime.startedAtNanos(), requestTime.computeDeadline(verb.expiresAfterNanos()), encodedFlags);
-    }
-
-    private static <T> Message<T> newWithFlags(long id, Verb verb, T payload, long startedAtNanos, long expiresAtNanos, int encodedFlags)
-    {
-        return new Message<>(new Header(id,
-                                        epochSupplier.get(),
-                                        verb,
-                                        getBroadcastAddressAndPort(),
-                                        startedAtNanos,
-                                        expiresAtNanos,
-                                        encodedFlags,
-                                        buildParams(null, null)),
-                             payload);
+        return outWithParam(nextId(), verb, requestTime.computeDeadline(verb.expiresAfterNanos()), payload, encodedFlags, null, null);
     }
 
     @VisibleForTesting
@@ -293,10 +277,11 @@ public class Message<T> implements ResponseContext
 
     private static <T> Message<T> outWithParam(long id, Verb verb, long expiresAtNanos, T payload, int flags, ParamType paramType, Object paramValue)
     {
-        return withParam(getBroadcastAddressAndPort(), id, verb, expiresAtNanos, payload, flags, paramType, paramValue);
+        assert !verb.isManagedResponse();
+        return withParam(id, verb, expiresAtNanos, payload, flags, paramType, paramValue);
     }
 
-    private static <T> Message<T> withParam(InetAddressAndPort from, long id, Verb verb, long expiresAtNanos, T payload, int flags, ParamType paramType, Object paramValue)
+    private static <T> Message<T> withParam(long id, Verb verb, long expiresAtNanos, T payload, int flags, ParamType paramType, Object paramValue)
     {
         if (payload == null)
             throw new IllegalArgumentException();
@@ -307,13 +292,14 @@ public class Message<T> implements ResponseContext
         if (ArtificialLatency.isEligibleForArtificialLatency())
             flags = ARTIFICIAL_LATENCY.addTo(flags);
 
+        InetAddressAndPort from = getBroadcastAddressAndPort();
         return new Message<>(new Header(id, epochSupplier.get(), verb, from, createdAtNanos, expiresAtNanos, flags, buildParams(paramType, paramValue)), payload);
     }
 
     public static <T> Message<T> internalResponse(Verb verb, T payload)
     {
-        assert verb.isResponse();
-        return outWithParam(0, verb, payload, null, null);
+        assert verb.isManagedResponse();
+        return withParam(0, verb, 0, payload, 0, null, null);
     }
 
     /**
@@ -323,7 +309,7 @@ public class Message<T> implements ResponseContext
     @VisibleForTesting
     public static <T> Message<T> remoteResponse(InetAddressAndPort from, Verb verb, T payload)
     {
-        assert verb.isResponse();
+        assert verb.isManagedResponse();
         long createdAtNanos = approxTime.now();
         long expiresAtNanos = verb.expiresAtNanos(createdAtNanos);
         return new Message<>(new Header(0, epochSupplier.get(), verb, from, createdAtNanos, expiresAtNanos, 0, NO_PARAMS), payload);
@@ -336,7 +322,7 @@ public class Message<T> implements ResponseContext
     @VisibleForTesting
     public static <T> Message<T> remoteResponseForTests(long id, InetAddressAndPort from, Verb verb, T payload)
     {
-        assert verb.isResponse();
+        assert verb.isManagedResponse();
         long createdAtNanos = approxTime.now();
         long expiresAtNanos = verb.expiresAtNanos(createdAtNanos);
         return new Message<>(new Header(id, epochSupplier.get(), verb, from, createdAtNanos, expiresAtNanos, 0, NO_PARAMS), payload);
@@ -368,7 +354,7 @@ public class Message<T> implements ResponseContext
     public static <T> Message<T> responseWith(T payload, ResponseContext respondTo)
     {
         int encodedFlags = respondTo.hasFlag(ARTIFICIAL_LATENCY) ? ARTIFICIAL_LATENCY.addTo(0) : 0;
-        return newWithFlags(respondTo.id(), respondTo.verb().responseVerb, payload, nanoTime(), respondTo.expiresAtNanos(), encodedFlags);
+        return withParam(respondTo.id(), respondTo.verb().responseVerb, respondTo.expiresAtNanos(), payload, encodedFlags, null, null);
     }
 
     /** Builds a response Message with no payload, and all the right fields inferred from request Message */
@@ -395,7 +381,7 @@ public class Message<T> implements ResponseContext
 
     static Message<RequestFailure> failureResponse(long id, long expiresAtNanos, RequestFailure reason)
     {
-        return outWithParam(id, Verb.FAILURE_RSP, expiresAtNanos, reason, null, null);
+        return withParam(id, Verb.FAILURE_RSP, expiresAtNanos, reason, 0, null, null);
     }
 
     public <V> Message<V> withPayload(V newPayload)
@@ -603,11 +589,6 @@ public class Message<T> implements ResponseContext
             return MessageFlag.TRACK_WARNINGS.isIn(flags);
         }
 
-        boolean isFinal()
-        {
-            return !MessageFlag.NOT_FINAL.isIn(flags);
-        }
-
         boolean permitsArtificialLatency()
         {
             return ARTIFICIAL_LATENCY.isIn(flags);
@@ -763,7 +744,7 @@ public class Message<T> implements ResponseContext
             this.verb = verb;
             if (expiresAtNanos == 0 && verb != null && createdAtNanos != 0)
                 expiresAtNanos = verb.expiresAtNanos(createdAtNanos);
-            if (!this.verb.isResponse() && from == null) // default to sending from self if we're a request verb
+            if (!this.verb.isManagedResponse() && from == null) // default to sending from self if we're a request verb
                 from = getBroadcastAddressAndPort();
             return this;
         }

src/java/org/apache/cassandra/net/MessageFlag.java

@@ -32,9 +32,7 @@ public enum MessageFlag
     TRACK_WARNINGS      (2),
     /** whether this message should be sent on an URGENT channel despite its Verb default priority */
     URGENT              (3),
-    /** Allow a single callback to receive multiple responses until a final response is received **/
-    NOT_FINAL           (4),
-    ARTIFICIAL_LATENCY  (5)
+    ARTIFICIAL_LATENCY  (4)
     ;
 
     private final int id;

src/java/org/apache/cassandra/net/OutboundConnection.java

@@ -40,6 +40,7 @@ import org.slf4j.LoggerFactory;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.io.util.DataOutputBufferFixed;
 import org.apache.cassandra.net.OutboundConnectionInitiator.Result.MessagingSuccess;
+import org.apache.cassandra.service.accord.debug.AccordRemoteTracing;
 import org.apache.cassandra.tracing.Tracing;
 import org.apache.cassandra.utils.Clock;
 import org.apache.cassandra.utils.FBUtilities;
@@ -829,6 +830,7 @@ public class OutboundConnection
                         }
 
                         Tracing.instance.traceOutgoingMessage(next, messageSize, settings.connectTo);
+                        AccordRemoteTracing.traceOnWire(next.header, settings.connectTo);
                         Message.serializer.serialize(next, out, messagingVersion);
 
                         if (sending.length() != sendingBytes + messageSize)

src/java/org/apache/cassandra/net/ParamType.java

@@ -20,6 +20,7 @@ package org.apache.cassandra.net;
 import javax.annotation.Nullable;
 
 import org.apache.cassandra.io.IVersionedSerializer;
+import org.apache.cassandra.service.accord.debug.AccordRemoteTracing;
 import org.apache.cassandra.service.writes.thresholds.WriteThresholdMapSerializer;
 import org.apache.cassandra.tracing.Tracing;
 import org.apache.cassandra.utils.Int32Serializer;
@@ -57,7 +58,9 @@ public enum ParamType
     TOO_MANY_REFERENCED_INDEXES_WARN (16, Int32Serializer.serializer),
     TOO_MANY_REFERENCED_INDEXES_FAIL (17, Int32Serializer.serializer),
     WRITE_SIZE_WARN                  (18, WriteThresholdMapSerializer.serializer),
-    WRITE_TOMBSTONE_WARN             (19, WriteThresholdMapSerializer.serializer);
+    WRITE_TOMBSTONE_WARN             (19, WriteThresholdMapSerializer.serializer),
+    ACCORD_TRACING                   (20, AccordRemoteTracing.tracingSerializer),
+    ;
 
     final int id;
     final IVersionedSerializer serializer;

src/java/org/apache/cassandra/net/ResponseContext.java

@@ -17,9 +17,18 @@
  */
 package org.apache.cassandra.net;
 
+import java.util.Map;
+import java.util.concurrent.TimeUnit;
+
+import accord.api.MessageSink;
+import accord.local.Node;
+import accord.messages.Reply;
 import accord.messages.ReplyContext;
 
 import org.apache.cassandra.locator.InetAddressAndPort;
+import org.apache.cassandra.service.accord.AccordMessageSink;
+
+import static java.util.concurrent.TimeUnit.NANOSECONDS;
 
 public interface ResponseContext extends ReplyContext
 {
@@ -28,4 +37,17 @@ public interface ResponseContext extends ReplyContext
     Verb verb();
     long expiresAtNanos();
     boolean hasFlag(MessageFlag flag);
+    Map<ParamType, Object> params();
+
+    @Override
+    default long expiresAt(TimeUnit units)
+    {
+        return units.convert(expiresAtNanos(), NANOSECONDS);
+    }
+
+    @Override
+    default void reply(Node.Id to, MessageSink sink, Reply success, Throwable failure)
+    {
+        ((AccordMessageSink)sink).reply(to, this, success, failure);
+    }
 }

src/java/org/apache/cassandra/net/ResponseVerbHandler.java

@@ -61,11 +61,7 @@ class ResponseVerbHandler implements IVerbHandler
     @Override
     public void doVerb(Message message)
     {
-        RequestCallbacks.CallbackInfo callbackInfo;
-        if (message.header.isFinal())
-            callbackInfo = MessagingService.instance().callbacks.remove(message.id(), message.from());
-        else
-            callbackInfo = MessagingService.instance().callbacks.get(message.id(), message.from());
+        RequestCallbacks.CallbackInfo callbackInfo = MessagingService.instance().callbacks.remove(message.id(), message.from());
         if (callbackInfo == null)
         {
             String msg = "Callback already removed for {} (from {})";

src/java/org/apache/cassandra/net/Verb.java

@@ -84,6 +84,7 @@ import org.apache.cassandra.schema.SchemaVersionVerbHandler;
 import org.apache.cassandra.service.EchoVerbHandler;
 import org.apache.cassandra.service.SnapshotVerbHandler;
 import org.apache.cassandra.service.accord.AccordService;
+import org.apache.cassandra.service.accord.debug.AccordRemoteTracing;
 import org.apache.cassandra.service.accord.interop.AccordInteropApply;
 import org.apache.cassandra.service.accord.interop.AccordInteropRead;
 import org.apache.cassandra.service.accord.interop.AccordInteropReadRepair;
@@ -104,6 +105,7 @@ import org.apache.cassandra.service.accord.serializers.LatestDepsSerializers;
 import org.apache.cassandra.service.accord.serializers.PreacceptSerializers;
 import org.apache.cassandra.service.accord.serializers.ReadDataSerializer;
 import org.apache.cassandra.service.accord.serializers.RecoverySerializers;
+import org.apache.cassandra.service.accord.serializers.RemoteSuccessSerializers;
 import org.apache.cassandra.service.accord.serializers.SetDurableSerializers;
 import org.apache.cassandra.service.accord.serializers.SimpleReplySerializer;
 import org.apache.cassandra.service.accord.serializers.Version;
@@ -373,9 +375,11 @@ public enum Verb
     ACCORD_INTEROP_APPLY_REQ        (166, P2, writeTimeout, IMMEDIATE,          () -> accordEmbedded(AccordInteropApply.serializer),             AccordService::requestHandlerOrNoop,     ACCORD_APPLY_RSP),
     ACCORD_FETCH_WATERMARKS_RSP     (167, P0, shortTimeout, FETCH_METADATA,     () -> accordEmbedded(WatermarkCollector.serializer),             RESPONSE_HANDLER),
     // NoPayload can not be prefixed with accord version as it is special cased in C* messaging
-    ACCORD_FETCH_WATERMARKS_REQ     (168, P0, shortTimeout, FETCH_METADATA,     () -> NoPayload.serializer,                      AccordService::watermarkHandlerOrNoop,   ACCORD_FETCH_WATERMARKS_RSP),
+    ACCORD_FETCH_WATERMARKS_REQ     (168, P0, shortTimeout, FETCH_METADATA,     () -> NoPayload.serializer,                                      AccordService::watermarkHandlerOrNoop,   ACCORD_FETCH_WATERMARKS_RSP),
     ACCORD_FETCH_TOPOLOGY_RSP       (169, P0, shortTimeout, FETCH_METADATA,     () -> accordEmbedded(FetchTopologies.responseSerializer),        RESPONSE_HANDLER),
     ACCORD_FETCH_TOPOLOGY_REQ       (170, P0, shortTimeout, FETCH_METADATA,     () -> accordEmbedded(FetchTopologies.serializer),                () -> FetchTopologies.handler,           ACCORD_FETCH_TOPOLOGY_RSP),
+    ACCORD_REMOTE_SUCCESS_REQ       (173, P0, shortTimeout, IMMEDIATE,          () -> accordEmbedded(RemoteSuccessSerializers.remoteSuccess),    AccordService::requestHandlerOrNoop),
+    ACCORD_REMOTE_TRACE(174, P0, shortTimeout, IMMEDIATE, () -> AccordRemoteTracing.traceSerializer, () -> AccordRemoteTracing.traceMessageHandler),
 
     DICTIONARY_UPDATE_RSP           (171, P1, rpcTimeout,   REQUEST_RESPONSE,  () -> NoPayload.serializer,                  RESPONSE_HANDLER                             ),
     DICTIONARY_UPDATE_REQ           (172, P1, rpcTimeout, MISC, () -> CompressionDictionaryUpdateMessage.serializer, () -> CompressionDictionaryUpdateVerbHandler.instance, DICTIONARY_UPDATE_RSP ),
@@ -440,6 +444,7 @@ public enum Verb
     private final Supplier<? extends IVerbHandler<?>> handler;
 
     public final Verb responseVerb;
+    private final boolean isResponse;
 
     private final ToLongFunction<TimeUnit> expiration;
 
@@ -488,6 +493,8 @@ public enum Verb
         this.responseVerb = responseVerb;
         this.expiration = expiration;
         this.kind = kind;
+        // this is a little hacky, but reduces the number of parameters up top
+        this.isResponse = name().endsWith("_RSP") || handler == RESPONSE_HANDLER;
     }
 
     public <In, Out> IVersionedAsymmetricSerializer<In, Out> serializer()
@@ -516,9 +523,14 @@ public enum Verb
     }
 
     // this is a little hacky, but reduces the number of parameters up top
+    public boolean isManagedResponse()
+    {
+        return handler == RESPONSE_HANDLER;
+    }
+
     public boolean isResponse()
     {
-        return handler.get() == ResponseVerbHandler.instance;
+        return isResponse;
     }
 
     @VisibleForTesting

src/java/org/apache/cassandra/service/StorageProxy.java

@@ -61,7 +61,7 @@ import org.apache.cassandra.batchlog.Batch;
 import org.apache.cassandra.batchlog.BatchlogManager;
 import org.apache.cassandra.concurrent.DebuggableTask.RunnableDebuggableTask;
 import org.apache.cassandra.concurrent.Stage;
-import org.apache.cassandra.config.AccordSpec;
+import org.apache.cassandra.config.AccordConfig;
 import org.apache.cassandra.config.CassandraRelevantProperties;
 import org.apache.cassandra.config.Config;
 import org.apache.cassandra.config.DatabaseDescriptor;
@@ -1355,7 +1355,7 @@ public class StorageProxy implements StorageProxyMBean
 
     private static void checkMixedTimeSourceHandling()
     {
-        AccordSpec.MixedTimeSourceHandling handling = DatabaseDescriptor.getAccord().mixedTimeSourceHandling;
+        AccordConfig.MixedTimeSourceHandling handling = DatabaseDescriptor.getAccord().mixedTimeSourceHandling;
         switch (handling)
         {
             case log:
@@ -1363,7 +1363,7 @@ public class StorageProxy implements StorageProxyMBean
             {
                 ClientWarn.instance.warn(UNSAFE_MIXED_MUTATIONS_MSG);
                 logger.warn(UNSAFE_MIXED_MUTATIONS_MSG);
-                if (handling == AccordSpec.MixedTimeSourceHandling.reject)
+                if (handling == AccordConfig.MixedTimeSourceHandling.reject)
                     throw new InvalidRequestException(UNSAFE_MIXED_MUTATIONS_MSG);
             }
             break;

src/java/org/apache/cassandra/service/accord/AccordCache.java

@@ -67,6 +67,7 @@ import org.apache.cassandra.metrics.ShardedHitRate;
 import org.apache.cassandra.service.accord.AccordCache.Adapter.Shrink;
 import org.apache.cassandra.service.accord.AccordCacheEntry.LoadExecutor;
 import org.apache.cassandra.service.accord.AccordCacheEntry.Status;
+import org.apache.cassandra.service.accord.AccordSafeCommandsForKey.CommandsForKeyCacheEntry;
 import org.apache.cassandra.service.accord.events.CacheEvents;
 import org.apache.cassandra.service.accord.journal.CommandChangeWriter;
 import org.apache.cassandra.service.accord.journal.CommandChanges;
@@ -1193,7 +1194,7 @@ public class AccordCache implements CacheSize
                 return null;
 
             TxnId last = value.size() == 0 ? null : value.get(value.size() - 1);
-            TxnId minUndecided = value.minUndecided();
+            TxnId minUndecided = value.minUndecidedManaged();
             int lastSize = (int) CommandSerializers.txnId.serializedSize(last);
             int minUndecidedSize = (int) CommandSerializers.txnId.serializedSize(minUndecided);
             ByteBuffer result = Serialize.toBytesWithoutKey(lastSize + minUndecidedSize, value.maximalPrune());
@@ -1249,11 +1250,19 @@ public class AccordCache implements CacheSize
         {
             return RoutingKey::compareAsRoutingKey;
         }
+
+        @Override
+        public AccordCacheEntry<RoutingKey, CommandsForKey> newEntry(RoutingKey key, Type<RoutingKey, CommandsForKey, ?>.Instance owner)
+        {
+            CommandsForKeyCacheEntry entry = new CommandsForKeyCacheEntry(key, owner);
+            entry.readyToLoad();
+            return entry;
+        }
     }
 
     public static class CommandAdapter implements Adapter<TxnId, Command, AccordSafeCommand>
     {
-        private static int SHRINK_WITHOUT_LOCK = -1;
+        private static final int SHRINK_WITHOUT_LOCK = -1;
 
         public static final CommandAdapter COMMAND_ADAPTER = new CommandAdapter();
 

src/java/org/apache/cassandra/service/accord/AccordCommandStore.java

@@ -60,6 +60,8 @@ import accord.local.CommandStores;
 import accord.local.CommandSummaries;
 import accord.local.MaxConflicts;
 import accord.local.MaxDecidedRX;
+import accord.local.MinimalCommand;
+import accord.local.MinimalCommand.MinimalWithDeps;
 import accord.local.NodeCommandStoreService;
 import accord.local.PreLoadContext;
 import accord.local.PreLoadContext.Empty;
@@ -67,7 +69,6 @@ import accord.local.RedundantBefore;
 import accord.local.RedundantBefore.Bounds;
 import accord.local.RedundantStatus.Property;
 import accord.local.RedundantStatus.SomeStatus;
-import accord.local.RejectBefore;
 import accord.local.SafeCommandStore;
 import accord.local.cfk.CommandsForKey;
 import accord.primitives.PartialTxn;
@@ -87,8 +88,8 @@ import accord.utils.async.AsyncChains;
 import accord.utils.async.AsyncResult;
 import accord.utils.async.AsyncResults.CountingResult;
 
-import org.apache.cassandra.config.AccordSpec;
-import org.apache.cassandra.config.AccordSpec.JournalSpec.ReplayMode;
+import org.apache.cassandra.config.AccordConfig;
+import org.apache.cassandra.config.AccordConfig.JournalConfig.ReplayMode;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.db.ColumnFamilyStore;
 import org.apache.cassandra.io.util.File;
@@ -255,7 +256,7 @@ public class AccordCommandStore extends CommandStore
 
         this.exclusiveExecutor = sharedExecutor.executor(id);
         {
-            AccordSpec.RangeIndexMode mode = getAccord().range_index_mode;
+            AccordConfig.RangeIndexMode mode = getAccord().range_index_mode;
             switch (mode)
             {
                 default: throw new UnhandledEnum(mode);
@@ -388,7 +389,7 @@ public class AccordCommandStore extends CommandStore
         RedundantBefore.QuickBounds bounds = safeGetRedundantBefore().get(key);
         if (!Invariants.expect(bounds != null, "No RedundantBefore information found when loading key %s", key))
             return cfk;
-        return cfk.withGcBeforeAtLeast(bounds.gcBefore, false);
+        return cfk.withCleanCfkBeforeAtLeast(bounds.cleanCfkBefore(), false);
     }
 
     boolean validateCommandsForKey(RoutableKey key, CommandsForKey evicting)
@@ -422,6 +423,18 @@ public class AccordCommandStore extends CommandStore
         return AccordTask.create(this, preLoadContext, consumer).chain();
     }
 
+    @Override
+    public AsyncChain<Void> priorityChain(PreLoadContext preLoadContext, Consumer<? super SafeCommandStore> consumer)
+    {
+        return AccordTask.create(this, preLoadContext, consumer).priorityChain();
+    }
+
+    @Override
+    public <T> AsyncChain<T> priorityChain(PreLoadContext preLoadContext, Function<? super SafeCommandStore, T> function)
+    {
+        return AccordTask.create(this, preLoadContext, function).priorityChain();
+    }
+
     @Override
     public <T> AsyncChain<T> chain(Callable<T> call)
     {
@@ -434,6 +447,12 @@ public class AccordCommandStore extends CommandStore
         taskExecutor().execute(run);
     }
 
+    @Override
+    public boolean tryExecuteImmediately(Runnable run)
+    {
+        return taskExecutor().tryExecuteImmediately(run);
+    }
+
     public AccordSafeCommandStore begin(AccordTask<?> operation, @Nullable CommandSummaries commandsForRanges)
     {
         require(current == null);
@@ -575,12 +594,12 @@ public class AccordCommandStore extends CommandStore
         return command;
     }
 
-    public Command.Minimal loadMinimal(TxnId txnId)
+    public MinimalCommand loadMinimal(TxnId txnId)
     {
         return journal.loadMinimal(id, txnId, safeGetRedundantBefore(), durableBefore());
     }
 
-    public Command.MinimalWithDeps loadMinimalWithDeps(TxnId txnId)
+    public MinimalWithDeps loadMinimalWithDeps(TxnId txnId)
     {
         return journal.loadMinimalWithDeps(id, txnId, safeGetRedundantBefore(), durableBefore());
     }
@@ -805,7 +824,6 @@ public class AccordCommandStore extends CommandStore
                 tmpSaveDir.createDirectoriesIfNotExists();
                 writeOne(new File(tmpSaveDir, "max_decidedrx"), unsafeGetMaxDecidedRX(), maxDecidedRX);
                 writeOne(new File(tmpSaveDir, "max_conflicts"), unsafeGetMaxConflicts(), maxConflicts);
-                writeOne(new File(tmpSaveDir, "reject_before"), unsafeGetRejectBefore(), rejectBefore);
                 writeList(new File(tmpSaveDir, "listeners"), ((DefaultLocalListeners)listeners).snapshot(), txnListener);
                 writeList(new File(tmpSaveDir, "progress_log"), ((DefaultProgressLog)progressLog).snapshot(), progressLogState);
                 rangeIndex.save(new File(tmpSaveDir, "range_index"));
@@ -872,7 +890,7 @@ public class AccordCommandStore extends CommandStore
 
             File savePoint = savePoints[savePoints.length - 1];
             long segment = Long.parseLong(savePoint.name());
-            MaxDecidedRX mxd; MaxConflicts mxc; RejectBefore rjb;
+            MaxDecidedRX mxd; MaxConflicts mxc;
             List<TxnListener> dll; List<TxnState> dpl; Object rgi;
             RedundantBefore rdb;
             try
@@ -880,7 +898,11 @@ public class AccordCommandStore extends CommandStore
                 logger.info("{} loading state from {}", this, savePoint);
                 mxd = readOne(new File(savePoint, "max_decidedrx"), maxDecidedRX);
                 mxc = readOne(new File(savePoint, "max_conflicts"), maxConflicts);
-                rjb = readOne(new File(savePoint, "reject_before"), rejectBefore);
+                {
+                    File rjbf = new File(savePoint, "reject_before");
+                    if (rjbf.exists())
+                        mxc = mxc.with(readOne(rjbf, rejectBefore));
+                }
                 dll = readList(new File(savePoint, "listeners"), txnListener);
                 dpl = readList(new File(savePoint, "progress_log"), progressLogState);
                 rgi = rangeIndex.load(new File(savePoint, "range_index"));
@@ -898,7 +920,6 @@ public class AccordCommandStore extends CommandStore
             rangeIndex.restore(rgi);
             unsafeSetMaxDecidedRX(mxd);
             unsafeSetMaxConflicts(mxc);
-            unsafeSetRejectBefore(rjb);
             ((DefaultLocalListeners) listeners).restore(dll);
             boolean unsetCatchup = ((DefaultProgressLog) progressLog).setModeExclusive(safeStore, CATCH_UP);
             ((DefaultProgressLog) progressLog).restore(safeStore, dpl);

src/java/org/apache/cassandra/service/accord/AccordCommandStores.java

@@ -48,14 +48,14 @@ import accord.utils.async.AsyncResults;
 import org.apache.cassandra.cache.CacheSize;
 import org.apache.cassandra.concurrent.ScheduledExecutors;
 import org.apache.cassandra.concurrent.Shutdownable;
-import org.apache.cassandra.config.AccordSpec.QueueShardModel;
+import org.apache.cassandra.config.AccordConfig.QueueShardModel;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.journal.Descriptor;
 import org.apache.cassandra.schema.TableId;
 import org.apache.cassandra.service.accord.AccordCommandStore.DurablyAppliedTo;
 import org.apache.cassandra.service.accord.AccordExecutor.AccordExecutorFactory;
 
-import static org.apache.cassandra.config.AccordSpec.QueueShardModel.THREAD_PER_SHARD;
+import static org.apache.cassandra.config.AccordConfig.QueueShardModel.THREAD_PER_SHARD;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordQueueShardCount;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordQueueSubmissionModel;
 import static org.apache.cassandra.service.accord.AccordExecutor.Mode.RUN_WITHOUT_LOCK;

src/java/org/apache/cassandra/service/accord/AccordExecutor.java

@@ -25,6 +25,7 @@ import java.util.Queue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.LockSupport;
@@ -46,6 +47,13 @@ import accord.local.Command;
 import accord.local.PreLoadContext;
 import accord.local.SequentialAsyncExecutor;
 import accord.local.cfk.CommandsForKey;
+import accord.messages.Accept;
+import accord.messages.Commit;
+import accord.messages.MessageType;
+import accord.messages.MessageType.StandardMessage;
+import accord.messages.Request;
+import accord.primitives.Ballot;
+import accord.primitives.SaveStatus;
 import accord.primitives.TxnId;
 import accord.utils.ArrayBuffers.BufferList;
 import accord.utils.IntrusivePriorityHeap;
@@ -69,6 +77,8 @@ import org.apache.cassandra.concurrent.DebuggableTask;
 import org.apache.cassandra.concurrent.DebuggableTask.DebuggableTaskRunner;
 import org.apache.cassandra.concurrent.ExecutorLocals;
 import org.apache.cassandra.concurrent.Shutdownable;
+import org.apache.cassandra.config.AccordConfig.QueuePriorityModel;
+import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.metrics.AccordCacheMetrics;
 import org.apache.cassandra.metrics.AccordExecutorMetrics;
 import org.apache.cassandra.metrics.AccordReplicaMetrics;
@@ -80,6 +90,9 @@ import org.apache.cassandra.metrics.ShardedDecayingHistograms.DecayingHistograms
 import org.apache.cassandra.service.accord.AccordCacheEntry.LoadExecutor;
 import org.apache.cassandra.service.accord.AccordCacheEntry.SaveExecutor;
 import org.apache.cassandra.service.accord.AccordCacheEntry.UniqueSave;
+import org.apache.cassandra.service.accord.debug.DebugExecution.DebugExecutor;
+import org.apache.cassandra.service.accord.debug.DebugExecution.DebugSequentialExecutor;
+import org.apache.cassandra.service.accord.debug.DebugExecution.DebugTask;
 import org.apache.cassandra.utils.Closeable;
 import org.apache.cassandra.utils.MonotonicClock;
 import org.apache.cassandra.utils.WithResources;
@@ -90,6 +103,7 @@ import org.apache.cassandra.utils.concurrent.Future;
 import io.netty.util.concurrent.FastThreadLocal;
 
 import static accord.utils.Invariants.createIllegalState;
+import static org.apache.cassandra.config.AccordConfig.QueuePriorityModel.PHASE_HLC_FIFO;
 import static org.apache.cassandra.service.accord.AccordCache.CommandAdapter.COMMAND_ADAPTER;
 import static org.apache.cassandra.service.accord.AccordCache.CommandsForKeyAdapter.CFK_ADAPTER;
 import static org.apache.cassandra.service.accord.AccordCache.registerJfrListener;
@@ -99,6 +113,8 @@ import static org.apache.cassandra.service.accord.AccordTask.State.RUNNING;
 import static org.apache.cassandra.service.accord.AccordTask.State.SCANNING_RANGES;
 import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_LOAD;
 import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_RUN;
+import static org.apache.cassandra.service.accord.debug.DebugExecution.DEBUG_EXECUTION;
+import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 
 /**
  * NOTE: We assume that NO BLOCKING TASKS are submitted to this executor AND WAITED ON by another task executing on this executor.
@@ -107,7 +123,12 @@ import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_RU
 public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTask<?>, Boolean>, SaveExecutor, Shutdownable, AbstractAsyncExecutor
 {
     private static final Logger logger = LoggerFactory.getLogger(AccordExecutor.class);
+
+    private static final long PRIORITY_BITS = 0x7000000000000000L;
+    private static final QueuePriorityModel PRIORITY_MODEL = DatabaseDescriptor.getAccord().queue_priority_model;
+    private static final long AGE_TO_FIFO = DatabaseDescriptor.getAccord().queue_priority_age_to_fifo.to(TimeUnit.MICROSECONDS);
     public static final ShardedDecayingHistograms HISTOGRAMS = new ShardedDecayingHistograms();
+    private static final FastThreadLocal<Lock> paranoidPriorityInversionCheck = new FastThreadLocal<>();
 
     public interface AccordExecutorFactory
     {
@@ -130,7 +151,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public void close()
         {
-            executor.beforeUnlock();
+            executor.beforeUnlockExternal();
             global.tryShrinkOrEvict(executor.lock);
             executor.unlock();
         }
@@ -150,6 +171,25 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
     }
 
+    private static class WaitForCompletion
+    {
+        final long position;
+        long maybeNotify;
+        final Runnable run;
+
+        private WaitForCompletion(long position, Runnable run)
+        {
+            this.position = position;
+            this.maybeNotify = position - 1;
+            this.run = run;
+        }
+
+        public String toString()
+        {
+            return run.toString() + " @" + position;
+        }
+    }
+
     private final Lock lock;
     final Agent agent;
     final int executorId;
@@ -173,28 +213,10 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     final LogLinearDecayingHistogram elapsedPreparingToRun;
     final LogLinearDecayingHistogram elapsedWaitingToRun;
     final LogLinearDecayingHistogram elapsedRunning;
+    final LogLinearDecayingHistogram elapsed;
     final LogLinearDecayingHistogram keys;
     public final AccordReplicaMetrics.Shard replicaMetrics;
 
-    private static class WaitForCompletion
-    {
-        final int position;
-        int maybeNotify;
-        final Runnable run;
-
-        private WaitForCompletion(int position, Runnable run)
-        {
-            this.position = position;
-            this.maybeNotify = position - 1;
-            this.run = run;
-        }
-
-        public String toString()
-        {
-            return run.toString() + " @" + position;
-        }
-    }
-
     /**
      * The maximum total number of loads we can queue at once - this includes loads for range transactions,
      * which are subject to this limit as well as that imposed by {@link #maxQueuedRangeLoads}
@@ -207,11 +229,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     private long maxWorkingSetSizeInBytes;
     private long maxWorkingCapacityInBytes;
-    private int nextPosition;
+    private long minPosition, nextPosition;
     private int activeLoads, activeRangeLoads;
     private boolean hasPausedLoading;
     int tasks;
     int runningThreads;
+    final DebugExecutor debug = DebugExecutor.maybeDebug();
 
     AccordExecutor(Lock lock, int executorId, Agent agent)
     {
@@ -235,6 +258,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         this.elapsedPreparingToRun = AccordExecutorMetrics.INSTANCE.elapsedPreparingToRun.forShard(histogramsShard);
         this.elapsedWaitingToRun = AccordExecutorMetrics.INSTANCE.elapsedWaitingToRun.forShard(histogramsShard);
         this.elapsedRunning = AccordExecutorMetrics.INSTANCE.elapsedRunning.forShard(histogramsShard);
+        this.elapsed = AccordExecutorMetrics.INSTANCE.elapsed.forShard(histogramsShard);
         this.keys = AccordExecutorMetrics.INSTANCE.keys.forShard(histogramsShard);
         this.replicaMetrics = new AccordReplicaMetrics.Shard(histogramsShard);
     }
@@ -250,7 +274,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         return caches;
     }
 
-    private static final FastThreadLocal<Lock> paranoidPriorityInversionCheck = new FastThreadLocal<>();
+    abstract boolean isInLoop();
 
     final Lock unsafeLock()
     {
@@ -259,24 +283,34 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     final void lock()
     {
-        if (Invariants.isParanoid())
-        {
-            Lock locked = paranoidPriorityInversionCheck.getAndSet(lock);
-            Invariants.require(locked == null || locked == lock, "Tried to take multiple AccordExecutor locks on same thread - this is dangerous for progress");
-        }
+        if (Invariants.isParanoid()) paranoidLockExclusive();
         //noinspection LockAcquiredButNotSafelyReleased
         lock.lock();
+        if (DEBUG_EXECUTION) debug.onEnterLock();
     }
 
     final void unlock()
     {
+        if (Invariants.isParanoid()) paranoidUnlockExclusive();
+        if (DEBUG_EXECUTION) debug.onExitLock();
         lock.unlock();
+    }
+
+    final void paranoidLockExclusive()
+    {
+        Lock locked = paranoidPriorityInversionCheck.getAndSet(lock);
+        Invariants.require(locked == null || locked == lock, "Tried to take multiple AccordExecutor locks on same thread - this is dangerous for progress");
+    }
+
+    final void paranoidUnlockExclusive()
+    {
         paranoidPriorityInversionCheck.set(null);
     }
 
     final boolean tryLock()
     {
         boolean result = lock.tryLock();
+        if (DEBUG_EXECUTION && result) debug.onEnterLock();
         if (Invariants.isParanoid())
         {
             if (result)
@@ -310,26 +344,30 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         return cache;
     }
 
-    boolean hasWaitingToRun()
+    final boolean hasWaitingToRun()
     {
         updateWaitingToRunExclusive();
         return !waitingToRun.isEmpty();
     }
 
-    Task pollWaitingToRunExclusive()
+    void updateWaitingToRunExclusive()
+    {
+        // TODO (expected): this should not be invoked on every update of waiting to run
+        maybeUnpauseLoading();
+    }
+
+    final Task pollWaitingToRunExclusive()
     {
         updateWaitingToRunExclusive();
         Task next = waitingToRun.poll();
         if (next != null)
+        {
+            if (DEBUG_EXECUTION) next.debug.onPolled();
             next.addToQueue(running);
+        }
         return next;
     }
 
-    void updateWaitingToRunExclusive()
-    {
-        maybeUnpauseLoading();
-    }
-
     public Stream<? extends DebuggableTaskRunner> active()
     {
         return Stream.of();
@@ -387,7 +425,8 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             if (waitingForCompletion == null)
                 waitingForCompletion = new ArrayDeque<>();
 
-            int position = nextPosition;
+            long position = nextPosition;
+            minPosition = position;
             waitingForCompletion.add(new WaitForCompletion(position, run));
         }
         finally
@@ -409,7 +448,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     }
 
     public abstract boolean hasTasks();
-    abstract void beforeUnlock();
+    abstract void beforeUnlockExternal();
     abstract boolean isOwningThread();
 
     private void enqueueLoadsExclusive()
@@ -569,27 +608,29 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     private void waitingToRun(AccordTask task)
     {
+        task.onWaitingToRun();
         task.addToQueue(task.commandStore.exclusiveExecutor);
     }
 
     private void waitingToRun(SubmittableTask task, @Nullable SequentialExecutor queue)
     {
+        task.onWaitingToRun();
         task.addToQueue(queue == null ? waitingToRun : queue);
     }
 
     public SequentialExecutor executor()
     {
-        return new SequentialExecutor();
+        return new SequentialExecutor(this);
     }
 
     public SequentialExecutor executor(int commandStoreId)
     {
-        return new SequentialExecutor(commandStoreId);
+        return new SequentialExecutor(this, commandStoreId);
     }
 
     public SequentialAsyncExecutor newSequentialExecutor()
     {
-        return new SequentialExecutor();
+        return new SequentialExecutor(this);
     }
 
     public <R> void cancel(AccordTask<R> task)
@@ -639,9 +680,25 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         submit(AccordExecutor::submitExclusive, i -> i, operation);
     }
 
+    <R> void submitPriority(AccordTask<R> operation)
+    {
+        submit(AccordExecutor::submitPriorityExclusive, i -> i, operation);
+    }
+
     void submitExclusive(AccordTask<?> task)
     {
         assignQueuePosition(task);
+        submitInternalExclusive(task);
+    }
+
+    void submitPriorityExclusive(AccordTask<?> task)
+    {
+        assignMinQueuePosition(task);
+        submitInternalExclusive(task);
+    }
+
+    private void submitInternalExclusive(AccordTask<?> task)
+    {
         task.setupExclusive();
         ++tasks;
         updateQueue(task);
@@ -670,21 +727,126 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         Invariants.require(isOwningThread());
         ++tasks;
         if (parent != null) inheritQueuePosition(parent, task);
-        else assignNewQueuePosition(task);
+        else assignFifoQueuePosition(task);
+        task.onWaitingToRun();
         waitingToRun.append(task);
         return task;
     }
 
     private void assignQueuePosition(Task task)
     {
-        if (task.queuePosition == 0)
-            assignNewQueuePosition(task);
+        if (task.queuePosition != 0) updateNextPosition(task);
+        else assignFifoQueuePosition(task);
     }
 
-    private void assignNewQueuePosition(Task task)
+    private void assignQueuePosition(AccordTask<?> task)
     {
-        if (nextPosition == 0) nextPosition++;
-        task.queuePosition = nextPosition++;
+        if (task.queuePosition != 0) updateNextPosition(task);
+        else
+        {
+            long priority_bits = PRIORITY_BITS;
+            TxnId txnId = null;
+            switch (PRIORITY_MODEL)
+            {
+                case ORIG_PHASE_HLC_FIFO:
+                case PHASE_HLC_FIFO:
+                {
+                    // TODO (expected): we should process messages for a TxnId together, to avoid processing delayed messages out of order
+                    PreLoadContext context = task.preLoadContext();
+                    if (context instanceof Request)
+                    {
+                        MessageType type = ((Request) context).type();
+                        if (type instanceof StandardMessage)
+                        {
+                            TxnId txnId0 = context.primaryTxnId();
+                            switch ((StandardMessage)type)
+                            {
+                                case APPLY_REQ:
+                                {
+                                    priority_bits = 0L;
+                                    txnId = txnId0;
+                                    break;
+                                }
+                                case READ_EPHEMERAL_REQ:
+                                case READ_REQ:
+                                case STABLE_THEN_READ_REQ:
+                                {
+                                    priority_bits = 1000000000000000L;
+                                    txnId = txnId0;
+                                    break;
+                                }
+                                case COMMIT_REQ:
+                                {
+                                    Commit commit = (Commit) context;
+                                    if (PRIORITY_MODEL == PHASE_HLC_FIFO || commit.ballot.equals(Ballot.ZERO))
+                                        txnId = commit.txnId;
+                                    if (commit.kind.saveStatus == SaveStatus.Stable) priority_bits = 1000000000000L;
+                                    else priority_bits = 2000000000000L;
+                                    break;
+                                }
+                                case ACCEPT_REQ:
+                                {
+                                    Accept accept = (Accept) context;
+                                    if (PRIORITY_MODEL == PHASE_HLC_FIFO || accept.ballot.equals(Ballot.ZERO))
+                                        txnId = accept.txnId;
+                                    priority_bits = 3000000000000L;
+                                    break;
+                                }
+                                case GET_EPHEMERAL_READ_DEPS_REQ:
+                                case PRE_ACCEPT_REQ:
+                                {
+                                    txnId = txnId0;
+                                    break;
+                                }
+                            }
+                        }
+                    }
+                    break;
+                }
+                case HLC_FIFO:
+                {
+                    txnId = task.preLoadContext().primaryTxnId();
+                    break;
+                }
+                case FIFO:
+                {
+                    break;
+                }
+            }
+
+            if (txnId != null)
+            {
+                long hlc = txnId.hlc();
+                long delta = nextPosition - hlc;
+                if (delta < AGE_TO_FIFO)
+                {
+                    long position = hlc;
+                    if (delta <= 0) nextPosition = position + 1;
+                    else if (position < minPosition) position = minPosition;
+                    position |= priority_bits;
+                    task.queuePosition = position;
+                    return;
+                }
+            }
+
+            assignFifoQueuePosition(task);
+        }
+
+    }
+
+    private void assignMinQueuePosition(Task task)
+    {
+        task.queuePosition = minPosition | PRIORITY_BITS;
+    }
+
+    private void assignFifoQueuePosition(Task task)
+    {
+        task.queuePosition = nextPosition++ | PRIORITY_BITS;
+    }
+
+    private void updateNextPosition(Task task)
+    {
+        nextPosition = Math.max(nextPosition, (task.queuePosition & ~PRIORITY_BITS) + 1);
     }
 
     private void inheritQueuePosition(Task parent, Task task)
@@ -698,18 +860,19 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         //  - first take the position so that represents the just-executed task
         //  - call cleanup to submit any following task on the relevant sub-queue
         //  - remove the previous task from the running collection only if still present (SequentialExecutor will have removed it)
-        int position = task.queuePosition;
+        long position = task.queuePosition;
         try
         {
-            task.cleanupExclusive();
+            task.cleanupExclusive(this);
         }
         finally
         {
+            if (DEBUG_EXECUTION) task.debug.onCompleted(task, debug);
             --tasks;
             if (running.contains(task))
                 running.remove(task);
 
-            if (waitingForCompletion != null && waitingForCompletion.peek().maybeNotify - position >= 0)
+            if (waitingForCompletion != null && waitingForCompletion.peek().maybeNotify <= position)
                 maybeNotifyWaitingForCompletion();
 
             cache.tryShrinkOrEvict(lock);
@@ -718,12 +881,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     private void maybeNotifyWaitingForCompletion()
     {
-        int min = minPosition(waitingToRun.peek(),
+        long min = minPosition(waitingToRun.peek(),
                     minPosition(waitingToLoad.peek(),
                       minPosition(waitingToLoadRangeTxns.peek(),
                         minPosition(running.peek(),
                           minPosition(loading.peek(),
-                            minPosition(scanningRanges.peek(), Integer.MAX_VALUE))))));
+                            minPosition(scanningRanges.peek(), Long.MAX_VALUE))))));
 
         while (!waitingForCompletion.isEmpty() && waitingForCompletion.peek().position - min <= 0)
             waitingForCompletion.poll().run.run();
@@ -733,20 +896,21 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             waitingForCompletion.peek().maybeNotify = min;
     }
 
-    private static int minPosition(@Nullable Task task, int min)
+    private static long minPosition(@Nullable Task task, long min)
     {
-        return task == null ? min : Integer.min(task.queuePosition, min);
+        return task == null ? min : Long.min(task.queuePosition, min);
     }
 
     void cancelExclusive(AccordTask<?> task)
     {
-        switch (task.state())
+        AccordTask.State state = task.state();
+        switch (state)
         {
-            default: throw new UnhandledEnum(task.state());
+            default: throw new UnhandledEnum(state);
             case INITIALIZED:
                 // we could be cancelled before we even reach the queue
                 try { task.cancelExclusive(); }
-                finally { task.cleanupExclusive(); }
+                finally { task.cleanupExclusive(this); }
                 break;
 
             case SCANNING_RANGES:
@@ -759,6 +923,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
                 finally { completeTaskExclusive(task); }
                 break;
 
+            case ASSIGNED:
             case RUNNING:
             case PERSISTING:
             case FINISHED:
@@ -869,7 +1034,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
         finally
         {
-            beforeUnlock();
+            beforeUnlockExternal();
             unlock();
         }
     }
@@ -947,24 +1112,31 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
     public static abstract class Task extends IntrusivePriorityHeap.Node
     {
-        int queuePosition;
-        Thread assigned;
+        long queuePosition;
+        public long createdAt = nanoTime(), waitingToRunAt, runningAt, cleanupAt;
+        public final DebugTask debug = DebugTask.maybeDebug();
 
         protected Task()
         {
         }
 
+        public void onWaitingToRun()
+        {
+            waitingToRunAt = nanoTime();
+        }
+
         public DebuggableTask debuggable() { return null; }
 
         /**
          * Prepare to run while holding the state cache lock
          */
-        abstract protected void preRunExclusive();
+        abstract protected void preRunExclusive(Thread assigned);
 
         /**
          * Run the command; the state cache lock may or may not be held depending on the executor implementation
          */
-        abstract protected void runInternal();
+        protected abstract void runInternal();
+
         /**
          * Fail the command; the state cache lock may or may not be held depending on the executor implementation
          */
@@ -973,7 +1145,19 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         /**
          * Cleanup the command while holding the state cache lock
          */
-        abstract protected void cleanupExclusive();
+        protected void cleanupExclusive(AccordExecutor executor)
+        {
+            cleanupAt = nanoTime();
+            if (runningAt != 0)
+            {
+                if (waitingToRunAt == 0)
+                    waitingToRunAt = runningAt;
+                executor.elapsedWaitingToRun.increment(runningAt - waitingToRunAt, runningAt);
+                executor.elapsedPreparingToRun.increment(waitingToRunAt - createdAt, runningAt);
+                executor.elapsedRunning.increment(cleanupAt - runningAt, cleanupAt);
+                executor.elapsed.increment(cleanupAt - createdAt, cleanupAt);
+            }
+        }
 
         void cancelExclusive(AccordExecutor owner) {}
 
@@ -1011,9 +1195,9 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
 
         @Override
-        protected void preRunExclusive()
+        protected void preRunExclusive(Thread assigned)
         {
-            queue.preRunTask();
+            queue.preRunTask(assigned);
         }
 
         @Override
@@ -1029,7 +1213,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
 
         @Override
-        protected void cleanupExclusive()
+        protected void cleanupExclusive(AccordExecutor executor)
         {
             queue.cleanupTask();
         }
@@ -1054,51 +1238,54 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         private volatile boolean visibleStopped;
         private boolean terminated;
 
-        SequentialExecutor()
+        final DebugSequentialExecutor debug;
+
+        SequentialExecutor(AccordExecutor executor)
         {
-            this(-1);
+            this(executor, -1);
         }
 
-        SequentialExecutor(int commandStoreId)
+        SequentialExecutor(AccordExecutor executor, int commandStoreId)
         {
-            super(WAITING_TO_RUN);
+            super(WAITING_TO_RUN, commandStoreId < 0);
             this.commandStoreId = commandStoreId;
             this.selfTask = new SequentialQueueTask(this);
+            this.debug = DebugSequentialExecutor.maybeDebug(executor.debug, commandStoreId);
         }
 
-        void preRunTask()
+        void preRunTask(Thread assigned)
         {
             Invariants.require(task != null);
-            assigned = Thread.currentThread();
-            task.preRunExclusive();
+            this.assigned = assigned;
+            task.preRunExclusive(assigned);
         }
 
         void runTask()
         {
-            outer: while (!ownerUpdater.compareAndSet(this, null, assigned))
+            if (!ownerUpdater.compareAndSet(this, null, assigned))
             {
+                if (DEBUG_EXECUTION) debug.onWaiting();
+                Invariants.require(assigned == Thread.currentThread());
                 waiting = assigned;
-                while (true)
+                outer: do
                 {
-                    Thread owner = this.owner;
-                    if (owner == assigned) break outer;
-                    if (owner == null) continue outer;
-                    LockSupport.park();
+                    while (true)
+                    {
+                        Thread owner = this.owner;
+                        if (owner == assigned) break outer;
+                        if (owner == null) continue outer;
+                        LockSupport.park();
+                    }
                 }
+                while (!ownerUpdater.compareAndSet(this, null, assigned));
             }
             waiting = null;
 
-            try
-            {
-                if (stopped && reject(task))
-                    task.fail(new RejectedExecutionException(commandStoreId + " is terminated. Cannot execute " + ((AccordTask<?>) task).preLoadContext()));
-                else
-                    task.runInternal();
-            }
-            finally
-            {
-                owner = null;
-            }
+            if (stopped && reject(task))
+                task.fail(new RejectedExecutionException(commandStoreId + " is terminated. Cannot execute " + ((AccordTask<?>) task).preLoadContext()));
+            else
+                task.runInternal();
+            // NOTE: cannot safely release owner here, in case an immediate-execution runs before we can release our references and store their changes to the cache
         }
 
         private boolean reject(Task task)
@@ -1118,11 +1305,13 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
 
         void cleanupTask()
         {
-            try { task.cleanupExclusive(); }
+            try { task.cleanupExclusive(AccordExecutor.this); }
             finally
             {
                 assigned = null;
+                owner = null;
                 task = super.poll();
+                if (DEBUG_EXECUTION) debug.onSetTask(task);
 
                 // it should only be possible for this method to be invoked once we're on the running queue
                 AccordExecutor.this.running.remove(selfTask);
@@ -1149,6 +1338,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
                 task = newTask;
                 selfTask.queuePosition = newTask.queuePosition;
                 waitingToRun.append(selfTask);
+                if (DEBUG_EXECUTION) debug.onSetTask(newTask);
             }
         }
 
@@ -1176,6 +1366,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             // but can for other tasks that don't track their own state
 
             task = super.poll();
+            if (DEBUG_EXECUTION) debug.onSetTask(task);
             if (waitingToRun.contains(selfTask))
             {
                 if (task == null) waitingToRun.remove(selfTask);
@@ -1242,7 +1433,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public AsyncChain<Void> chain(Runnable run)
         {
-            int position = inheritQueuePosition();
+            long position = inheritQueuePosition();
             return new AsyncChains.Head<>()
             {
                 @Override
@@ -1256,7 +1447,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public <T> AsyncChain<T> chain(Callable<T> call)
         {
-            int position = inheritQueuePosition();
+            long position = inheritQueuePosition();
             return new AsyncChains.Head<>()
             {
                 @Override
@@ -1270,7 +1461,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public <T> AsyncChain<T> flatChain(Callable<? extends AsyncChain<T>> call)
         {
-            int position = inheritQueuePosition();
+            long position = inheritQueuePosition();
             return new AsyncChains.Head<>()
             {
                 @Override
@@ -1287,12 +1478,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             return execute(runOrFail, inheritQueuePosition());
         }
 
-        private int inheritQueuePosition()
+        private long inheritQueuePosition()
         {
             return inExecutor() && task != null ? task.queuePosition : 0;
         }
 
-        private Cancellable execute(RunOrFail runOrFail, int queuePosition)
+        private Cancellable execute(RunOrFail runOrFail, long queuePosition)
         {
             PlainChain submit = new PlainChain(runOrFail, SequentialExecutor.this, queuePosition);
             return AccordExecutor.this.submit(submit);
@@ -1319,12 +1510,13 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             {
                 if (owner == null)
                 {
-                    this.owner = null;
+                    Thread waiting = this.waiting;
+                    Invariants.require(waiting != self);
+                    this.owner = waiting;
+                    if (waiting == null) // recheck, to ensure happens-before relation with a new waiter that expects any non-null owner to notify it
+                        waiting = this.waiting;
                     if (waiting != null)
-                    {
                         LockSupport.unpark(waiting);
-                        ownerUpdater.compareAndSet(this, null, waiting);
-                    }
                 }
             }
             return true;
@@ -1340,6 +1532,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             this.kind = kind;
         }
 
+        TaskQueue(AccordTask.State kind, boolean tiny)
+        {
+            super(tiny);
+            this.kind = kind;
+        }
+
         @Override
         public int compare(T o1, T o2)
         {
@@ -1421,10 +1619,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         abstract SequentialExecutor executor();
 
         @Override
-        protected void preRunExclusive() {}
-
-        @Override
-        protected void cleanupExclusive() {}
+        protected void preRunExclusive(Thread assigned) {}
 
         @Override
         protected final void addToQueue(TaskQueue queue)
@@ -1475,7 +1670,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             this(result, run, null, 0);
         }
 
-        PlainRunnable(AsyncPromise<Void> result, Runnable run, @Nullable SequentialExecutor executor, int queuePosition)
+        PlainRunnable(AsyncPromise<Void> result, Runnable run, @Nullable SequentialExecutor executor, long queuePosition)
         {
             this.result = result;
             this.run = run;
@@ -1486,12 +1681,14 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected void runInternal()
         {
+            runningAt = nanoTime();
             try (Closeable close = locals.get())
             {
                 run.run();
             }
             if (result != null)
                 result.trySuccess(null);
+            if (DEBUG_EXECUTION) debug.onRunComplete();
         }
 
         @Override
@@ -1518,14 +1715,15 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         abstract void postRunExclusive();
 
         @Override
-        protected void preRunExclusive()
+        protected void preRunExclusive(Thread assigned)
         {
             startedAtNanos = MonotonicClock.Global.approxTime.now();
         }
 
         @Override
-        protected void cleanupExclusive()
+        protected void cleanupExclusive(AccordExecutor executor)
         {
+            super.cleanupExclusive(executor);
             postRunExclusive();
         }
 
@@ -1586,10 +1784,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public void runInternal()
         {
+            runningAt = nanoTime();
             try (Closeable close = locals.get())
             {
                 result = entry.owner.parent().adapter().load(entry.owner.commandStore, entry.key());
             }
+            if (DEBUG_EXECUTION) debug.onRunComplete();
         }
 
         @Override
@@ -1631,10 +1831,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected void runInternal()
         {
+            runningAt = nanoTime();
             try (Closeable close = locals.get())
             {
                 wrapped.runInternal();
             }
+            if (DEBUG_EXECUTION) debug.onRunComplete();
         }
 
         @Override
@@ -1680,10 +1882,12 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         public void runInternal()
         {
+            runningAt = nanoTime();
             try (Closeable close = locals.get())
             {
                 run.run();
             }
+            if (DEBUG_EXECUTION) debug.onRunComplete();
             failure = null;
         }
 
@@ -1710,7 +1914,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             this(runOrFail, null, 0);
         }
 
-        PlainChain(RunOrFail runOrFail, @Nullable SequentialExecutor executor, int queuePosition)
+        PlainChain(RunOrFail runOrFail, @Nullable SequentialExecutor executor, long queuePosition)
         {
             this.runOrFail = runOrFail;
             this.executor = executor;
@@ -1726,6 +1930,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         @Override
         protected void runInternal()
         {
+            runningAt = nanoTime();
             try (Closeable close = locals.get())
             {
                 runOrFail.run();
@@ -1735,6 +1940,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
                 // shouldn't throw exceptions
                 agent.onException(t);
             }
+            if (DEBUG_EXECUTION) debug.onRunComplete();
         }
 
         @Override
@@ -1778,16 +1984,11 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         }
 
         @Override
-        protected void preRunExclusive()
+        protected void preRunExclusive(Thread assigned)
         {
             startedAtNanos = MonotonicClock.Global.approxTime.now();
         }
 
-        @Override
-        protected void cleanupExclusive()
-        {
-        }
-
         @Override
         public String description()
         {
@@ -1829,7 +2030,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
             return commandStoreId >= 0 ? commandStoreId : null;
         }
 
-        public int position()
+        public long position()
         {
             return task.queuePosition;
         }
@@ -1858,7 +2059,7 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
         public int compareTo(TaskInfo that)
         {
             int c = this.status.compareTo(that.status);
-            if (c == 0) c = Integer.compare(this.position(), that.position());
+            if (c == 0) c = Long.compare(this.position(), that.position());
             return c;
         }
     }
@@ -1918,4 +2119,5 @@ public abstract class AccordExecutor implements CacheSize, LoadExecutor<AccordTa
     {
         return running.size();
     }
+
 }

src/java/org/apache/cassandra/service/accord/AccordExecutorAbstractLockLoop.java

@@ -26,15 +26,18 @@ import accord.utils.QuadFunction;
 import accord.utils.QuintConsumer;
 
 import org.apache.cassandra.concurrent.DebuggableTask.DebuggableTaskRunner;
+import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.service.accord.AccordExecutorLoops.LoopTask;
+import org.apache.cassandra.service.accord.debug.DebugExecution.DebugExecutorLoop;
 import org.apache.cassandra.utils.concurrent.ConcurrentLinkedStack;
 
 import static org.apache.cassandra.service.accord.AccordExecutor.Mode.RUN_WITH_LOCK;
+import static org.apache.cassandra.service.accord.debug.DebugExecution.DEBUG_EXECUTION;
 
 abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
 {
+    private static final int YIELD_INTERVAL = DatabaseDescriptor.getAccord().queue_yield_interval;
     final ConcurrentLinkedStack<Submittable> submitted = new ConcurrentLinkedStack<>();
-    boolean isHeldByExecutor;
     boolean shutdown;
 
     AccordExecutorAbstractLockLoop(Lock lock, int executorId, Agent agent)
@@ -44,9 +47,9 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
 
     abstract void notifyWork();
     abstract void notifyWorkExclusive();
+    void loopYieldExclusive() throws InterruptedException {}
     abstract void awaitExclusive() throws InterruptedException;
     abstract AccordExecutorLoops loops();
-    abstract boolean isInLoop();
     abstract <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4);
 
     <P1s, P1a, P2, P3, P4> void submit(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
@@ -57,7 +60,7 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
         else submitExternal(sync, async, p1s, p1a, p2, p3, p4);
     }
 
-    <P1s, P1a, P2, P3, P4> void submitExternalExclusive(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
+    <P1s, P2, P3, P4> void submitExternalExclusive(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, P1s p1s, P2 p2, P3 p3, P4 p4)
     {
         try
         {
@@ -95,50 +98,54 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
         }
     }
 
-    @Override
-    void beforeUnlock()
-    {
-        if (!isInLoop())
-            notifyIfMoreWorkExclusive();
-    }
-
-    void updateWaitingToRunExclusive()
+    final void updateWaitingToRunExclusive()
     {
         drainSubmittedExclusive();
         super.updateWaitingToRunExclusive();
     }
 
-    void drainSubmittedExclusive()
+    final void drainSubmittedExclusive()
     {
         submitted.drain(AccordExecutor::consumeExclusive, this, true);
     }
 
-    void notifyIfMoreWorkExclusive()
+    final void notifyIfMoreWorkExclusive()
     {
         if (hasWaitingToRun())
             notifyWorkExclusive();
     }
 
-    private void enterLockExclusive()
+    @Override
+    final void beforeUnlockExternal()
     {
-        isHeldByExecutor = true;
+        beforeUnlockLoop();
     }
 
-    private void exitLockExclusive()
+    final void beforeUnlockLoop()
     {
         notifyIfMoreWorkExclusive();
     }
 
-    private void pauseExclusive()
+    private void enterLockLoop()
+    {
+        resumeLoop();
+    }
+
+    private void exitLockLoop()
+    {
+        pauseLoop();
+        notifyIfMoreWorkExclusive();
+    }
+
+    final void pauseLoop()
     {
-        isHeldByExecutor = false;
         if (--runningThreads == 0 && tasks == 0)
             notifyQuiescentExclusive();
     }
 
-    private void resumeExclusive()
+    final void resumeLoop()
     {
-        isHeldByExecutor = true;
+        if (DEBUG_EXECUTION) debug.onEnterLock();
         ++runningThreads;
     }
 
@@ -154,14 +161,14 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
             @Override
             public void run()
             {
+                Thread self = Thread.currentThread();
                 Task task;
                 while (true)
                 {
                     lock();
                     try
                     {
-                        resumeExclusive();
-                        enterLockExclusive();
+                        enterLockLoop();
                         while (true)
                         {
                             task = pollWaitingToRunExclusive();
@@ -171,7 +178,7 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
                                 setRunning(task);
                                 try
                                 {
-                                    task.preRunExclusive();
+                                    task.preRunExclusive(self);
                                     task.runInternal();
                                 }
                                 catch (Throwable t)
@@ -188,22 +195,21 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
                             {
                                 if (shutdown)
                                 {
-                                    pauseExclusive();
-                                    exitLockExclusive();
+                                    pauseLoop();
+                                    exitLockLoop();
                                     notifyWorkExclusive(); // always notify on shutdown
                                     return;
                                 }
 
-                                pauseExclusive();
+                                pauseLoop();
                                 awaitExclusive();
-                                resumeExclusive();
+                                resumeLoop();
                             }
                         }
                     }
                     catch (Throwable t)
                     {
-                        pauseExclusive();
-                        exitLockExclusive();
+                        exitLockLoop();
 
                         try { agent.onException(t); }
                         catch (Throwable t2) { }
@@ -221,15 +227,21 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
     {
         return new LoopTask(name)
         {
+            final DebugExecutorLoop debug = DEBUG_EXECUTION ? new DebugExecutorLoop(AccordExecutorAbstractLockLoop.this.debug) : null;
             @Override
             public void run()
             {
+                Thread self = Thread.currentThread();
+                int count = 0;
                 Task task = null;
                 while (true)
                 {
+                    if (DEBUG_EXECUTION) debug.onLock();
                     lock();
                     try
                     {
+                        if (DEBUG_EXECUTION) debug.onEnterLock();
+                        enterLockLoop();
                         if (task != null)
                         {
                             Task tmp = task;
@@ -237,31 +249,40 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
                             completeTaskExclusive(tmp);
                             clearRunning();
                         }
-                        else resumeExclusive();
-                        enterLockExclusive();
+
+                        if (count >= YIELD_INTERVAL)
+                        {
+                            loopYieldExclusive();
+                            count = 0;
+                        }
 
                         while (true)
                         {
                             task = pollWaitingToRunExclusive();
+
                             if (task != null)
                             {
                                 setRunning(task);
-                                task.preRunExclusive();
-                                exitLockExclusive();
+                                task.preRunExclusive(self);
+                                if (DEBUG_EXECUTION) debug.onExitLock();
+                                exitLockLoop();
                                 break;
                             }
 
-                            pauseExclusive();
-
                             if (shutdown)
                             {
-                                exitLockExclusive();
+                                if (DEBUG_EXECUTION) debug.onExitLock();
+                                exitLockLoop();
                                 notifyWorkExclusive();
                                 return;
                             }
 
+                            pauseLoop();
+                            if (DEBUG_EXECUTION) debug.onExitLock();
                             awaitExclusive();
-                            resumeExclusive();
+                            if (DEBUG_EXECUTION) debug.onEnterLock();
+                            resumeLoop();
+                            count = 0;
                         }
                     }
                     catch (Throwable t)
@@ -281,18 +302,18 @@ abstract class AccordExecutorAbstractLockLoop extends AccordExecutor
                             try { agent.onException(t); }
                             catch (Throwable t2) { /* nothing we can sensibly do after already reporting */ }
                         }
-                        if (isHeldByExecutor)
-                            pauseExclusive();
-                        exitLockExclusive();
+                        exitLockLoop();
                         continue;
                     }
                     finally
                     {
+                        if (DEBUG_EXECUTION) debug.onExitLock();
                         unlock();
                     }
 
                     try
                     {
+                        ++count;
                         task.runInternal();
                     }
                     catch (Throwable t)

src/java/org/apache/cassandra/service/accord/AccordExecutorAbstractSemiSyncSubmit.java

@@ -35,20 +35,7 @@ abstract class AccordExecutorAbstractSemiSyncSubmit extends AccordExecutorAbstra
 
     <P1s, P1a, P2, P3, P4> void submitExternal(QuintConsumer<AccordExecutor, P1s, P2, P3, P4> sync, QuadFunction<P1a, P2, P3, P4, Submittable> async, P1s p1s, P1a p1a, P2 p2, P3 p3, P4 p4)
     {
-        if (!tryLock())
-        {
-            submitted.push(async.apply(p1a, p2, p3, p4));
+        if (submitted.push(async.apply(p1a, p2, p3, p4)) && !isInLoop())
             notifyWork();
-            return;
-        }
-
-        try
-        {
-            submitExternalExclusive(sync, async, p1s, p1a, p2, p3, p4);
-        }
-        finally
-        {
-            unlock();
-        }
     }
 }

src/java/org/apache/cassandra/service/accord/AccordExecutorSemiSyncSubmit.java

@@ -31,6 +31,7 @@ class AccordExecutorSemiSyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
     private final AccordExecutorLoops loops;
     private final ReentrantLock lock;
     private final Condition hasWork;
+    private int waiting;
 
     public AccordExecutorSemiSyncSubmit(int executorId, Mode mode, int threads, IntFunction<String> name, Agent agent)
     {
@@ -45,11 +46,30 @@ class AccordExecutorSemiSyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
         this.loops = new AccordExecutorLoops(mode, threads, name, this::task);
     }
 
+    @Override
+    void loopYieldExclusive() throws InterruptedException
+    {
+        if (waiting > 0 && hasWaitingToRun())
+        {
+            pauseLoop();
+            hasWork.signal();
+            awaitWork();
+            resumeLoop();
+        }
+    }
+
     @Override
     void awaitExclusive() throws InterruptedException
     {
         if (submitted.isEmpty())
-            hasWork.await();
+            awaitWork();
+    }
+
+    private void awaitWork() throws InterruptedException
+    {
+        waiting++;
+        try { hasWork.await(); }
+        finally { waiting--; }
     }
 
     @Override
@@ -67,19 +87,9 @@ class AccordExecutorSemiSyncSubmit extends AccordExecutorAbstractSemiSyncSubmit
     @Override
     void notifyWork()
     {
-        // we check running both sides of tryLock for ordering guarantees
-        boolean hadRunning = isHeldByExecutor;
-        if (lock.tryLock())
-        {
-            try { hasWork.signal(); }
-            finally { lock.unlock(); }
-        }
-        else if (!hadRunning || !isHeldByExecutor)
-        {
-            lock.lock();
-            try { hasWork.signal(); }
-            finally { lock.unlock(); }
-        }
+        lock.lock();
+        try { hasWork.signal(); }
+        finally { lock.unlock(); }
     }
     
     @Override

src/java/org/apache/cassandra/service/accord/AccordExecutorSimple.java

@@ -62,6 +62,12 @@ class AccordExecutorSimple extends AccordExecutor
         this.executor = executorFactory().sequential(name.apply(0));
     }
 
+    @Override
+    boolean isInLoop()
+    {
+        return executor.inExecutor();
+    }
+
     @Override
     public boolean hasTasks()
     {
@@ -69,7 +75,7 @@ class AccordExecutorSimple extends AccordExecutor
     }
 
     @Override
-    void beforeUnlock()
+    void beforeUnlockExternal()
     {
         if (hasWaitingToRun())
             executor.execute(this::run);
@@ -77,6 +83,7 @@ class AccordExecutorSimple extends AccordExecutor
 
     protected void run()
     {
+        Thread self = Thread.currentThread();
         lock.lock();
         try
         {
@@ -92,7 +99,7 @@ class AccordExecutorSimple extends AccordExecutor
                     return;
                 }
 
-                try { task.preRunExclusive(); task.runInternal(); }
+                try { task.preRunExclusive(self); task.runInternal(); }
                 catch (Throwable t) { task.fail(t); }
                 finally
                 {

src/java/org/apache/cassandra/service/accord/AccordExecutorSyncSubmit.java

@@ -79,14 +79,8 @@ class AccordExecutorSyncSubmit extends AccordExecutorAbstractLockLoop
     void notifyWork()
     {
         lock.lock();
-        try
-        {
-            hasWork.signal();
-        }
-        finally
-        {
-            lock.unlock();
-        }
+        try { hasWork.signal(); }
+        finally { lock.unlock(); }
     }
 
     @Override
@@ -100,7 +94,7 @@ class AccordExecutorSyncSubmit extends AccordExecutorAbstractLockLoop
         lock.lock();
         try
         {
-            submitExternalExclusive(sync, async, p1s, p1a, p2, p3, p4);
+            submitExternalExclusive(sync, p1s, p2, p3, p4);
         }
         finally
         {

src/java/org/apache/cassandra/service/accord/AccordKeyspace.java

@@ -116,7 +116,7 @@ import org.apache.cassandra.utils.vint.VIntCoding;
 
 import static java.lang.String.format;
 import static java.util.Collections.emptyMap;
-import static org.apache.cassandra.config.AccordSpec.RangeIndexMode.journal_sai;
+import static org.apache.cassandra.config.AccordConfig.RangeIndexMode.journal_sai;
 import static org.apache.cassandra.db.partitions.PartitionUpdate.singleRowUpdate;
 import static org.apache.cassandra.db.rows.BTreeRow.singleCellRow;
 import static org.apache.cassandra.schema.SchemaConstants.ACCORD_KEYSPACE_NAME;
@@ -307,7 +307,7 @@ public class AccordKeyspace
                 return null;
 
             // TODO (desired): consider whether better to not compact any validation failures, since we expect is already overwritten
-            CommandsForKey updated = current.withGcBeforeAtLeast(redundantBefore.gcBefore(), false);
+            CommandsForKey updated = current.withCleanCfkBeforeAtLeast(redundantBefore.cleanCfkBefore(), false);
             if (current == updated)
                 return row;
 

src/java/org/apache/cassandra/service/accord/AccordMessageSink.java

@@ -28,7 +28,9 @@ import com.google.common.collect.ImmutableMap;
 
 import accord.api.AsyncExecutor;
 import accord.api.MessageSink;
+import accord.api.Tracing;
 import accord.impl.RequestCallbacks;
+import accord.local.MapReduceCommandStores;
 import accord.local.Node;
 import accord.messages.Callback;
 import accord.messages.MessageType;
@@ -36,19 +38,19 @@ import accord.messages.Reply;
 import accord.messages.ReplyContext;
 import accord.messages.Request;
 import accord.primitives.TxnId;
+import accord.utils.Invariants;
 import accord.utils.async.Cancellable;
 
 import org.apache.cassandra.exceptions.RequestFailureReason;
 import org.apache.cassandra.locator.InetAddressAndPort;
 import org.apache.cassandra.net.Message;
 import org.apache.cassandra.net.MessageDelivery;
-import org.apache.cassandra.net.MessageFlag;
 import org.apache.cassandra.net.MessagingService;
+import org.apache.cassandra.net.ParamType;
 import org.apache.cassandra.net.ResponseContext;
 import org.apache.cassandra.net.Verb;
 import org.apache.cassandra.service.TimeoutStrategy;
 import org.apache.cassandra.service.accord.topology.AccordEndpointMapper;
-import org.apache.cassandra.utils.Clock;
 
 import static accord.messages.MessageType.StandardMessage.ACCEPT_REQ;
 import static accord.messages.MessageType.StandardMessage.ACCEPT_RSP;
@@ -86,6 +88,7 @@ import static accord.messages.MessageType.StandardMessage.READ_REQ;
 import static accord.messages.MessageType.StandardMessage.READ_RSP;
 import static accord.messages.MessageType.StandardMessage.RECOVER_AWAIT_REQ;
 import static accord.messages.MessageType.StandardMessage.RECOVER_AWAIT_RSP;
+import static accord.messages.MessageType.StandardMessage.REMOTE_SUCCESS_REQ;
 import static accord.messages.MessageType.StandardMessage.SET_GLOBALLY_DURABLE_REQ;
 import static accord.messages.MessageType.StandardMessage.SET_SHARD_DURABLE_REQ;
 import static accord.messages.MessageType.StandardMessage.SIMPLE_RSP;
@@ -96,6 +99,7 @@ import static java.util.concurrent.TimeUnit.NANOSECONDS;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.expire;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.slowPreaccept;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.slowRead;
+import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 
 public class AccordMessageSink implements MessageSink
 {
@@ -167,6 +171,7 @@ public class AccordMessageSink implements MessageSink
             builder.put(SET_GLOBALLY_DURABLE_REQ,                 Verb.ACCORD_SET_GLOBALLY_DURABLE_REQ);
             builder.put(GET_DURABLE_BEFORE_REQ,                   Verb.ACCORD_GET_DURABLE_BEFORE_REQ);
             builder.put(GET_DURABLE_BEFORE_RSP,                   Verb.ACCORD_GET_DURABLE_BEFORE_RSP);
+            builder.put(REMOTE_SUCCESS_REQ,                       Verb.ACCORD_REMOTE_SUCCESS_REQ);
             builder.put(FAILURE_RSP,                              Verb.FAILURE_RSP);
             Map<StandardMessage, Verb> mapping = builder.build();
             StandardMessage.initialise(mapping);
@@ -209,6 +214,7 @@ public class AccordMessageSink implements MessageSink
     {
         Verb verb = VerbMapping.getVerb(request);
         Preconditions.checkNotNull(verb, "Verb is null for type %s", request.type());
+
         Message<Request> message = Message.out(verb, request);
         InetAddressAndPort endpoint = endpointMapper.mappedEndpointOrNull(to, message);
         if (endpoint == null)
@@ -224,7 +230,7 @@ public class AccordMessageSink implements MessageSink
         Verb verb = VerbMapping.getVerb(request);
         Preconditions.checkNotNull(verb, "Verb is null for type %s", request.type());
 
-        long nowNanos = Clock.Global.nanoTime();
+        long nowNanos = nanoTime();
         TxnId txnId = request.primaryTxnId();
         long slowAtNanos = Long.MAX_VALUE;
         long expiresAtNanos = nowNanos + expire(txnId, verb).computeWait(attempt, NANOSECONDS);
@@ -251,6 +257,13 @@ public class AccordMessageSink implements MessageSink
         }
 
         Message<Request> message = Message.out(verb, request, expiresAtNanos);
+        if (request instanceof MapReduceCommandStores<?, ?>)
+        {
+            Tracing tracing =  ((MapReduceCommandStores<?, ?>) request).tracing();
+            if (tracing != null) tracing = tracing.send();
+            if (tracing != null) message = message.withParam(ParamType.ACCORD_TRACING, tracing);
+        }
+
         InetAddressAndPort endpoint = endpointMapper.mappedEndpointOrNull(to, message);
         if (endpoint == null)
         {
@@ -263,26 +276,24 @@ public class AccordMessageSink implements MessageSink
         return cancellable;
     }
 
-    @Override
-    public void reply(Node.Id replyingTo, ReplyContext replyContext, Reply reply)
+    public void reply(Node.Id replyingTo, ReplyContext replyContext, Reply reply, Throwable failure)
     {
         ResponseContext respondTo = (ResponseContext) replyContext;
-        Message<?> message = Message.responseWith(reply, respondTo);
-        if (!reply.isFinal())
-            message = message.withFlag(MessageFlag.NOT_FINAL);
-        checkReplyType(reply, respondTo);
-        InetAddressAndPort endpoint = endpointMapper.mappedEndpointOrNull(replyingTo, message);
-        if (endpoint == null)
-            return;
+        Message<?> message;
+        if (failure != null) message = Message.failureResponse(RequestFailureReason.UNKNOWN, failure, respondTo);
+        else
+        {
+            message = Message.responseWith(reply, respondTo);
+            if (Invariants.isParanoid()) checkReplyType(reply, respondTo);
+        }
+        Object tracing = respondTo.params().get(ParamType.ACCORD_TRACING);
+        if (tracing != null)
+        {
+            tracing = ((Tracing)tracing).send();
+            if (tracing != null)
+                message = message.withParam(ParamType.ACCORD_TRACING, tracing);
+        }
 
-        messaging.send(message, endpoint);
-    }
-
-    @Override
-    public void replyWithUnknownFailure(Node.Id replyingTo, ReplyContext replyContext, Throwable failure)
-    {
-        ResponseContext respondTo = (ResponseContext) replyContext;
-        Message<?> message = Message.failureResponse(RequestFailureReason.UNKNOWN, failure, respondTo);
         InetAddressAndPort endpoint = endpointMapper.mappedEndpointOrNull(replyingTo, message);
         if (endpoint == null)
             return;

src/java/org/apache/cassandra/service/accord/AccordObjectSizes.java

@@ -25,7 +25,7 @@ import accord.api.Key;
 import accord.api.Result;
 import accord.api.RoutingKey;
 import accord.local.Command;
-import accord.local.ICommand;
+import accord.local.CommandBuilder;
 import accord.local.Node;
 import accord.local.StoreParticipants;
 import accord.local.cfk.CommandsForKey;
@@ -39,11 +39,9 @@ import accord.primitives.FullKeyRoute;
 import accord.primitives.FullRangeRoute;
 import accord.primitives.KeyDeps;
 import accord.primitives.Keys;
-import accord.primitives.PartialDeps;
 import accord.primitives.PartialKeyRoute;
 import accord.primitives.PartialRangeRoute;
 import accord.primitives.PartialTxn;
-import accord.primitives.Participants;
 import accord.primitives.Range;
 import accord.primitives.RangeDeps;
 import accord.primitives.Ranges;
@@ -61,6 +59,7 @@ import accord.utils.ImmutableBitSet;
 import accord.utils.UnhandledEnum;
 
 import org.apache.cassandra.config.DatabaseDescriptor;
+import org.apache.cassandra.db.BufferDecoratedKey;
 import org.apache.cassandra.dht.Murmur3Partitioner;
 import org.apache.cassandra.schema.TableId;
 import org.apache.cassandra.service.accord.api.PartitionKey;
@@ -73,18 +72,15 @@ import org.apache.cassandra.service.accord.txn.TxnQuery;
 import org.apache.cassandra.service.accord.txn.TxnRead;
 import org.apache.cassandra.service.accord.txn.TxnResult;
 import org.apache.cassandra.service.accord.txn.TxnWrite;
+import org.apache.cassandra.utils.ByteBufferUtil;
 import org.apache.cassandra.utils.ObjectSizes;
 
-import static accord.local.Command.Accepted.accepted;
-import static accord.local.Command.Committed.committed;
-import static accord.local.Command.Executed.executed;
-import static accord.local.Command.NotAcceptedWithoutDefinition.notAccepted;
-import static accord.local.Command.NotDefined.notDefined;
-import static accord.local.Command.PreAccepted.preaccepted;
 import static accord.local.Command.Truncated.WaitingOn;
-import static accord.local.Command.Truncated.invalidated;
-import static accord.local.Command.Truncated.vestigial;
 import static accord.local.cfk.CommandsForKey.InternalStatus.ACCEPTED;
+import static accord.primitives.SaveStatus.Invalidated;
+import static accord.primitives.SaveStatus.NotDefined;
+import static accord.primitives.SaveStatus.PreAccepted;
+import static accord.primitives.SaveStatus.TruncatedUnapplied;
 import static accord.primitives.Status.Durability.NotDurable;
 import static accord.primitives.TxnId.NO_TXNIDS;
 import static org.apache.cassandra.utils.ObjectSizes.measure;
@@ -306,23 +302,24 @@ public class AccordObjectSizes
 
     private static class CommandEmptySizes
     {
-        private final static TokenKey EMPTY_KEY = new TokenKey(EMPTY_ID, null);
+        private final static PartitionKey EMPTY_KEY = new PartitionKey(EMPTY_ID, new BufferDecoratedKey(new Murmur3Partitioner.LongToken(1), ByteBufferUtil.EMPTY_BYTE_BUFFER));
+        private final static TokenKey EMPTY_TOKEN_KEY = new TokenKey(EMPTY_ID, new Murmur3Partitioner.LongToken(1));
         private final static TxnId EMPTY_TXNID = new TxnId(42, 42, 0, Kind.Read, Domain.Key, new Node.Id(42));
 
-        private static ICommand attrs(boolean hasDeps, boolean hasTxn, boolean executes)
+        private static Command build(SaveStatus saveStatus, boolean hasDeps, boolean hasTxn, boolean executes)
         {
-            FullKeyRoute route = new FullKeyRoute(EMPTY_KEY, new RoutingKey[]{ EMPTY_KEY });
-            Participants<?> empty = route.slice(0, 0);
-            ICommand.Builder builder = new ICommand.Builder(EMPTY_TXNID)
-                                       .setParticipants(StoreParticipants.create(route, empty, executes ? empty : null, executes ? empty : null, empty, route))
+            Keys keys = Keys.of(EMPTY_KEY);
+            FullKeyRoute route = new FullKeyRoute(EMPTY_TOKEN_KEY, new RoutingKey[]{ EMPTY_TOKEN_KEY });
+            CommandBuilder builder = new CommandBuilder(EMPTY_TXNID)
+                                       .participants(StoreParticipants.create(route, route, executes ? route : null, executes ? route : null, route, route))
                                        .durability(NotDurable)
                                        .executeAt(EMPTY_TXNID)
                                        .promised(Ballot.ZERO);
             if (hasDeps)
-                builder.partialDeps(PartialDeps.NONE);
+                builder.partialDeps(new Deps(KeyDeps.none(route.toParticipants()), RangeDeps.NONE).intersecting(route));
 
             if (hasTxn)
-                builder.partialTxn(new PartialTxn.InMemory(Kind.Read, null, null, null, null, TableMetadatasAndKeys.none(Domain.Key)));
+                builder.partialTxn(new PartialTxn.InMemory(Kind.Read, keys, TxnRead.empty(Domain.Key), null, null, TableMetadatasAndKeys.none(Domain.Key)));
 
             if (executes)
             {
@@ -330,18 +327,20 @@ public class AccordObjectSizes
                 builder.result(new TxnData());
             }
 
-            return builder;
+            return builder.build(saveStatus);
         }
 
-        final static long NOT_DEFINED = measure(notDefined(attrs(false, false, false)));
-        final static long PREACCEPTED = measure(preaccepted(attrs(false, true, false), SaveStatus.PreAccepted));
-        final static long NOTACCEPTED = measure(notAccepted(attrs(false, false, false), SaveStatus.AcceptedInvalidate));
-        final static long ACCEPTED = measure(accepted(attrs(true, false, false), SaveStatus.AcceptedMedium));
-        final static long COMMITTED = measure(committed(attrs(true, true, false), SaveStatus.Committed));
-        final static long EXECUTED = measure(executed(attrs(true, true, true), SaveStatus.Applied));
+        final static long NOT_DEFINED = measure(build(NotDefined, false, false, false));
+        final static long PREACCEPTED = measure(build(PreAccepted, false, true, false));
+        final static long NOTACCEPTED = measure(build(SaveStatus.AcceptedInvalidate, false, false, false));
+        final static long ACCEPTED = measure(build(SaveStatus.AcceptedMedium, true, false, false));
+        final static long COMMITTED = measure(build(SaveStatus.Committed, true, true, false));
+        final static long EXECUTED = measure(build(SaveStatus.Applied, true, true, true));
         // TODO (expected): TruncatedAwaitsOnlyDeps
-        final static long TRUNCATED = measure(vestigial(EMPTY_TXNID, attrs(false, false, false).participants()));
-        final static long INVALIDATED = measure(invalidated(EMPTY_TXNID, attrs(false, false, false).participants()));
+        final static long TRUNCATED = measure(build(TruncatedUnapplied, false, false, false).participants());
+        final static long INVALIDATED = measure(build(Invalidated, false, false, false).participants());
+
+        private static void touch() {}
 
         private static long emptySize(Command command)
         {

src/java/org/apache/cassandra/service/accord/AccordSafeCommandStore.java

@@ -81,11 +81,11 @@ public class AccordSafeCommandStore extends AbstractSafeCommandStore<AccordSafeC
         return commandStore.unsafeGetRangesForEpoch();
     }
 
-    public static AccordSafeCommandStore create(AccordTask<?> operation,
+    public static AccordSafeCommandStore create(AccordTask<?> task,
                                                 @Nullable CommandSummaries commandsForRanges,
                                                 AccordCommandStore commandStore)
     {
-        return new AccordSafeCommandStore(operation, commandsForRanges, commandStore);
+        return new AccordSafeCommandStore(task, commandsForRanges, commandStore);
     }
 
     @VisibleForTesting

src/java/org/apache/cassandra/service/accord/AccordSafeCommandsForKey.java

@@ -24,10 +24,21 @@ import com.google.common.annotations.VisibleForTesting;
 
 import accord.api.RoutingKey;
 import accord.local.cfk.CommandsForKey;
+import accord.local.cfk.NotifySink;
 import accord.local.cfk.SafeCommandsForKey;
 
 public class AccordSafeCommandsForKey extends SafeCommandsForKey implements AccordSafeState<RoutingKey, CommandsForKey>
 {
+    public static class CommandsForKeyCacheEntry extends AccordCacheEntry<RoutingKey, CommandsForKey>
+    {
+        private NotifySink overrideSink;
+
+        CommandsForKeyCacheEntry(RoutingKey key, AccordCache.Type<RoutingKey, CommandsForKey, ?>.Instance owner)
+        {
+            super(key, owner);
+        }
+    }
+
     private boolean invalidated;
     private final AccordCacheEntry<RoutingKey, CommandsForKey> global;
     private CommandsForKey original;
@@ -39,6 +50,8 @@ public class AccordSafeCommandsForKey extends SafeCommandsForKey implements Acco
         this.global = global;
         this.original = null;
         this.current = null;
+//        if (overrideSink() == null)
+//            overrideSink(new RecordingNotifySink());
     }
 
     @Override
@@ -103,6 +116,18 @@ public class AccordSafeCommandsForKey extends SafeCommandsForKey implements Acco
         this.current = cfk;
     }
 
+    @Override
+    public void overrideSink(NotifySink overrideSink)
+    {
+        ((CommandsForKeyCacheEntry)global).overrideSink = overrideSink;
+    }
+
+    @Override
+    public NotifySink overrideSink()
+    {
+        return ((CommandsForKeyCacheEntry)global).overrideSink;
+    }
+
     public CommandsForKey original()
     {
         checkNotInvalidated();

src/java/org/apache/cassandra/service/accord/AccordService.java

@@ -32,6 +32,7 @@ import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.BiConsumer;
 import java.util.function.BiFunction;
 import java.util.function.Function;
+import java.util.stream.Stream;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
@@ -72,6 +73,8 @@ import accord.primitives.Seekables;
 import accord.primitives.Timestamp;
 import accord.primitives.Txn;
 import accord.primitives.TxnId;
+import accord.primitives.TxnId.FastPath;
+import accord.primitives.TxnId.FastPaths;
 import accord.topology.ActiveEpochs;
 import accord.topology.EpochReady;
 import accord.topology.Shard;
@@ -87,9 +90,9 @@ import accord.utils.async.AsyncResult;
 import accord.utils.async.AsyncResults;
 
 import org.apache.cassandra.concurrent.Shutdownable;
-import org.apache.cassandra.config.AccordSpec;
-import org.apache.cassandra.config.AccordSpec.CatchupMode;
-import org.apache.cassandra.config.AccordSpec.JournalSpec.ReplaySavePoint;
+import org.apache.cassandra.config.AccordConfig;
+import org.apache.cassandra.config.AccordConfig.CatchupMode;
+import org.apache.cassandra.config.AccordConfig.JournalConfig.ReplaySavePoint;
 import org.apache.cassandra.config.CassandraRelevantProperties;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.db.ColumnFamilyStore;
@@ -158,23 +161,24 @@ import org.apache.cassandra.utils.concurrent.ImmediateFuture;
 import org.apache.cassandra.utils.concurrent.UncheckedInterruptedException;
 
 import static accord.api.Journal.TopologyUpdate;
-import static accord.api.ProtocolModifiers.Toggles.FastExec.MAY_BYPASS_SAFESTORE;
+import static accord.api.ProtocolModifiers.FastExecution.MAY_BYPASS_SAFESTORE;
 import static accord.impl.progresslog.DefaultProgressLog.ModeFlag.CATCH_UP;
 import static accord.local.durability.DurabilityService.SyncLocal.Self;
 import static accord.local.durability.DurabilityService.SyncRemote.All;
 import static accord.messages.SimpleReply.Ok;
 import static accord.primitives.Txn.Kind.ExclusiveSyncPoint;
 import static accord.primitives.Txn.Kind.Write;
+import static accord.primitives.TxnId.MediumPath.NoMediumPath;
+import static accord.primitives.TxnId.MediumPath.TrackStable;
 import static java.util.concurrent.TimeUnit.MINUTES;
 import static java.util.concurrent.TimeUnit.NANOSECONDS;
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.apache.cassandra.concurrent.ExecutorFactory.Global.executorFactory;
 import static org.apache.cassandra.concurrent.ExecutorFactory.SimulatorThreadTag.JOB;
 import static org.apache.cassandra.concurrent.ExecutorFactory.SystemThreadTag.DAEMON;
-import static org.apache.cassandra.config.AccordSpec.CatchupMode.DISABLED;
-import static org.apache.cassandra.config.AccordSpec.CatchupMode.FALLBACK_TO_HARD;
-import static org.apache.cassandra.config.AccordSpec.CatchupMode.HARD;
-import static org.apache.cassandra.config.AccordSpec.JournalSpec.ReplayMode.RESET;
+import static org.apache.cassandra.config.AccordConfig.CatchupMode.FALLBACK_TO_HARD;
+import static org.apache.cassandra.config.AccordConfig.CatchupMode.HARD;
+import static org.apache.cassandra.config.AccordConfig.JournalConfig.ReplayMode.RESET;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccord;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordCommandStoreShardCount;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordGlobalDurabilityCycle;
@@ -276,12 +280,10 @@ public class AccordService implements IAccordService, Shutdownable
     private static final Future<Void> EPOCH_READY = ImmediateFuture.success(null);
     static
     {
-        ProtocolModifiers.Toggles.setPermitLocalExecution(true);
-        ProtocolModifiers.Toggles.setRequiresUniqueHlcs(true);
-        ProtocolModifiers.Toggles.setFastReadExecMayResendTxn(true);
-        ProtocolModifiers.Toggles.setFastReadExec(MAY_BYPASS_SAFESTORE);
-        ProtocolModifiers.Toggles.setFastWriteExec(MAY_BYPASS_SAFESTORE);
-        ProtocolModifiers.Toggles.setDataStoreDetectsFutureReads(true);
+        ProtocolModifiers.Configure.setDataStoreRequiresUniqueHlcs(true);
+        ProtocolModifiers.Configure.setDataStoreDetectsFutureReads(true);
+        ProtocolModifiers.Configure.setFastReadExecution(MAY_BYPASS_SAFESTORE);
+        ProtocolModifiers.Configure.setFastWriteExecution(MAY_BYPASS_SAFESTORE);
     }
 
     private enum State { INIT, STARTING, STARTED, STOPPED, SHUTTING_DOWN, SHUTDOWN }
@@ -322,6 +324,10 @@ public class AccordService implements IAccordService, Shutdownable
         unsafeInstance = instance = requestInstance = replyInstance = NOOP_SERVICE;
     }
 
+    public static void touch()
+    {
+    }
+
     public static IAccordService tryGetUnsafe()
     {
         return unsafeInstance;
@@ -345,8 +351,7 @@ public class AccordService implements IAccordService, Shutdownable
     public static IVerbHandler<Void> watermarkHandlerOrNoop()
     {
         if (!isSetup()) return ignore -> {};
-        AccordService i = (AccordService) instance();
-        return i.topologyService().watermarkCollector().handler;
+        return instance().topologyService().watermarkCollector().handler;
     }
 
     public static IVerbHandler<? extends Request> requestHandlerOrNoop()
@@ -448,14 +453,14 @@ public class AccordService implements IAccordService, Shutdownable
         AccordAgent agent = FBUtilities.construct(CassandraRelevantProperties.ACCORD_AGENT_CLASS.getString(AccordAgent.class.getName()), "AccordAgent");
         agent.setup(localId);
         AccordTimeService time = new AccordTimeService();
-        final RequestCallbacks callbacks = new RequestCallbacks(time);
         this.scheduler = new AccordScheduler();
+        final RequestCallbacks callbacks = new RequestCallbacks(time, scheduler);
         this.dataStore = new AccordDataStore();
         this.journal = new AccordJournal(DatabaseDescriptor.getAccord().journal);
         this.endpointMapper = new EndpointMapping.Updateable();
+        this.messageSink = new AccordMessageSink(endpointMapper, callbacks);
         this.topologyService = new AccordTopologyService(localId, endpointMapper);
         this.fastPathCoordinator = AccordFastPathCoordinator.create(localId, endpointMapper);
-        this.messageSink = new AccordMessageSink(endpointMapper, callbacks);
         this.node = new Node(localId,
                              messageSink,
                              topologyService,
@@ -479,6 +484,38 @@ public class AccordService implements IAccordService, Shutdownable
         this.responseHandler = new AccordResponseVerbHandler<>(callbacks, endpointMapper);
     }
 
+    public static void applyProtocolModifiers(AccordConfig config)
+    {
+        if (config.coordinator_backlog_execution != null)
+            ProtocolModifiers.Configure.setCoordinatorBacklogExecution(config.coordinator_backlog_execution);
+        if (config.permit_coordinator_local_execution != null)
+            ProtocolModifiers.Configure.setPermitCoordinatorLocalExecution(config.permit_coordinator_local_execution);
+        if (config.permit_local_delivery != null)
+            ProtocolModifiers.Configure.setPermitLocalDelivery(config.permit_local_delivery);
+        if (config.permit_fast_path != null)
+            ProtocolModifiers.Configure.setPermittedFastPaths(new FastPaths(Stream.of(FastPath.values()).filter(fp -> fp.compareTo(config.permit_fast_path) <= 0).toArray(FastPath[]::new)));
+        if (config.permit_track_stable_medium_path != null)
+            ProtocolModifiers.Configure.setDefaultMediumPath(config.permit_track_stable_medium_path ? TrackStable : NoMediumPath);
+        if (config.permit_fast_quorum_medium_path != null)
+            ProtocolModifiers.Configure.setPermitFastQuorumMediumPath(config.permit_fast_quorum_medium_path);
+        if (config.always_inform_durable_single_key != null)
+            ProtocolModifiers.Configure.setInformOfSingleKeyDurabilityIfDepsSizeAtLeast(0);
+        if (config.replica_execution != null)
+            ProtocolModifiers.Configure.setReplicaExecution(config.replica_execution);
+        if (config.replica_execution_distributed_persist_chance != null)
+            ProtocolModifiers.Configure.setReplicaExecuteDistributedPersistChance(config.replica_execution_distributed_persist_chance);
+        if (config.fast_read_execution != null)
+            ProtocolModifiers.Configure.setFastReadExecution(config.fast_read_execution);
+        if (config.fast_write_execution != null)
+            ProtocolModifiers.Configure.setFastWriteExecution(config.fast_write_execution);
+        if (config.clean_cfk_before != null)
+            ProtocolModifiers.Configure.setCleanCfkBefore(config.clean_cfk_before);
+        if (config.send_stable != null)
+            ProtocolModifiers.Configure.setSendStableMessages(config.send_stable);
+        if (config.send_minimal != null)
+            ProtocolModifiers.Configure.setSendMinimalCommits(config.send_minimal);
+    }
+
     @Override
     public synchronized void localStartup()
     {
@@ -497,6 +534,7 @@ public class AccordService implements IAccordService, Shutdownable
                     case EXIT:
                         throw new RuntimeException("Stop marker is older than start marker (" + stopMarker + '<' + startMarker + ") , so cannot assume we have a complete log of our votes in any consensus groups. Exiting.");
 
+                    case ALLOW_UNSAFE_STARTUP:
                     case UNSAFE_STARTUP:
                         logger.warn("Stop marker is older than start marker ({}<{}), so cannot assume we have a complete log of our votes in any consensus groups. Continuing to startup as configured.", stopMarker, startMarker);
                         break;
@@ -611,7 +649,11 @@ public class AccordService implements IAccordService, Shutdownable
     private void distributedStartupInternal()
     {
         finishTopologyInitialization();
-        WatermarkCollector.fetchAndReportWatermarksAsync(topology());
+        WatermarkCollector.fetchAndReportWatermarksAsync(topology())
+                          .addCallback((success, failure) -> {
+                              topologyService.afterStartup(node);
+
+                          });
 
         fastPathCoordinator.start();
         ClusterMetadataService.instance().log().addListener(fastPathCoordinator);
@@ -638,8 +680,8 @@ public class AccordService implements IAccordService, Shutdownable
 
     void catchup()
     {
-        AccordSpec spec = DatabaseDescriptor.getAccord();
-        if (spec.catchup_on_start == DISABLED)
+        AccordConfig spec = DatabaseDescriptor.getAccord();
+        if (spec.catchup_on_start == CatchupMode.DISABLED)
         {
             logger.info("Catchup disabled; continuing to startup");
             return;

src/java/org/apache/cassandra/service/accord/AccordTask.java

@@ -85,9 +85,8 @@ import static accord.local.LoadKeysFor.RECOVERY;
 import static accord.local.LoadKeysFor.WRITE;
 import static accord.primitives.Routable.Domain.Key;
 import static accord.primitives.Txn.Kind.EphemeralRead;
-import static accord.utils.Invariants.illegalState;
-import static org.apache.cassandra.config.CassandraRelevantProperties.DTEST_ACCORD_JOURNAL_SANITY_CHECK_ENABLED;
 import static org.apache.cassandra.config.DatabaseDescriptor.getPartitioner;
+import static org.apache.cassandra.service.accord.AccordTask.State.ASSIGNED;
 import static org.apache.cassandra.service.accord.AccordTask.State.CANCELLED;
 import static org.apache.cassandra.service.accord.AccordTask.State.FAILED;
 import static org.apache.cassandra.service.accord.AccordTask.State.FINISHED;
@@ -99,13 +98,14 @@ import static org.apache.cassandra.service.accord.AccordTask.State.SCANNING_RANG
 import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_LOAD;
 import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_RUN;
 import static org.apache.cassandra.service.accord.AccordTask.State.WAITING_TO_SCAN_RANGES;
+import static org.apache.cassandra.service.accord.debug.DebugExecution.DEBUG_EXECUTION;
+import static org.apache.cassandra.service.accord.debug.DebugExecution.DebugTask.SANITY_CHECK;
 import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 
 public abstract class AccordTask<R> extends SubmittableTask implements Function<SafeCommandStore, R>, Cancellable, DebuggableTask
 {
     private static final Logger logger = LoggerFactory.getLogger(AccordTask.class);
     private static final NoSpamLogger noSpamLogger = NoSpamLogger.getLogger(logger, 1, TimeUnit.MINUTES);
-    private static final boolean SANITY_CHECK = DTEST_ACCORD_JOURNAL_SANITY_CHECK_ENABLED.getBoolean();
 
     static class ForFunction<R> extends AccordTask<R>
     {
@@ -161,11 +161,12 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
         WAITING_TO_LOAD(INITIALIZED, SCANNING_RANGES),
         LOADING(INITIALIZED, SCANNING_RANGES, WAITING_TO_LOAD),
         WAITING_TO_RUN(INITIALIZED, SCANNING_RANGES, WAITING_TO_LOAD, LOADING),
-        RUNNING(WAITING_TO_RUN),
+        ASSIGNED(WAITING_TO_RUN),
+        RUNNING(ASSIGNED),
         PERSISTING(RUNNING),
         FINISHED(RUNNING, PERSISTING),
-        CANCELLED(WAITING_TO_SCAN_RANGES, SCANNING_RANGES, WAITING_TO_LOAD, LOADING, WAITING_TO_RUN),
-        FAILED(WAITING_TO_SCAN_RANGES, SCANNING_RANGES, WAITING_TO_LOAD, LOADING, WAITING_TO_RUN, RUNNING, PERSISTING);
+        CANCELLED(WAITING_TO_SCAN_RANGES, SCANNING_RANGES, WAITING_TO_LOAD, LOADING, WAITING_TO_RUN, ASSIGNED),
+        FAILED(WAITING_TO_SCAN_RANGES, SCANNING_RANGES, WAITING_TO_LOAD, LOADING, WAITING_TO_RUN, ASSIGNED, RUNNING, PERSISTING);
 
         private final int permittedFrom;
 
@@ -218,8 +219,6 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     @Nullable private TaskQueue queued;
 
     private BiConsumer<? super R, Throwable> callback;
-    private List<Command> sanityCheck;
-    public long createdAt = nanoTime(), waitingToRunAt, runningAt, completedAt;
 
     public AccordTask(@Nonnull AccordCommandStore commandStore, PreLoadContext preLoadContext)
     {
@@ -311,23 +310,6 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
         {
             Invariants.require(rangeScanner == null || rangeScanner.scanned);
             Invariants.require(loading == null && waitingToLoad == null, "WAITING_TO_RUN => no loading or waiting; found %s", this, AccordTask::toDescription);
-            waitingToRunAt = nanoTime();
-            commandStore.executor().elapsedPreparingToRun.increment(waitingToRunAt - createdAt, runningAt);
-        }
-        else if (state == RUNNING)
-        {
-            runningAt = nanoTime();
-            if (waitingToRunAt == 0)
-            {
-                waitingToRunAt = runningAt;
-                commandStore.executor().elapsedPreparingToRun.increment(waitingToRunAt - createdAt, runningAt);
-            }
-            commandStore.executor().elapsedWaitingToRun.increment(runningAt - waitingToRunAt, runningAt);
-            commandStore.executor().keys.increment(commandsForKey == null ? 0 : commandsForKey.size(), runningAt);
-        }
-        else if (state.isExecuted() && completedAt == 0)
-        {
-            completedAt = nanoTime();
         }
     }
 
@@ -345,15 +327,34 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
             @Override
             protected Cancellable start(BiConsumer<? super R, Throwable> callback)
             {
-                Invariants.require(AccordTask.this.callback == null);
-                AccordTask.this.callback = callback;
-                commandStore.tryPreSetup(AccordTask.this);
+                preSetup(callback);
                 commandStore.executor().submit(AccordTask.this);
                 return AccordTask.this;
             }
         };
     }
 
+    public AsyncChain<R> priorityChain()
+    {
+        return new AsyncChains.Head<>()
+        {
+            @Override
+            protected Cancellable start(BiConsumer<? super R, Throwable> callback)
+            {
+                preSetup(callback);
+                commandStore.executor().submitPriority(AccordTask.this);
+                return AccordTask.this;
+            }
+        };
+    }
+
+    private void preSetup(BiConsumer<? super R, Throwable> callback)
+    {
+        Invariants.require(this.callback == null);
+        this.callback = callback;
+        commandStore.tryPreSetup(this);
+    }
+
     // to be invoked only by the CommandStore owning thread, to take references to objects already in use by the current execution
     public void presetup(AccordTask<?> parent)
     {
@@ -628,22 +629,21 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     {
         if (SANITY_CHECK)
         {
-            if (sanityCheck == null)
-                sanityCheck = new ArrayList<>(commands.size());
-            sanityCheck.add(safeCommand.current());
+            if (debug.sanityCheck == null)
+                debug.sanityCheck = new ArrayList<>(commands.size());
+            debug.sanityCheck.add(safeCommand.current());
         }
     }
 
     private void save(List<Journal.CommandUpdate> diffs, Runnable onFlush)
     {
-        if (sanityCheck != null)
+        if (SANITY_CHECK && debug.sanityCheck != null)
         {
-            Invariants.require(SANITY_CHECK);
             Condition condition = Condition.newOneTimeCondition();
             this.commandStore.appendCommands(diffs, condition::signal);
             condition.awaitUninterruptibly();
 
-            for (Command check : sanityCheck)
+            for (Command check : debug.sanityCheck)
                 this.commandStore.sanityCheckCommand(commandStore.unsafeGetRedundantBefore(), check);
 
             if (onFlush != null) onFlush.run();
@@ -655,9 +655,9 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     }
 
     @Override
-    protected void preRunExclusive()
+    protected void preRunExclusive(Thread assigned)
     {
-        state(RUNNING);
+        state(ASSIGNED);
         queued = null;
         if (rangeScanner != null)
         {
@@ -673,6 +673,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     @Override
     public void runInternal()
     {
+        runningAt = nanoTime();
         logger.trace("Running {} with state {}", this, state);
         AccordSafeCommandStore safeStore = null;
         try (Closeable close = locals.get())
@@ -680,8 +681,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
             if (Tracing.isTracing())
                 Tracing.trace(preLoadContext.describe());
 
-            if (state != RUNNING)
-                throw illegalState("Unexpected state " + toDescription());
+            state(RUNNING);
 
             safeStore = commandStore.begin(this, commandsForRanges);
             R result = apply(safeStore);
@@ -717,6 +717,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
 
             commandStore.complete(safeStore);
             safeStore = null;
+            if (DEBUG_EXECUTION) debug.onRunComplete();
             if (!flush)
                 finish(result, null);
         }
@@ -729,10 +730,6 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
             }
             throw t;
         }
-        finally
-        {
-            logger.trace("Exiting {}", this);
-        }
     }
 
     public void fail(Throwable throwable)
@@ -742,8 +739,8 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
 
         try
         {
-            commandStore.agent().onException(throwable);
             state(FAILED);
+            commandStore.agent().onException(throwable);
         }
         finally
         {
@@ -757,17 +754,16 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
         fail(throwable);
     }
 
-    protected void cleanupExclusive()
+    @Override
+    protected void cleanupExclusive(AccordExecutor executor)
     {
+        super.cleanupExclusive(executor);
         Invariants.expect(state.isExecuted());
         releaseResources(commandStore.cachesExclusive());
-        if (runningAt != 0)
-        {
-            commandStore.executor().elapsedRunning.increment(completedAt - runningAt, completedAt);
-        }
+        executor.keys.increment(commandsForKey == null ? 0 : commandsForKey.size(), runningAt);
         if (histogramBuffer != null)
         {
-            histogramBuffer.flush(completedAt);
+            histogramBuffer.flush(cleanupAt);
             histogramBuffer = null;
         }
     }
@@ -786,16 +782,21 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     @Override
     public void cancel()
     {
-        commandStore.executor().cancel(this);
+        if (!state.isComplete())
+            commandStore.executor().cancel(this);
     }
 
-    public void cancelExclusive()
+    void cancelExclusive()
     {
+        logger.info("Cancelling {}", preLoadContext);
         state(CANCELLED);
         if (rangeScanner != null)
             rangeScanner.cancelled = true;
         if (callback != null)
-            commandStore.executor().submit(() -> callback.accept(null, new CancellationException()));
+        {
+            if (commandStore.executor().isInLoop()) callback.accept(null, new CancellationException());
+            else commandStore.executor().submit(() -> callback.accept(null, new CancellationException()));
+        }
     }
 
     void cancelExclusive(AccordExecutor owner)
@@ -1064,7 +1065,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
     public class RangeTxnScanner extends AccordExecutor.AbstractIOTask
     {
         final Map<Timestamp, Summary> summaries = new HashMap<>();
-        final Map<Timestamp, Summary> mutexSummaries = Collections.synchronizedMap(summaries);
+        final Map<Timestamp, Summary> guardedSummaries = Collections.synchronizedMap(summaries);
 
         RangeIndex.Loader loader;
         boolean scanned;
@@ -1074,7 +1075,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
 
         protected void runInternal()
         {
-            loader.load(mutexSummaries, () -> cancelled);
+            loader.load(guardedSummaries, () -> cancelled);
         }
 
         PreLoadContext preLoadContext()
@@ -1105,7 +1106,7 @@ public abstract class AccordTask<R> extends SubmittableTask implements Function<
         void startInternal(Caches caches)
         {
             loader = commandStore.rangeIndex().loader(preLoadContext.primaryTxnId(), preLoadContext.executeAt(), preLoadContext.loadKeysFor(), preLoadContext.keys());
-            loader.loadExclusive(mutexSummaries, caches);
+            loader.loadExclusive(guardedSummaries, caches);
         }
 
         public void scannedExclusive()

src/java/org/apache/cassandra/service/accord/AccordVerbHandler.java

@@ -23,14 +23,21 @@ import java.util.concurrent.TimeUnit;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import accord.api.Tracing;
+import accord.local.MapReduceCommandStores;
 import accord.local.Node;
+import accord.messages.AbstractRequest;
 import accord.messages.Request;
+import accord.utils.Invariants;
 
 import org.apache.cassandra.net.IVerbHandler;
 import org.apache.cassandra.net.Message;
+import org.apache.cassandra.service.accord.debug.AccordRemoteTracing;
 import org.apache.cassandra.service.accord.topology.AccordEndpointMapper;
 import org.apache.cassandra.utils.NoSpamLogger;
 
+import static org.apache.cassandra.net.ParamType.ACCORD_TRACING;
+
 public class AccordVerbHandler<T extends Request> implements IVerbHandler<T>
 {
     private static final Logger logger = LoggerFactory.getLogger(AccordVerbHandler.class);
@@ -51,6 +58,12 @@ public class AccordVerbHandler<T extends Request> implements IVerbHandler<T>
         logger.trace("Receiving {} from {}", message.payload, message.from());
 
         T request = message.payload;
+        Tracing tracing = (Tracing) message.header.params().get(ACCORD_TRACING);
+        if (tracing != null && request instanceof AbstractRequest<?, ?>)
+        {
+            Invariants.require(tracing instanceof AccordRemoteTracing);
+            ((MapReduceCommandStores<?, ?>) request).setTracing(tracing);
+        }
 
         /*
          * TODO (desired): messages are retained on heap until the node catches up to waitForEpoch,

src/java/org/apache/cassandra/service/accord/RangeIndex.java

@@ -27,6 +27,8 @@ import accord.local.Command;
 import accord.local.CommandSummaries;
 import accord.local.LoadKeysFor;
 import accord.local.MaxDecidedRX;
+import accord.local.MinimalCommand;
+import accord.local.MinimalCommand.MinimalWithDeps;
 import accord.local.RedundantBefore;
 import accord.primitives.Ranges;
 import accord.primitives.Routable;
@@ -66,13 +68,13 @@ public interface RangeIndex
         {
             if (loadKeysFor != RECOVERY)
             {
-                Command.Minimal cmd = commandStore().loadMinimal(txnId);
+                MinimalCommand cmd = commandStore().loadMinimal(txnId);
                 if (cmd != null)
                     return ifRelevant(cmd);
             }
             else
             {
-                Command.MinimalWithDeps cmd = commandStore().loadMinimalWithDeps(txnId);
+                MinimalWithDeps cmd = commandStore().loadMinimalWithDeps(txnId);
                 if (cmd != null)
                     return ifRelevant(cmd);
             }

src/java/org/apache/cassandra/service/accord/TokenRange.java

@@ -22,7 +22,6 @@ import java.io.IOException;
 
 import com.google.common.annotations.VisibleForTesting;
 
-import accord.api.ProtocolModifiers.RangeSpec;
 import accord.api.RoutingKey;
 import accord.primitives.Range;
 import accord.utils.Invariants;
@@ -38,12 +37,14 @@ import org.apache.cassandra.schema.TableId;
 import org.apache.cassandra.service.accord.api.TokenKey;
 import org.apache.cassandra.utils.ObjectSizes;
 
+import static accord.api.ProtocolModifiers.isRangeEndInclusive;
+
 public class TokenRange extends Range
 {
     public static final long EMPTY_SIZE = ObjectSizes.measure(new TokenRange(TokenKey.min(TableId.fromLong(0), Murmur3Partitioner.instance), TokenKey.max(TableId.fromLong(0), Murmur3Partitioner.instance)));
     static
     {
-        Invariants.require(RangeSpec.isEndInclusive());
+        Invariants.require(isRangeEndInclusive());
     }
 
     // Don't make this public use create or createUnsafe

src/java/org/apache/cassandra/service/accord/api/AccordAgent.java

@@ -35,6 +35,7 @@ import accord.api.CoordinatorEventListener;
 import accord.api.OwnershipEventListener;
 import accord.api.ProgressLog.BlockedUntil;
 import accord.api.ReplicaEventListener;
+import accord.api.Result;
 import accord.api.RoutingKey;
 import accord.api.Tracing;
 import accord.coordinate.Coordination;
@@ -47,12 +48,11 @@ import accord.local.Node;
 import accord.local.SafeCommand;
 import accord.local.SafeCommandStore;
 import accord.local.TimeService;
-import accord.messages.ReplyContext;
+import accord.messages.MessageType;
 import accord.primitives.Keys;
 import accord.primitives.Participants;
 import accord.primitives.Ranges;
 import accord.primitives.Routable;
-import accord.primitives.Status;
 import accord.primitives.Timestamp;
 import accord.primitives.Txn;
 import accord.primitives.Txn.Kind;
@@ -68,19 +68,19 @@ import accord.utils.async.AsyncChain;
 import accord.utils.async.AsyncChains;
 import accord.utils.async.Cancellable;
 
-import org.apache.cassandra.config.AccordSpec;
+import org.apache.cassandra.config.AccordConfig;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.db.SystemKeyspace;
 import org.apache.cassandra.exceptions.RequestTimeoutException;
 import org.apache.cassandra.metrics.AccordReplicaMetrics;
 import org.apache.cassandra.metrics.AccordSystemMetrics;
-import org.apache.cassandra.net.ResponseContext;
 import org.apache.cassandra.service.RetryStrategy;
 import org.apache.cassandra.service.accord.AccordService;
 import org.apache.cassandra.service.accord.debug.AccordTracing;
 import org.apache.cassandra.service.accord.serializers.TableMetadatasAndKeys;
 import org.apache.cassandra.service.accord.txn.TxnQuery;
 import org.apache.cassandra.service.accord.txn.TxnRead;
+import org.apache.cassandra.service.accord.txn.TxnResult;
 import org.apache.cassandra.utils.Clock;
 import org.apache.cassandra.utils.JVMStabilityInspector;
 import org.apache.cassandra.utils.NoSpamLogger;
@@ -95,6 +95,8 @@ import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordScheduleDurabilityTxnIdLag;
 import static org.apache.cassandra.config.DatabaseDescriptor.getReadRpcTimeout;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.expireEpochWait;
+import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.expireSyncPoint;
+import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.expireTxn;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.fetch;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.recover;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.retryBootstrap;
@@ -104,7 +106,7 @@ import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.retry
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.retrySyncPoint;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.slowRead;
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.slowTxnPreaccept;
-import static org.apache.cassandra.utils.Clock.Global.nanoTime;
+import static org.apache.cassandra.service.accord.txn.TxnResult.Kind.txn_data;
 
 // TODO (expected): merge with AccordService
 public class AccordAgent implements Agent, OwnershipEventListener
@@ -124,7 +126,7 @@ public class AccordAgent implements Agent, OwnershipEventListener
     private final AccordTracing tracing = new AccordTracing();
     private final RandomSource random = new DefaultRandom();
     protected Node.Id self;
-    protected AccordSpec config;
+    protected AccordConfig config;
 
     public AccordAgent()
     {
@@ -248,27 +250,19 @@ public class AccordAgent implements Agent, OwnershipEventListener
     @Override
     public long cfkHlcPruneDelta()
     {
-        return SECONDS.toMicros(10L);
+        return config.commands_for_key_prune_delta.to(MICROSECONDS);
     }
 
     @Override
     public int cfkPruneInterval()
     {
-        return 32;
+        return config.commands_for_key_prune_interval;
     }
 
-    // TODO (expected): we probably want additional configuration here
     @Override
     public long maxConflictsHlcPruneDelta()
     {
-        return SECONDS.toMicros(1);
-    }
-
-    // TODO (expected): I don't think we even need this - just prune each time we have doubled in size
-    @Override
-    public long maxConflictsPruneInterval()
-    {
-        return 1024;
+        return config.max_conflicts_prune_delta.to(MICROSECONDS);
     }
 
     @Override
@@ -456,40 +450,29 @@ public class AccordAgent implements Agent, OwnershipEventListener
     }
 
     @Override
-    public long expiresAt(ReplyContext replyContext, TimeUnit unit)
+    public long selfSlowAt(TxnId txnId, MessageType type, TimeUnit unit)
     {
-        return unit.convert(((ResponseContext)replyContext).expiresAtNanos(), NANOSECONDS);
+        if (type.getClass() == MessageType.StandardMessage.class)
+        {
+            switch ((MessageType.StandardMessage)type)
+            {
+                case PRE_ACCEPT_REQ:
+                    return unit.convert(slowTxnPreaccept.computeWaitUntil(1), unit);
+                case READ_EPHEMERAL_REQ:
+                case READ_REQ:
+                case STABLE_THEN_READ_REQ:
+                    return unit.convert(slowRead.computeWaitUntil(1), NANOSECONDS);
+            }
+        }
+        return -1;
     }
 
     @Override
-    public long selfSlowAt(TxnId txnId, Status.Phase phase, TimeUnit unit)
+    public long selfExpiresAt(TxnId txnId, MessageType type, TimeUnit unit)
     {
-        switch (phase)
-        {
-            default: throw new UnhandledEnum(phase);
-            case PreAccept: return unit.convert(slowTxnPreaccept.computeWaitUntil(1), NANOSECONDS);
-            case Execute:   return unit.convert(slowRead.computeWaitUntil(1), NANOSECONDS);
-        }
-    }
-
-    @Override
-    public long selfExpiresAt(TxnId txnId, Status.Phase phase, TimeUnit unit)
-    {
-        long delayNanos;
-        switch (txnId.kind())
-        {
-            default: throw new UnhandledEnum(txnId.kind());
-            case Write:
-                delayNanos = DatabaseDescriptor.getWriteRpcTimeout(NANOSECONDS);
-                break;
-            case EphemeralRead:
-            case Read:
-                delayNanos = DatabaseDescriptor.getReadRpcTimeout(NANOSECONDS);
-                break;
-            case ExclusiveSyncPoint:
-                delayNanos = DatabaseDescriptor.getAccordRangeSyncPointTimeoutNanos();
-        }
-        return unit.convert(nanoTime() + delayNanos, NANOSECONDS);
+        if (txnId == null || txnId.equals(TxnId.NONE))
+            return DatabaseDescriptor.getRpcTimeout(unit);
+        return unit.convert((txnId.isSyncPoint() ? expireSyncPoint : expireTxn).computeWaitUntil(1), NANOSECONDS);
     }
 
     @Override
@@ -516,4 +499,10 @@ public class AccordAgent implements Agent, OwnershipEventListener
     {
         return node.now() - (100 + getAccordScheduleDurabilityTxnIdLag(MICROSECONDS));
     }
+
+    @Override
+    public boolean reportRemoteSuccess(Result success)
+    {
+        return success instanceof TxnResult && ((TxnResult) success).kind() == txn_data;
+    }
 }

src/java/org/apache/cassandra/service/accord/api/AccordScheduler.java

@@ -19,6 +19,7 @@
 package org.apache.cassandra.service.accord.api;
 
 import java.util.List;
+import java.util.concurrent.Executor;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
@@ -29,7 +30,7 @@ import org.apache.cassandra.concurrent.ExecutorFactory;
 import org.apache.cassandra.concurrent.ScheduledExecutorPlus;
 import org.apache.cassandra.concurrent.Shutdownable;
 
-public class AccordScheduler implements Scheduler, Shutdownable
+public class AccordScheduler implements Scheduler, Shutdownable, Executor
 {
     private final ScheduledExecutorPlus scheduledExecutor = ExecutorFactory.Global.executorFactory().scheduled(false, "AccordScheduled");
 
@@ -112,4 +113,10 @@ public class AccordScheduler implements Scheduler, Shutdownable
     {
         return scheduledExecutor.awaitTermination(timeout, units);
     }
+
+    @Override
+    public void execute(Runnable command)
+    {
+        scheduledExecutor.execute(command);
+    }
 }

src/java/org/apache/cassandra/service/accord/api/AccordTimeService.java

@@ -22,13 +22,22 @@ import java.util.concurrent.TimeUnit;
 
 import accord.local.TimeService;
 
-import org.apache.cassandra.utils.Clock;
+import org.apache.cassandra.config.DatabaseDescriptor;
+import org.apache.cassandra.utils.FBUtilities;
 
 import static java.util.concurrent.TimeUnit.NANOSECONDS;
+import static org.apache.cassandra.utils.Clock.Global.currentTimeMicros;
 import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 
 public class AccordTimeService implements TimeService
 {
+    private static final boolean PRECISE_MICROS;
+    static
+    {
+        Boolean precise = DatabaseDescriptor.getAccord().precise_micros;
+        PRECISE_MICROS = precise == null || precise;
+    }
+
     @Override
     public long now()
     {
@@ -37,7 +46,7 @@ public class AccordTimeService implements TimeService
 
     public static long nowMicros()
     {
-        return TimeUnit.MILLISECONDS.toMicros(Clock.Global.currentTimeMillis());
+        return PRECISE_MICROS ? currentTimeMicros() : FBUtilities.timestampMicros();
     }
 
     @Override

src/java/org/apache/cassandra/service/accord/api/AccordWaitStrategies.java

@@ -22,7 +22,7 @@ import javax.annotation.Nullable;
 
 import accord.primitives.TxnId;
 
-import org.apache.cassandra.config.AccordSpec;
+import org.apache.cassandra.config.AccordConfig;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.config.StringRetryStrategy;
 import org.apache.cassandra.net.Verb;
@@ -84,7 +84,7 @@ public class AccordWaitStrategies
 
     static
     {
-        AccordSpec config = DatabaseDescriptor.getAccord();
+        AccordConfig config = DatabaseDescriptor.getAccord();
         setSlowRead(config.slow_read);
         setSlowTxnPreaccept(config.slow_txn_preaccept);
         setSlowSyncPointPreaccept(config.slow_syncpoint_preaccept);

src/java/org/apache/cassandra/service/accord/debug/AccordRemoteTracing.java

@@ -0,0 +1,385 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.service.accord.debug;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+import accord.api.Tracing;
+import accord.local.CommandStore;
+import accord.primitives.TxnId;
+
+import org.apache.cassandra.db.TypeSizes;
+import org.apache.cassandra.io.IVersionedSerializer;
+import org.apache.cassandra.io.util.DataInputPlus;
+import org.apache.cassandra.io.util.DataOutputPlus;
+import org.apache.cassandra.locator.InetAddressAndPort;
+import org.apache.cassandra.net.IVerbHandler;
+import org.apache.cassandra.net.Message;
+import org.apache.cassandra.net.MessagingService;
+import org.apache.cassandra.net.ParamType;
+import org.apache.cassandra.net.Verb;
+import org.apache.cassandra.service.accord.AccordService;
+import org.apache.cassandra.service.accord.api.AccordAgent;
+import org.apache.cassandra.service.accord.api.AccordTimeService;
+import org.apache.cassandra.service.accord.debug.AccordTracing.TxnEvent;
+import org.apache.cassandra.service.accord.serializers.CommandSerializers;
+import org.apache.cassandra.tcm.ClusterMetadata;
+import org.apache.cassandra.tcm.membership.NodeId;
+
+import static org.apache.cassandra.utils.Clock.Global.nanoTime;
+import static org.apache.cassandra.utils.MonotonicClock.Global.preciseTime;
+
+public class AccordRemoteTracing implements Tracing
+{
+    public static class AccordRemoteTrace {}
+    public static final class AccordTraceOut extends AccordRemoteTrace
+    {
+        final TxnId txnId;
+        final long idMicros;
+        final long receivedAtMicros;
+        final long nanosSinceReceived;
+        final int commandStoreId;
+        final String message;
+
+        AccordTraceOut(TxnId txnId, long idMicros, long receivedAtMicros, long nanosSinceReceived, int commandStoreId, String message)
+        {
+            this.txnId = txnId;
+            this.idMicros = idMicros;
+            this.receivedAtMicros = receivedAtMicros;
+            this.nanosSinceReceived = nanosSinceReceived;
+            this.commandStoreId = commandStoreId;
+            this.message = message;
+        }
+    }
+
+    public static final class AccordTraceIn extends AccordRemoteTrace
+    {
+        final TxnId txnId;
+        final long idMicros;
+        final long atNanos;
+        final int commandStoreId;
+        final String message;
+
+        AccordTraceIn(TxnId txnId, long idMicros, long atNanos, int commandStoreId, String message)
+        {
+            this.txnId = txnId;
+            this.idMicros = idMicros;
+            this.atNanos = atNanos;
+            this.commandStoreId = commandStoreId;
+            this.message = message;
+        }
+    }
+
+    public static final class AccordTracingIn implements Tracing
+    {
+        final TxnId txnId;
+        final long idMicros;
+        final long onWireAtNanos;
+        final List<AccordTraceIn> messages;
+
+        public AccordTracingIn(TxnId txnId, long idMicros, long onWireAtNanos, List<AccordTraceIn> messages)
+        {
+            this.txnId = txnId;
+            this.idMicros = idMicros;
+            this.onWireAtNanos = onWireAtNanos;
+            this.messages = messages;
+        }
+
+        void report(NodeId from)
+        {
+            AccordTracing tracing = ((AccordAgent)AccordService.unsafeInstance().agent()).tracing();
+            long offWireAtNanos = nanoTime();
+            long onWireAtNanos = Math.min(offWireAtNanos - 1, this.onWireAtNanos);
+            int fromId = from == null ? Integer.MAX_VALUE : from.id();
+            for (AccordTraceIn message : messages)
+                tracing.report(message, fromId);
+            tracing.report(new AccordTraceIn(txnId, idMicros, onWireAtNanos, -1, "Reply on wire"), fromId);
+            tracing.report(new AccordTraceIn(txnId, idMicros, offWireAtNanos, -1, "Reply off wire from " + from) , -1);
+        }
+
+        @Override
+        public void trace(CommandStore commandStore, String message)
+        {
+            throw new UnsupportedOperationException();
+        }
+    }
+
+    public static final class AccordTracingOut implements Tracing
+    {
+        final TxnId txnId;
+        final long idMicros;
+        final long receivedAtMicros;
+        final long receivedAtNanos;
+        final List<BufferedMessage> messages;
+
+        AccordTracingOut(TxnId txnId, long idMicros, long receivedAtMicros, long receivedAtNanos, List<BufferedMessage> messages)
+        {
+            this.txnId = txnId;
+            this.idMicros = idMicros;
+            this.receivedAtMicros = receivedAtMicros;
+            this.receivedAtNanos = receivedAtNanos;
+            this.messages = messages;
+        }
+
+        @Override
+        public void trace(CommandStore commandStore, String message)
+        {
+            if (message.length() > 100)
+                message = message.substring(0, 100);
+            messages.add(new BufferedMessage(nanoTime(), commandStore == null ? -1 : commandStore.id(), message));
+        }
+    }
+
+    static class BufferedMessage
+    {
+        final long atNanos;
+        final int commandStoreId;
+        final String message;
+
+        BufferedMessage(long atNanos, int commandStoreId, String message)
+        {
+            this.atNanos = atNanos;
+            this.commandStoreId = commandStoreId;
+            this.message = message;
+        }
+    }
+
+    final TxnId txnId;
+    final long idMicros;
+    private InetAddressAndPort replyTo;
+    final long receivedAtMicros = AccordTimeService.nowMicros();
+    final long receivedAtNanos = nanoTime();
+    List<BufferedMessage> messages = new ArrayList<>();
+
+    private AccordRemoteTracing(TxnId txnId, long idMicros)
+    {
+        this.txnId = txnId;
+        this.idMicros = idMicros;
+    }
+
+    public void setReplyTo(InetAddressAndPort from)
+    {
+        replyTo = from;
+    }
+
+    @Override
+    public synchronized void trace(CommandStore commandStore, String message)
+    {
+        long atNanos = nanoTime();
+        int commandStoreId = commandStore == null ? -1 : commandStore.id();
+        // TODO (expected): make this configurable
+        if (message.length() > 100)
+            message = message.substring(0, 100);
+
+        if (messages == null)
+        {
+            long nanosSinceReceived = atNanos - receivedAtNanos;
+            Message<?> reply = Message.out(Verb.ACCORD_REMOTE_TRACE, new AccordTraceOut(txnId, idMicros, receivedAtMicros, nanosSinceReceived, commandStoreId, message));
+            MessagingService.instance().send(reply, replyTo);
+        }
+        else
+        {
+            messages.add(new BufferedMessage(atNanos, commandStoreId, message));
+        }
+    }
+
+    @Override
+    public synchronized Tracing send()
+    {
+        List<BufferedMessage> messages = this.messages;
+        if (messages == null) messages = new ArrayList<>();
+        else this.messages = null;
+        return new AccordTracingOut(txnId, idMicros, receivedAtMicros, receivedAtNanos, messages);
+    }
+
+    public static final IVerbHandler<AccordRemoteTrace> traceMessageHandler = message -> {
+        AccordTracing tracing = ((AccordAgent)AccordService.unsafeInstance().agent()).tracing();
+        AccordTraceIn trace = (AccordTraceIn) message.payload;
+        NodeId nodeId = ClusterMetadata.current().directory.peerId(message.from());
+        tracing.report(trace, nodeId == null ? Integer.MAX_VALUE : nodeId.id());
+    };
+
+    static final int REPLY_FLAG = 1;
+    public static final IVersionedSerializer<Tracing> tracingSerializer = new IVersionedSerializer<>()
+    {
+        @Override
+        public void serialize(Tracing t, DataOutputPlus out, int version) throws IOException
+        {
+            int flags = 0;
+            if (t.getClass() == TxnEvent.class)
+            {
+                TxnEvent event = (TxnEvent) t;
+                out.writeUnsignedVInt32(flags);
+                CommandSerializers.txnId.serialize(event.txnId(), out);
+                out.writeLong(event.idMicros);
+            }
+            else
+            {
+                AccordTracingOut tracing = (AccordTracingOut) t;
+                flags |= REPLY_FLAG;
+                out.writeUnsignedVInt32(flags);
+                CommandSerializers.txnId.serialize(tracing.txnId, out);
+                out.writeLong(tracing.idMicros);
+                out.writeLong(tracing.receivedAtMicros);
+                out.writeLong(nanoTime() - tracing.receivedAtNanos);
+                out.writeUnsignedVInt32(tracing.messages.size());
+                for (BufferedMessage message : tracing.messages)
+                {
+                    out.writeUnsignedVInt(message.atNanos - tracing.receivedAtNanos);
+                    out.writeUnsignedVInt32(1 + message.commandStoreId);
+                    out.writeUTF(message.message);
+                }
+            }
+        }
+
+        @Override
+        public Tracing deserialize(DataInputPlus in, int version) throws IOException
+        {
+            int flags = in.readUnsignedVInt32();
+            TxnId txnId = CommandSerializers.txnId.deserialize(in);
+            long idMicros = in.readLong();
+            if ((flags & REPLY_FLAG) == 0)
+                return new AccordRemoteTracing(txnId, idMicros);
+
+            long remoteReceivedAtMicros = in.readLong();
+            long remoteReceivedAtNanos = preciseTime.translate().fromMicrosSinceEpoch(remoteReceivedAtMicros);
+            long onWireAtNanos = remoteReceivedAtNanos + in.readLong();
+            int messageCount = in.readUnsignedVInt32();
+            List<AccordTraceIn> messages = new ArrayList<>(messageCount);
+            for (int i = 0 ; i < messageCount ; ++i)
+            {
+                long atNanos = remoteReceivedAtNanos + in.readUnsignedVInt();
+                int commandStoreId = in.readUnsignedVInt32() - 1;
+                String message = in.readUTF();
+                messages.add(new AccordTraceIn(txnId, idMicros, atNanos, commandStoreId, message));
+            }
+            return new AccordTracingIn(txnId, idMicros, onWireAtNanos, messages);
+        }
+
+        @Override
+        public long serializedSize(Tracing t, int version)
+        {
+            int flags = 0;
+            if (t.getClass() == TxnEvent.class)
+            {
+                TxnEvent event = (TxnEvent) t;
+                long size = TypeSizes.sizeofUnsignedVInt(flags);
+                size += CommandSerializers.txnId.serializedSize(event.txnId());
+                size += TypeSizes.LONG_SIZE;
+                return size;
+            }
+            else
+            {
+                AccordTracingOut tracing = (AccordTracingOut) t;
+                flags |= REPLY_FLAG;
+                long size = TypeSizes.sizeofUnsignedVInt(flags);
+                size += CommandSerializers.txnId.serializedSize(tracing.txnId);
+                size += 3 * TypeSizes.LONG_SIZE;
+                size += TypeSizes.sizeofUnsignedVInt(tracing.messages.size());
+                for (BufferedMessage message : tracing.messages)
+                {
+                    size += TypeSizes.sizeofUnsignedVInt(message.atNanos - tracing.receivedAtNanos);
+                    size += TypeSizes.sizeofUnsignedVInt(1 + message.commandStoreId);
+                    size += TypeSizes.sizeof(message.message);
+                }
+                return size;
+            }
+        }
+    };
+
+    public static final IVersionedSerializer<AccordRemoteTrace> traceSerializer = new IVersionedSerializer<>()
+    {
+        @Override
+        public void serialize(AccordRemoteTrace t, DataOutputPlus out, int version) throws IOException
+        {
+            out.writeUnsignedVInt32(0);
+            AccordTraceOut trace = (AccordTraceOut) t;
+            CommandSerializers.txnId.serialize(trace.txnId, out);
+            out.writeUnsignedVInt(trace.idMicros);
+            out.writeUnsignedVInt(trace.receivedAtMicros);
+            out.writeUnsignedVInt(trace.nanosSinceReceived);
+            out.writeUnsignedVInt32(1 + trace.commandStoreId);
+            out.writeUTF(trace.message);
+        }
+
+        @Override
+        public AccordRemoteTrace deserialize(DataInputPlus in, int version) throws IOException
+        {
+            in.readUnsignedVInt32();
+            TxnId txnId = CommandSerializers.txnId.deserialize(in);
+            long idMicros = in.readUnsignedVInt();
+            long receivedAtMicros = in.readUnsignedVInt();
+            long nanosSinceReceived = in.readUnsignedVInt();
+            long atNanos = nanosSinceReceived + preciseTime.translate().fromMicrosSinceEpoch(receivedAtMicros);
+            int commandStoreId = 1 + in.readUnsignedVInt32();
+            String message = in.readUTF();
+            return new AccordTraceIn(txnId, idMicros, atNanos, commandStoreId, message);
+        }
+
+        @Override
+        public long serializedSize(AccordRemoteTrace t, int version)
+        {
+            AccordTraceOut trace = (AccordTraceOut) t;
+            long size = TypeSizes.sizeofUnsignedVInt(0);
+            size += CommandSerializers.txnId.serializedSize(trace.txnId);
+            size += TypeSizes.sizeofUnsignedVInt(trace.idMicros);
+            size += TypeSizes.sizeofUnsignedVInt(trace.receivedAtMicros);
+            size += TypeSizes.sizeofUnsignedVInt(trace.nanosSinceReceived);
+            size += TypeSizes.sizeofUnsignedVInt(1 + trace.commandStoreId);
+            size += TypeSizes.sizeof(trace.message);
+            return size;
+        }
+    };
+
+    public static void traceOnWire(Message.Header out, InetAddressAndPort to)
+    {
+        Object obj = out.params().get(ParamType.ACCORD_TRACING);
+        if (obj == null)
+            return;
+
+        Tracing tracing = (Tracing) obj;
+        if (tracing instanceof TxnEvent)
+        {
+            NodeId id = ClusterMetadata.current().directory.peerId(to);
+            tracing.trace(null, "Request on wire to %s", id);
+        }
+    }
+
+    public static void traceOffWire(Message.Header in)
+    {
+        Object obj = in.params().get(ParamType.ACCORD_TRACING);
+        if (obj == null)
+            return;
+
+        if (obj instanceof AccordRemoteTracing)
+        {
+            AccordRemoteTracing tracing = (AccordRemoteTracing) obj;
+            tracing.setReplyTo(in.from);
+            tracing.trace(null, "Request off wire");
+        }
+        else
+        {
+            AccordTracingIn tracing = (AccordTracingIn) obj;
+            NodeId id = ClusterMetadata.current().directory.peerId(in.from);
+            tracing.report(id);
+        }
+    }
+}

src/java/org/apache/cassandra/service/accord/debug/AccordTracing.java

@@ -22,8 +22,11 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.EnumMap;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.TreeSet;
+import java.util.WeakHashMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.ThreadLocalRandom;
@@ -46,6 +49,7 @@ import accord.coordinate.Coordination.CoordinationKind;
 import accord.local.CommandStore;
 import accord.local.Node;
 import accord.primitives.Participants;
+import accord.primitives.Routables;
 import accord.primitives.TxnId;
 import accord.utils.Invariants;
 import accord.utils.SortedListMap;
@@ -54,11 +58,14 @@ import accord.utils.UnhandledEnum;
 
 import org.apache.cassandra.metrics.AccordCoordinatorMetrics;
 import org.apache.cassandra.service.ClientWarn;
+import org.apache.cassandra.service.accord.debug.AccordRemoteTracing.AccordTraceIn;
 import org.apache.cassandra.utils.Clock;
 import org.apache.cassandra.utils.NoSpamLogger;
 
 import static org.apache.cassandra.service.accord.debug.AccordTracing.BucketMode.LEAKY;
 import static org.apache.cassandra.service.accord.debug.AccordTracing.BucketMode.SAMPLE;
+import static org.apache.cassandra.service.accord.debug.AccordTracing.BucketMode.SLOWEST;
+import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 
 public class AccordTracing extends AccordCoordinatorMetrics.Listener
 {
@@ -68,16 +75,20 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
 
     public enum BucketMode
     {
-        LEAKY, SAMPLE, RING;
+        LEAKY, SAMPLE, RING, SLOWEST;
 
         int position(int permits, int total)
         {
             switch (this)
             {
                 default: throw UnhandledEnum.unknown(this);
-                case LEAKY: return Integer.MAX_VALUE;
-                case RING: return total % permits;
-                case SAMPLE: return ThreadLocalRandom.current().nextInt(total);
+                case SLOWEST:
+                case LEAKY:
+                    return Integer.MAX_VALUE;
+                case RING:
+                    return total % permits;
+                case SAMPLE:
+                    return ThreadLocalRandom.current().nextInt(total);
             }
         }
     }
@@ -90,14 +101,26 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
     public static class Message
     {
         public final long atNanos;
+        public final int nodeId;
         public final int commandStoreId;
         public final String message;
 
-        Message(int commandStoreId, String message, long atLeastNanos)
+        static Message withAtLeastNanos(int nodeId, int commandStoreId, String message, long atLeastNanos)
         {
+            return new Message(nodeId, commandStoreId, message, Math.max(atLeastNanos, nanoTime()));
+        }
+
+        static Message withExactNanos(int nodeId, int commandStoreId, String message, long atNanos)
+        {
+            return new Message(nodeId, commandStoreId, message, atNanos);
+        }
+
+        private Message(int nodeId, int commandStoreId, String message, long atNanos)
+        {
+            this.atNanos = atNanos;
+            this.nodeId = nodeId;
             this.commandStoreId = commandStoreId;
             this.message = message;
-            this.atNanos = Math.max(atLeastNanos, Clock.Global.nanoTime());
         }
 
         @Override
@@ -109,25 +132,29 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
 
     public static class TxnEvent implements Tracing, Comparable<TxnEvent>
     {
+        public final TxnEvents parent;
         public final CoordinationKind kind;
         public final long idMicros = uniqueNowMicros();
-        public final long atNanos = Clock.Global.nanoTime();
+        public final long atNanos = nanoTime();
         final List<Message> messages = new ArrayList<>();
         int index = -1, subIndex = -1;
+        long elapsedNanos;
 
-        public TxnEvent(CoordinationKind kind)
+        public TxnEvent(TxnEvents parent, CoordinationKind kind)
         {
+            this.parent = parent;
             this.kind = kind;
         }
 
         @Override
-        public void trace(CommandStore commandStore, String s)
+        public synchronized void trace(CommandStore commandStore, String s)
         {
             long prevNanos = messages.isEmpty() ? 0 : messages.get(messages.size() - 1).atNanos;
             int id = commandStore == null ? -1 : commandStore.id();
+            // TODO (expected): make this configurable
             if (s.length() > 1000)
                 s = s.substring(0, 1000);
-            messages.add(new Message(id, s, prevNanos + 1));
+            messages.add(Message.withAtLeastNanos(-1, id, s, prevNanos + 1));
         }
 
         @Override
@@ -140,6 +167,79 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         {
             return Collections.unmodifiableList(messages);
         }
+
+        @Override
+        public Tracing send()
+        {
+            return this;
+        }
+
+        public long elapsedNanos()
+        {
+            return elapsedNanos;
+        }
+
+        public long doneAtNanos()
+        {
+            return atNanos + elapsedNanos;
+        }
+
+        public long doneAtMicros()
+        {
+            return idMicros + elapsedNanos/1000;
+        }
+
+        @Override
+        public void done()
+        {
+            elapsedNanos = nanoTime() - atNanos;
+            TracePatternState pattern = parent.owner;
+            if (pattern != null && pattern.bucketMode == SLOWEST && !pattern.updateSlowest(parent, elapsedNanos))
+                return;
+
+            if (index < 0)
+            {
+                parent.parent.txnIdMap.compute(parent.txnId, (id, events) -> {
+                    TracePatternState owner = parent.owner;
+                    if (owner != null)
+                    {
+                        synchronized (owner)
+                        {
+                            owner.detachedEvent.remove(idMicros);
+                        }
+                    }
+
+                    if (events != parent)
+                        return events;
+
+                    TxnEventsList subList = events.subLists.get(kind);
+                    TxnEventsList list = subList == null ? events : subList;
+                    if (list.size < events.bucketSize && (subList == null || subList.size < events.bucketSubSize))
+                    {
+                        if (events.parent.globalCount.admit())
+                            events.add(kind, subList, this);
+                        return events;
+                    }
+
+                    for (int i = 0 ; i < list.size ; ++i)
+                    {
+                        TxnEvent event = list.get(i);
+                        if (event.elapsedNanos < elapsedNanos)
+                        {
+                            events.remove(event.index);
+                            events.add(kind, subList, this);
+                            break;
+                        }
+                    }
+                    return events;
+                });
+            }
+        }
+
+        public TxnId txnId()
+        {
+            return parent.txnId;
+        }
     }
 
     private static class TxnEventsList
@@ -189,11 +289,28 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
     public static class TxnEvents extends TxnEventsList
     {
         private final EnumMap<CoordinationKind, TxnEventsList> subLists = new EnumMap<>(CoordinationKind.class);
+
+        final AccordTracing parent;
+        final TxnId txnId;
+
         private CoordinationKinds traceEvents = CoordinationKinds.ALL;
         private BucketMode mode = LEAKY;
         private TracePatternState owner;
         private int bucketSize, bucketSubSize;
         private float chance = 1.0f;
+        private boolean detached;
+
+        public TxnEvents(AccordTracing parent, TxnId txnId)
+        {
+            this.parent = parent;
+            this.txnId = txnId;
+        }
+
+        private TxnEvents setDetached()
+        {
+            detached = true;
+            return this;
+        }
 
         void remove(int index)
         {
@@ -287,7 +404,11 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
                 subList.incrementSeen();
                 int position = mode.position(bucketSubSize, subList.bucketSeen);
                 if (position >= bucketSubSize)
+                {
+                    if (mode == SLOWEST)
+                        return new TxnEvent(this, kind);
                     return null;
+                }
 
                 remove(subList.get(position).index);
             }
@@ -296,13 +417,17 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
                 incrementSeen();
                 int position = mode.position(bucketSize, bucketSeen);
                 if (position >= bucketSize)
+                {
+                    if (mode == SLOWEST)
+                        return new TxnEvent(this, kind);
                     return null;
+                }
 
                 remove(position);
             }
             else
             {
-                if (!globalCount.admit())
+                if (globalCount != null && !globalCount.admit())
                     return null;
             }
 
@@ -320,16 +445,21 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         }
 
         private TxnEvent newTrace(CoordinationKind kind, TxnEventsList subList)
+        {
+            TxnEvent event = new TxnEvent(this, kind);
+            add(kind, subList, event);
+            return event;
+        }
+
+        private void add(CoordinationKind kind, TxnEventsList subList, TxnEvent event)
         {
             if (subList == null)
                 subLists.put(kind, subList = new TxnEventsList());
 
-            TxnEvent event = new TxnEvent(kind);
             event.subIndex = subList.size;
             subList.addInternal(event);
             event.index = size;
             addInternal(event);
-            return event;
         }
 
         public boolean hasOwner()
@@ -370,7 +500,12 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         public void forEach(Consumer<TxnEvent> forEach)
         {
             for (int i = 0 ; i < size ; ++i)
-                forEach.accept(events[i]);
+            {
+                synchronized (events[i])
+                {
+                    forEach.accept(events[i]);
+                }
+            }
         }
     }
 
@@ -423,7 +558,7 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             return new TracePattern(kinds, intersects, traceNew, traceFailures, chance);
         }
 
-        boolean matches(TxnId txnId, @Nullable Participants<?> participants, CoordinationKind kind, NewOrFailure newOrFailure)
+        boolean matches(TxnId txnId, @Nullable Routables<?> participants, CoordinationKind kind, NewOrFailure newOrFailure)
         {
             if (kinds != null && !kinds.matches(txnId))
                 return false;
@@ -432,13 +567,33 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             if (testKind == null || !testKind.contains(kind))
                 return false;
 
-            if (intersects != null && (participants == null || !intersects.intersects(participants)))
+            if (intersects != null && (participants == null || !participants.intersects(intersects)))
                 return false;
 
             return chance >= 1.0f || ThreadLocalRandom.current().nextFloat() <= chance;
         }
     }
 
+    private static class SlowestTxnId implements Comparable<SlowestTxnId>
+    {
+        final TxnId txnId;
+        long elapsedNanos;
+
+        private SlowestTxnId(TxnId txnId, long elapsedNanos)
+        {
+            this.txnId = txnId;
+            this.elapsedNanos = elapsedNanos;
+        }
+
+        @Override
+        public int compareTo(SlowestTxnId that)
+        {
+            int c = Long.compare(this.elapsedNanos, that.elapsedNanos);
+            if (c == 0) c = this.txnId.compareTo(that.txnId);
+            return c;
+        }
+    }
+
     public class TracePatternState
     {
         final int id;
@@ -452,6 +607,10 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         private CoordinationKinds traceEvents = new CoordinationKinds(false, 0);
 
         private final List<TxnId> txnIds = new ArrayList<>();
+        private final WeakHashMap<TxnId, TxnEvents> detachedEvents = new WeakHashMap<>();
+        private final WeakHashMap<Long, TxnEvent> detachedEvent = new WeakHashMap<>();
+        private final Map<TxnId, SlowestTxnId> slowestLookup = new HashMap<>();
+        private final TreeSet<SlowestTxnId> slowest = new TreeSet<>();
 
         public TracePatternState(int id)
         {
@@ -479,7 +638,7 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             return txnIds.get(index);
         }
 
-        TxnEvents maybeAdd(TxnId txnId, @Nullable Participants<?> participants, CoordinationKind kind, NewOrFailure newOrFailure)
+        TxnEvents maybeAdd(TxnId txnId, @Nullable Routables<?> participants, CoordinationKind kind, NewOrFailure newOrFailure)
         {
             if (!pattern.matches(txnId, participants, kind, newOrFailure))
                 return null;
@@ -487,37 +646,160 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             return maybeAdd(txnId);
         }
 
-        private synchronized TxnEvents maybeAdd(TxnId txnId)
+        private TxnEvents maybeAdd(TxnId txnId)
         {
-            if (bucketSize == 0)
-                return null;
-
-            if (++bucketSeen < 0)
-                bucketSeen = Integer.MAX_VALUE;
-
-            if (bucketSize > txnIds.size())
+            class MaybeAdd implements BiFunction<TxnId, TxnEvents, TxnEvents>
             {
-                TxnEvents added = trace(txnId);
-                if (added != null)
-                    txnIds.add(txnId);
-                return added;
+                TxnEvents result;
+                TxnId untrace;
+
+                @Override
+                public TxnEvents apply(TxnId txnId, TxnEvents in)
+                {
+                    synchronized (TracePatternState.this)
+                    {
+                        if (in != null)
+                            return null; // already tracing
+
+                        if (bucketSize == 0)
+                            return null;
+
+                        if (++bucketSeen < 0)
+                            bucketSeen = Integer.MAX_VALUE;
+
+                        if (bucketSize > txnIds.size())
+                        {
+                            result = newEvents(txnId);
+                            txnIds.add(txnId);
+                            if (bucketMode == SLOWEST)
+                                initSlowest(result);
+                            return result;
+                        }
+
+                        int position = bucketMode.position(bucketSize, bucketSeen);
+                        if (position >= bucketSize)
+                        {
+                            if (bucketMode == SLOWEST)
+                                result = detachedEvents.computeIfAbsent(txnId, id -> newEvents(id).setDetached());
+                            return null;
+                        }
+
+                        result = newEvents(txnId);
+                        untrace = txnIds.get(position);
+                        txnIds.set(position, txnId);
+                        if (bucketMode == SLOWEST)
+                            initSlowest(result);
+                        return result;
+                    }
+                }
             }
 
-            int position = bucketMode.position(bucketSize, bucketSeen);
-
-            if (position >= bucketSize)
-                return null;
-
-            TxnEvents added = trace(txnId);
-            if (added == null)
-                return null;
-
-            untrace(txnIds.get(position));
-            txnIds.set(position, txnId);
-            return added;
+            MaybeAdd maybeAdd = new MaybeAdd();
+            txnIdMap.compute(txnId, maybeAdd);
+            if (maybeAdd.untrace != null)
+                untrace(maybeAdd.untrace);
+            return maybeAdd.result;
         }
 
-        private synchronized void untrace(TxnId txnId)
+        private boolean initSlowest(TxnEvents events)
+        {
+            SlowestTxnId entry = new SlowestTxnId(events.txnId, Long.MAX_VALUE);
+            if (null != slowestLookup.putIfAbsent(events.txnId, entry))
+                return false;
+            slowest.add(entry);
+            return true;
+        }
+
+        private boolean updateSlowest(TxnEvents events, long elapsedNanos)
+        {
+            synchronized (this)
+            {
+                SlowestTxnId entry = slowestLookup.get(events.txnId);
+                if (entry != null)
+                {
+                    if (entry.elapsedNanos < elapsedNanos)
+                    {
+                        slowest.remove(entry);
+                        entry.elapsedNanos = elapsedNanos;
+                        slowest.add(entry);
+                    }
+                    return true;
+                }
+            }
+
+            class UpdateSlowest implements BiFunction<TxnId, TxnEvents, TxnEvents>
+            {
+                boolean result;
+                TxnId untrace;
+
+                @Override
+                public TxnEvents apply(TxnId txnId, TxnEvents cur)
+                {
+                    if (cur != null && cur != events)
+                        return cur;
+
+                    synchronized (TracePatternState.this)
+                    {
+                        SlowestTxnId entry = slowestLookup.get(events.txnId);
+                        if (entry != null)
+                        {
+                            if (entry.elapsedNanos < elapsedNanos)
+                            {
+                                slowest.remove(entry);
+                                entry.elapsedNanos = elapsedNanos;
+                                slowest.add(entry);
+                            }
+                            result = true;
+                            return events;
+                        }
+
+                        int index = txnIds.size();
+                        if (index >= bucketSize && !slowest.isEmpty())
+                        {
+                            SlowestTxnId leastSlow = slowest.first();
+                            if (leastSlow.elapsedNanos >= elapsedNanos)
+                                return cur;
+
+                            SlowestTxnId removed;
+                            removed = slowest.pollFirst();
+                            Invariants.expect(leastSlow.equals(removed), "%s != %s", entry, removed);
+                            removed = slowestLookup.remove(leastSlow.txnId);
+                            Invariants.expect(leastSlow.equals(removed), "%s != %s", entry, removed);
+                            index = txnIds.indexOf(leastSlow.txnId);
+                            Invariants.expect(index >= 0);
+                            if (index < 0)
+                                return cur;
+
+                            untrace = leastSlow.txnId;
+                            txnIds.set(index, events.txnId);
+                        }
+                        else
+                        {
+                            if (index > bucketSize)
+                                txnIds.remove(0);
+                            txnIds.add(events.txnId);
+                        }
+
+                        if (null != slowestLookup.put(events.txnId, entry = new SlowestTxnId(events.txnId, elapsedNanos)))
+                            return cur;
+
+                        slowest.add(entry);
+                        detachedEvents.remove(entry.txnId);
+                        events.detached = false;
+                        result = true;
+                        return events;
+                    }
+                }
+            }
+
+            UpdateSlowest updateSlowest = new UpdateSlowest();
+            txnIdMap.compute(events.txnId, updateSlowest);
+            if (updateSlowest.untrace != null)
+                untrace(updateSlowest.untrace);
+            return updateSlowest.result;
+        }
+
+        private void untrace(TxnId txnId)
         {
             txnIdMap.compute(txnId, (ignore, cur) -> {
                 if (cur == null || cur.owner != this)
@@ -528,16 +810,14 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             });
         }
 
-        private synchronized TxnEvents trace(TxnId txnId)
+        private TxnEvents newEvents(TxnId txnId)
         {
-            TxnEvents events = new TxnEvents();
+            TxnEvents events = new TxnEvents(AccordTracing.this, txnId);
             events.mode = traceBucketMode;
             events.bucketSize = traceBucketSize;
             events.bucketSubSize = traceBucketSubSize;
             events.owner = this;
-            if (null == txnIdMap.putIfAbsent(txnId, events))
-                return events;
-            return null;
+            return events;
         }
 
         synchronized void set(Function<TracePattern, TracePattern> pattern, BucketMode newBucketMode, int newBucketSeen, int newBucketSize, BucketMode newTraceBucketMode, int newTraceBucketSize, int newTraceBucketSubSize, CoordinationKinds newTraceEvents)
@@ -545,8 +825,16 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             Invariants.require(newBucketSize != 0);
             Invariants.require(newTraceBucketSize != 0);
             this.pattern = pattern.apply(this.pattern);
-            if (newBucketMode != null)
+            if (newBucketMode != null && newBucketMode != bucketMode)
+            {
+                if (bucketMode == SLOWEST)
+                {
+                    slowest.clear();
+                    slowestLookup.clear();
+                    detachedEvents.clear();
+                }
                 this.bucketMode = newBucketMode;
+            }
             if (newBucketSize >= 0)
                 this.bucketSize = newBucketSize;
             if (newBucketSeen >= 0)
@@ -561,11 +849,19 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
                 this.traceEvents = newTraceEvents;
         }
 
-        synchronized void clear()
+        void clear()
         {
-            for (TxnId txnId : txnIds)
+            List<TxnId> untrace;
+            synchronized (this)
+            {
+                untrace = new ArrayList<>(txnIds);
+                txnIds.clear();
+                slowest.clear();
+                slowestLookup.clear();
+                detachedEvents.clear();
+            }
+            for (TxnId txnId : untrace)
                 untrace(txnId);
-            txnIds.clear();
         }
     }
 
@@ -602,20 +898,54 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
     final CopyOnWriteArrayList<TracePatternState> traceNewPatterns = new CopyOnWriteArrayList<>();
     final GlobalCount globalCount = new GlobalCount();
 
-    public Tracing trace(TxnId txnId, @Nullable Participants<?> participants, CoordinationKind kind)
+    public Tracing trace(TxnId txnId, @Nullable Routables<?> participants, CoordinationKind kind)
     {
         if (kind == CoordinationKind.FetchDurableBefore)
             return (cs, msg) -> logger.info("Catchup/FetchDurableBefore: {}", msg.length() <= 100 ? msg : msg.substring(0, 100));
 
-        if (!txnIdMap.containsKey(txnId) && null == maybeTrace(txnId, participants, kind, NewOrFailure.NEW, traceNewPatterns))
-            return null;
+        if (!txnIdMap.containsKey(txnId))
+        {
+            TxnEvents events = maybeTrace(txnId, participants, kind, NewOrFailure.NEW, traceNewPatterns);
+            if (events == null)
+                return null;
+
+            if (events.detached)
+            {
+                class RegisterDetached implements BiFunction<TxnId, TxnEvents, TxnEvents>
+                {
+                    TxnEvent event;
+
+                    @Override
+                    public TxnEvents apply(TxnId txnId, TxnEvents state)
+                    {
+                        if (state != null) event = state.trace(kind, globalCount);
+                        else
+                        {
+                            TracePatternState owner = events.owner;
+                            if (owner != null)
+                            {
+                                synchronized (owner)
+                                {
+                                    event = new TxnEvent(events, kind);
+                                    owner.detachedEvent.put(event.idMicros, event);
+                                }
+                            }
+                        }
+                        return state;
+                    }
+                }
+                RegisterDetached register = new RegisterDetached();
+                txnIdMap.compute(txnId, register);
+                return register.event;
+            }
+        }
 
         class Register implements BiFunction<TxnId, TxnEvents, TxnEvents>
         {
             TxnEvent event;
 
             @Override
-            public TxnEvents apply(TxnId id, TxnEvents state)
+            public TxnEvents apply(TxnId txnId, TxnEvents state)
             {
                 if (state == null)
                     return null;
@@ -629,6 +959,65 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         return register.event;
     }
 
+    public void report(AccordTraceIn in, int nodeId)
+    {
+        txnIdMap.compute(in.txnId, (ignore, events) -> {
+            if (events != null)
+            {
+                report(in, nodeId, events);
+                return events;
+            }
+            else
+            {
+                for (TracePatternState patternState : allPatterns)
+                {
+                    synchronized (patternState)
+                    {
+                        TxnEvent detached = patternState.detachedEvent.get(in.idMicros);
+                        if (detached != null)
+                        {
+                            report(in, nodeId, detached);
+                            return null;
+                        }
+                    }
+                }
+                return null;
+            }
+        });
+
+    }
+
+    private boolean report(AccordTraceIn in, int nodeId, TxnEvents events)
+    {
+        for (int i = 0 ; i < events.size ; ++i)
+        {
+            TxnEvent event = events.get(i);
+            if (event.idMicros == in.idMicros)
+            {
+                report(in, nodeId, event);
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private void report(AccordTraceIn in, int nodeId, TxnEvent event)
+    {
+        synchronized (event)
+        {
+            long atNanos = Math.max(in.atNanos, event.atNanos);
+            int j = event.messages.size();
+            while (j > 0 && event.messages.get(j - 1).atNanos >= atNanos)
+                --j;
+            while (j < event.messages.size() && event.messages.get(j).atNanos == atNanos)
+            {
+                atNanos++;
+                j++;
+            }
+            event.messages.add(j, Message.withExactNanos(nodeId, in.commandStoreId, in.message, atNanos));
+        }
+    }
+
     // null values, or values < 0, are ignored
     public void set(TxnId txnId, CoordinationKinds trace, BucketMode newBucketMode, int newBucketSize, int newBucketSubSize, int newBucketSeen, float newChance, boolean unsetManagedByPattern)
     {
@@ -641,7 +1030,7 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
                 if (newBucketSize < 0)
                     throw new IllegalArgumentException("Must specify bucket size for new trace config.");
 
-                cur = new TxnEvents();
+                cur = new TxnEvents(this, id);
                 if (newBucketSubSize < 0)
                     cur.bucketSubSize = newBucketSize;
             }
@@ -714,6 +1103,15 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         txnIdMap.keySet().forEach(this::stopTracing);
     }
 
+    public void eraseAll()
+    {
+        allPatterns.forEach(TracePatternState::clear);
+        txnIdMap.keySet().forEach(txnId -> {
+            eraseEvents(txnId);
+            stopTracing(txnId);
+        });
+    }
+
     public void forEach(Predicate<TxnId> include, ConsumeState forEach)
     {
         txnIdMap.forEach((txnId, state) -> {
@@ -721,13 +1119,24 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
             {
                 // ensure lock is held for duration of callback
                 txnIdMap.compute(txnId, (id, cur) -> {
-                    forEach.accept(txnId, cur);
+                    if (cur != null)
+                        forEach.accept(txnId, cur);
                     return cur;
                 });
             }
         });
     }
 
+    public void forEach(TxnId txnId, Consumer<TxnEvents> forEach)
+    {
+        // ensure lock is held for duration of callback
+        txnIdMap.compute(txnId, (id, cur) -> {
+            if (cur != null)
+                forEach.accept(cur);
+            return cur;
+        });
+    }
+
     public void setPattern(int id, Function<TracePattern, TracePattern> pattern, BucketMode newBucketMode, int newBucketSeen, int newBucketSize, BucketMode newTraceBucketMode, int newTraceBucketSize, int newTraceBucketSubSize, CoordinationKinds newTraceEvents)
     {
         synchronized (allPatterns)
@@ -815,6 +1224,7 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
                 if (cur != tracing)
                     return cur;
 
+                // TODO (desired): assign an idMicros that is based on coordinationId
                 TxnEvent event = tracing.forceTrace(coordination.kind(), globalCount);
                 if (event == null) // we still honour global limit
                     return cur;
@@ -833,7 +1243,7 @@ public class AccordTracing extends AccordCoordinatorMetrics.Listener
         }
     }
 
-    private TxnEvents maybeTrace(TxnId txnId, @Nullable Participants<?> participants, CoordinationKind kind, NewOrFailure newOrFailure, List<TracePatternState> patterns)
+    private TxnEvents maybeTrace(TxnId txnId, @Nullable Routables<?> participants, CoordinationKind kind, NewOrFailure newOrFailure, List<TracePatternState> patterns)
     {
         if (patterns.isEmpty())
             return null;

src/java/org/apache/cassandra/service/accord/debug/DebugExecution.java

@@ -0,0 +1,242 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.service.accord.debug;
+
+import java.lang.management.ManagementFactory;
+import java.lang.management.ThreadMXBean;
+import java.util.List;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import accord.local.Command;
+
+import org.apache.cassandra.config.CassandraRelevantProperties;
+import org.apache.cassandra.metrics.LogLinearHistogram;
+import org.apache.cassandra.service.accord.AccordExecutor;
+
+import static org.apache.cassandra.config.CassandraRelevantProperties.DTEST_ACCORD_JOURNAL_SANITY_CHECK_ENABLED;
+import static org.apache.cassandra.utils.Clock.Global.nanoTime;
+
+public class DebugExecution
+{
+    private static final Logger logger = LoggerFactory.getLogger(DebugExecution.class);
+    public static final boolean DEBUG_EXECUTION = CassandraRelevantProperties.ACCORD_DEBUG_EXECUTION.getBoolean(false);
+    private static final long REPORT_MIN_LATENCY_MICROS = 10_000;
+    private static final long REPORT_CPU_RATIO = 2;
+    private static final long REPORT_MAX_LATENCY_MICROS = 50_000;
+
+    // TODO (expected): use sharded histogram so we can report global stats
+    public static class DebugExecutor
+    {
+        public static DebugExecutor maybeDebug() { return DEBUG_EXECUTION ? new DebugExecutor() : null; }
+
+        private DebugExecutor() {}
+
+        final LogLinearHistogram waitingToLock = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram locked = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram sequentialExecutorWaitingToRunLatency = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram sequentialExecutorSetHeadToRunLatency = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram pollToRun = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram applying = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram running = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram cleanup = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+        final LogLinearHistogram taskTotal = new LogLinearHistogram(REPORT_MAX_LATENCY_MICROS);
+
+        long lockedAt, lockedAtCpu;
+        long unlockedAt, unlockedAtCpu;
+
+        public void onEnterLock()
+        {
+            onEnterLock(0);
+        }
+
+        public void onEnterLock(long lockAt)
+        {
+            lockedAt = nanoTime();
+            lockedAtCpu = nowCpu();
+            if (lockAt > 0)
+            {
+                long waitingToLockForMicros = (lockedAt - lockAt)/1000;
+                waitingToLock.increment(waitingToLockForMicros);
+                if (waitingToLockForMicros > REPORT_MAX_LATENCY_MICROS)
+                {
+                    report("Took {}us to aquire executor lock", waitingToLockForMicros);
+                }
+            }
+        }
+
+        public void onExitLock()
+        {
+            unlockedAt = nanoTime();
+            unlockedAtCpu = nowCpu();
+            long lockedForMicros = (unlockedAt - lockedAt)/1000;
+            long lockedForCpuMicros = (unlockedAtCpu - lockedAtCpu)/1000;
+            if (lockedForMicros >= REPORT_MAX_LATENCY_MICROS)
+            {
+                report("Held lock for {}us (cpu:{}us)\n", lockedForMicros, lockedForCpuMicros);
+            }
+            else if (lockedForMicros >= REPORT_MIN_LATENCY_MICROS && (lockedForMicros / lockedForCpuMicros) >= REPORT_CPU_RATIO)
+            {
+                report("Held lock for {}us with cpu time only {}us\n", lockedForMicros, lockedForCpuMicros);
+            }
+            locked.increment(lockedForMicros);
+        }
+    }
+
+    public static class DebugExecutorLoop
+    {
+        final DebugExecutor owner;
+        long lockAt;
+
+        public DebugExecutorLoop(DebugExecutor owner)
+        {
+            this.owner = owner;
+        }
+
+        public void onLock()
+        {
+            lockAt = nanoTime();
+        }
+
+        public void onEnterLock()
+        {
+            owner.onEnterLock(lockAt);
+            lockAt = 0;
+        }
+
+        public void onExitLock()
+        {
+            owner.onExitLock();
+        }
+    }
+
+    public static class DebugSequentialExecutor
+    {
+        public static DebugSequentialExecutor maybeDebug(DebugExecutor owner, int commandStoreId)
+        {
+            return DEBUG_EXECUTION ? new DebugSequentialExecutor(owner, commandStoreId) : null;
+        }
+
+        final DebugExecutor owner;
+        final int commandStoreId;
+
+        long setTaskAt, waitingAt;
+        AccordExecutor.Task prev;
+
+        public DebugSequentialExecutor(DebugExecutor owner, int commandStoreId)
+        {
+            this.owner = owner;
+            this.commandStoreId = commandStoreId;
+        }
+
+        public void onSetTask(AccordExecutor.Task next)
+        {
+            if (next == null) setTaskAt = 0;
+            else setTaskAt = nanoTime();
+        }
+
+        public void onComplete(AccordExecutor.Task completed)
+        {
+            long readyAt = setTaskAt;
+            if (waitingAt > setTaskAt)
+            {
+                readyAt = waitingAt;
+                long waitingMicros = (completed.runningAt - waitingAt)/1000;
+                owner.sequentialExecutorWaitingToRunLatency.increment(waitingMicros);
+                if (waitingMicros > REPORT_MAX_LATENCY_MICROS)
+                    report("{} spent {}us blocked by a direct execution on queue {}", completed, waitingMicros, commandStoreId);
+            }
+            long atHeadMicros = (completed.runningAt - readyAt)/1000;
+            owner.sequentialExecutorSetHeadToRunLatency.increment(atHeadMicros);
+            if (atHeadMicros > REPORT_MAX_LATENCY_MICROS)
+            {
+                report("{} spent {}us at head of queue {}", completed, atHeadMicros, commandStoreId);
+            }
+            this.prev = completed;
+        }
+
+        public void onWaiting()
+        {
+            waitingAt = nanoTime();
+        }
+    }
+
+    public static class DebugTask
+    {
+        public static final boolean SANITY_CHECK = DTEST_ACCORD_JOURNAL_SANITY_CHECK_ENABLED.getBoolean();
+        private static final boolean DEBUG = DEBUG_EXECUTION || SANITY_CHECK;
+        public static DebugTask maybeDebug() { return DEBUG ? new DebugTask() : null; }
+
+        public List<Command> sanityCheck; // for AccordTask only
+        long polledAt, appliedAt, completedAt;
+        long polledAtCpu, completedAtCpu;
+
+        public void onPolled()
+        {
+            polledAt = nanoTime();
+            polledAtCpu = ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime();
+        }
+
+        public void onRunComplete()
+        {
+            appliedAt = nanoTime();
+        }
+
+        public void onCompleted(AccordExecutor.Task task, DebugExecutor owner)
+        {
+            completedAt = nanoTime();
+            completedAtCpu = ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime();
+            if (task.runningAt > 0 && polledAt > 0)
+            {
+                long pollToRunMicros = (task.runningAt - polledAt)/1000;
+                owner.pollToRun.increment(pollToRunMicros);
+                long applyingMicros = -1;
+                if (appliedAt > 0)
+                {
+                    applyingMicros = (appliedAt - task.runningAt)/1000;
+                    owner.applying.increment(applyingMicros);
+                }
+                long runningMicros = (task.cleanupAt - task.runningAt)/1000;
+                owner.running.increment(runningMicros);
+                long cleanupMicros = (completedAt - task.cleanupAt)/1000;
+                owner.cleanup.increment(cleanupMicros);
+                long totalMicros = (completedAt - polledAt)/1000;
+                owner.taskTotal.increment(totalMicros);
+                long totalCpu = (completedAtCpu - polledAtCpu)/1000;
+                if (totalMicros > REPORT_MAX_LATENCY_MICROS || (totalMicros > REPORT_MIN_LATENCY_MICROS && (totalMicros/totalCpu) >= REPORT_CPU_RATIO))
+                {
+                    report("{}: total {}us {}cpu, running {}us{}, cleanup {}us, pollToRun {}us", task, totalMicros, totalCpu,
+                           runningMicros, (applyingMicros >= 0 ? ", applying " + applyingMicros + "us" : ""), cleanupMicros, pollToRunMicros);
+                }
+            }
+        }
+    }
+
+    private static void report(String message, Object ... params)
+    {
+        logger.warn(Thread.currentThread() + " " + message, params);
+    }
+
+    private static final ThreadMXBean runtime = ManagementFactory.getThreadMXBean();
+    private static long nowCpu()
+    {
+        return runtime.getCurrentThreadCpuTime();
+    }
+}

src/java/org/apache/cassandra/service/accord/interop/AccordInteropApply.java

@@ -165,7 +165,7 @@ public class AccordInteropApply extends Apply implements LocalListeners.ComplexL
         // and prevents races where we respond before dispatching all the required reads (if the reads are
         // completing faster than the reads can be setup on all required shards)
         if (-1 == --waitingOnCount)
-            node.reply(replyTo, replyContext, ApplyReply.Applied, null);
+            node.reply(replyTo, replyContext, ApplyReply.Applied, null, tracing());
     }
 
     @Override
@@ -175,11 +175,11 @@ public class AccordInteropApply extends Apply implements LocalListeners.ComplexL
         {
             // Respond with insufficient which should make the coordinator send us the commit
             // we need to respond
-            node.reply(replyTo, replyContext, reply, failure);
+            node.reply(replyTo, replyContext, reply, failure, tracing());
         }
         else if (failure != null)
         {
-            node.reply(replyTo, replyContext, null, failure);
+            node.reply(replyTo, replyContext, null, failure, tracing());
             node.agent().onException(failure);
             fail();
         }

src/java/org/apache/cassandra/service/accord/interop/AccordInteropExecution.java

@@ -219,7 +219,7 @@ public class AccordInteropExecution implements ReadCoordinator
         //  Also this read scope doesn't reflect the contents of this particular read and is larger than it needs to be
         // TODO (required): understand interop and whether StableFastPath is appropriate
         AccordInteropStableThenRead commit = new AccordInteropStableThenRead(id, allTopologies, txnId, Kind.StableFastPath, executeAt, txn, deps, route, message.payload);
-        node.send(id, commit, executor, new AccordInteropRead.ReadCallback(id, to, message, callback, this));
+        node.send(id, commit, executor, new AccordInteropRead.ReadCallback(id, to, message, callback, this), null);
     }
 
     @Override
@@ -235,7 +235,7 @@ public class AccordInteropExecution implements ReadCoordinator
         }
         Participants<?> readScope = Participants.singleton(txn.read().keys().domain(), new TokenKey(message.payload.getTableIds().iterator().next(), message.payload.key().getToken()));
         AccordInteropReadRepair readRepair = new AccordInteropReadRepair(id, executes, txnId, readScope, executeAt.epoch(), message.payload);
-        node.send(id, readRepair, executor, new AccordInteropReadRepair.ReadRepairCallback(id, to, message, callback, this));
+        node.send(id, readRepair, executor, new AccordInteropReadRepair.ReadRepairCallback(id, to, message, callback, this), null);
     }
 
     private List<AsyncChain<Data>> readChains(Dispatcher.RequestTime requestTime)
@@ -376,7 +376,7 @@ public class AccordInteropExecution implements ReadCoordinator
         {
             InetAddressAndPort endpoint = endpointMapper.mappedEndpointOrNull(to);
             if (endpoint != null && !contacted.contains(endpoint))
-                node.send(to, new Commit(Kind.StableFastPath, to, allTopologies, txnId, txn, route, Ballot.ZERO, executeAt, deps));
+                node.send(to, new Commit(Kind.StableFastPath, to, allTopologies, txnId, txn, route, Ballot.ZERO, executeAt, deps), null);
         }
     }
 
@@ -387,7 +387,7 @@ public class AccordInteropExecution implements ReadCoordinator
             for (Node.Id to : allTopologies.nodes())
             {
                 if (!executeTopology.contains(to))
-                    node.send(to, new Commit(Kind.StableFastPath, to, allTopologies, txnId, txn, route, Ballot.ZERO, executeAt, deps));
+                    node.send(to, new Commit(Kind.StableFastPath, to, allTopologies, txnId, txn, route, Ballot.ZERO, executeAt, deps), null);
             }
         }
         AsyncChain<Data> result;
@@ -423,7 +423,7 @@ public class AccordInteropExecution implements ReadCoordinator
             // and can be extended similar to MessageType which allows additional types not from Accord to be added
             // This commit won't necessarily execute before the interop read repair message so there could be an insufficient which is fine
             for (Node.Id to : executeTopology.nodes())
-                    node.send(to, new Commit(Kind.StableFastPath, to, allTopologies, txnId, txn, route, Ballot.ZERO, executeAt, deps));
+                    node.send(to, new Commit(Kind.StableFastPath, to, allTopologies, txnId, txn, route, Ballot.ZERO, executeAt, deps), null);
             repairUpdate.runBRR(AccordInteropExecution.this);
             return new TxnData();
         });
@@ -455,7 +455,7 @@ public class AccordInteropExecution implements ReadCoordinator
     // Provide request callbacks with a way to send maximal commits on Insufficient responses
     public void sendMaximalCommit(Id to)
     {
-        node.send(to, new Commit(Kind.StableWithTxnAndDeps, to, allTopologies, txnId, txn, route, ballot, executeAt, deps));
+        node.send(to, new Commit(Kind.StableWithTxnAndDeps, to, allTopologies, txnId, txn, route, ballot, executeAt, deps), null);
     }
 
     public void maybeUpdateUniqueHlc(long uniqueHlc)

src/java/org/apache/cassandra/service/accord/interop/AccordInteropPersist.java

@@ -25,8 +25,6 @@ import accord.coordinate.ExecuteFlag;
 import accord.coordinate.Persist;
 import accord.coordinate.tracking.AllTracker;
 import accord.coordinate.tracking.QuorumTracker;
-import accord.coordinate.tracking.RequestStatus;
-import accord.coordinate.tracking.ResponseTracker;
 import accord.local.Node;
 import accord.local.SequentialAsyncExecutor;
 import accord.messages.Apply;
@@ -40,10 +38,13 @@ import accord.primitives.TxnId;
 import accord.primitives.Writes;
 import accord.topology.Topologies;
 import accord.utils.Invariants;
-import accord.utils.UnhandledEnum;
 
 import org.apache.cassandra.db.ConsistencyLevel;
-import org.apache.cassandra.utils.Throwables;
+
+import static org.apache.cassandra.db.ConsistencyLevel.ALL;
+import static org.apache.cassandra.db.ConsistencyLevel.ONE;
+import static org.apache.cassandra.db.ConsistencyLevel.QUORUM;
+import static org.apache.cassandra.db.ConsistencyLevel.SERIAL;
 
 /**
  * Similar to Accord persist, but can wait on a configurable number of responses and sends AccordInteropApply messages
@@ -52,110 +53,13 @@ import org.apache.cassandra.utils.Throwables;
  */
 public class AccordInteropPersist extends Persist
 {
-    private static class CallbackHolder
+    public AccordInteropPersist(Node node, SequentialAsyncExecutor executor, Topologies topologies, TxnId txnId, Route<?> sendTo, Ballot ballot, Txn txn, Timestamp executeAt, Deps deps, Writes writes, Result result, FullRoute<?> fullRoute, ConsistencyLevel consistencyLevel, ExecuteFlag.CoordinationFlags flags, boolean informDurableOnDone, Apply.Kind applyKind, BiConsumer<? super Result, Throwable> callback)
     {
-        boolean isDone = false;
-        private final ResponseTracker tracker;
-        private final Result result;
-        private final BiConsumer<? super Result, Throwable> clientCallback;
-        private Throwable failure = null;
-
-        public CallbackHolder(ResponseTracker tracker, Result result, BiConsumer<? super Result, Throwable> clientCallback)
-        {
-            this.tracker = tracker;
-            this.result = result;
-            this.clientCallback = clientCallback;
-        }
-
-        private void handleStatus(RequestStatus status)
-        {
-            if (isDone)
-                return;
-
-            switch (status)
-            {
-                default: throw new IllegalStateException("Unhandled request status " + status);
-                case Success:
-                    isDone = true;
-                    clientCallback.accept(result, null);
-                    return;
-                case Failed:
-                    isDone = true;
-                    clientCallback.accept(null, failure);
-                    return;
-                case NoChange:
-                    // noop
-            }
-        }
-
-        public void recordSuccess(Node.Id node)
-        {
-            handleStatus(tracker.recordSuccess(node));
-        }
-
-        public void recordFailure(Node.Id node, Throwable throwable)
-        {
-            failure = Throwables.merge(failure, throwable);
-            handleStatus(tracker.recordFailure(node));
-        }
-
-        boolean recordCallbackFailure(Throwable throwable)
-        {
-            if (isDone)
-                return false;
-            isDone = true;
-            failure = Throwables.merge(failure, throwable);
-            clientCallback.accept(null, failure);
-            return true;
-        }
-    }
-
-    private final ConsistencyLevel consistencyLevel;
-    private CallbackHolder callback;
-
-    public AccordInteropPersist(Node node, SequentialAsyncExecutor executor, Topologies topologies, TxnId txnId, Route<?> sendTo, Ballot ballot, Txn txn, Timestamp executeAt, Deps deps, Writes writes, Result result, FullRoute<?> fullRoute, ConsistencyLevel consistencyLevel, ExecuteFlag.CoordinationFlags flags, boolean informDurableOnDone, Apply.Kind applyKind, BiConsumer<? super Result, Throwable> clientCallback)
-    {
-        super(node, executor, topologies, txnId, ballot, sendTo, txn, executeAt, deps, writes, result, fullRoute, flags, informDurableOnDone, AccordInteropApply.FACTORY, applyKind);
-        Invariants.requireArgument(consistencyLevel == ConsistencyLevel.QUORUM || consistencyLevel == ConsistencyLevel.ALL || consistencyLevel == ConsistencyLevel.SERIAL || consistencyLevel == ConsistencyLevel.ONE);
-        this.consistencyLevel = consistencyLevel;
-        registerClientCallback(result, clientCallback);
-    }
-
-    public void registerClientCallback(Result result, BiConsumer<? super Result, Throwable> clientCallback)
-    {
-        Invariants.require(callback == null);
-        switch (consistencyLevel)
-        {
-            case ONE: // Can safely upgrade ONE to QUORUM/SERIAL to get a synchronous commit
-            case SERIAL:
-            case QUORUM:
-                callback = new CallbackHolder(new QuorumTracker(tracker.topologies()), result, clientCallback);
-                break;
-            case ALL:
-                callback = new CallbackHolder(new AllTracker(tracker.topologies()), result, clientCallback);
-                break;
-            default:
-                throw new IllegalArgumentException("Unhandled consistency level: " + consistencyLevel);
-        }
-    }
-
-    @Override
-    public void onSuccess(Node.Id from, Apply.ApplyReply reply)
-    {
-        super.onSuccess(from, reply);
-        switch (reply.kind)
-        {
-            case InsufficientEpochs: throw UnhandledEnum.invalid(reply.kind);
-            case Redundant:
-            case Applied:
-                callback.recordSuccess(from);
-                return;
-            case Insufficient:
-                // On insufficient Persist will send a commit with the missing information
-                // which will allow a final response to be returned later that could be successful
-                return;
-            default: throw UnhandledEnum.unknown(reply.kind);
-        }
+        super(node, executor, topologies, txnId, ballot, sendTo, txn, executeAt, deps, writes, result, fullRoute, flags, informDurableOnDone, AccordInteropApply.FACTORY, applyKind, consistencyLevel == ALL ? AllTracker::new : QuorumTracker::new, (ignore, fail) -> {
+            if (fail != null) callback.accept(null, fail);
+            else callback.accept(result, null);
+        });
+        Invariants.requireArgument(consistencyLevel == QUORUM || consistencyLevel == ALL || consistencyLevel == SERIAL || consistencyLevel == ONE);
     }
 
     @Override
@@ -163,18 +67,4 @@ public class AccordInteropPersist extends Persist
     {
         super.start();
     }
-
-    @Override
-    public void onFailure(Node.Id from, Throwable failure)
-    {
-        callback.recordFailure(from, failure);
-        super.onFailure(from, failure);
-    }
-
-    @Override
-    public boolean onCallbackFailure(Node.Id from, Throwable failure)
-    {
-        super.onCallbackFailure(from, failure);
-        return callback.recordCallbackFailure(failure);
-    }
 }

src/java/org/apache/cassandra/service/accord/interop/AccordInteropRead.java

@@ -245,6 +245,8 @@ public class AccordInteropRead extends ReadData
         @Override
         ReadResponse convertResponse(ReadOk ok)
         {
+            LocalReadData data = (LocalReadData) ok.data;
+            data.ensureRemoteResponse();
             return ((LocalReadData)ok.data).remoteResponse;
         }
     }

src/java/org/apache/cassandra/service/accord/journal/AccordJournal.java

@@ -41,6 +41,8 @@ import accord.local.CommandStore;
 import accord.local.CommandStores;
 import accord.local.CommandStores.RangesForEpoch;
 import accord.local.DurableBefore;
+import accord.local.MinimalCommand;
+import accord.local.MinimalCommand.MinimalWithDeps;
 import accord.local.Node;
 import accord.local.RedundantBefore;
 import accord.primitives.EpochSupplier;
@@ -50,8 +52,8 @@ import accord.primitives.TxnId;
 import accord.utils.Invariants;
 import accord.utils.PersistentField;
 
-import org.apache.cassandra.config.AccordSpec.JournalSpec;
-import org.apache.cassandra.config.AccordSpec.JournalSpec.ReplayMode;
+import org.apache.cassandra.config.AccordConfig.JournalConfig;
+import org.apache.cassandra.config.AccordConfig.JournalConfig.ReplayMode;
 import org.apache.cassandra.db.ColumnFamilyStore;
 import org.apache.cassandra.db.Keyspace;
 import org.apache.cassandra.db.marshal.LongType;
@@ -77,7 +79,7 @@ import org.apache.cassandra.utils.concurrent.OpOrder;
 import static accord.api.Journal.Load.MINIMAL;
 import static accord.api.Journal.Load.MINIMAL_WITH_DEPS;
 import static accord.local.Cleanup.Input.FULL;
-import static org.apache.cassandra.config.AccordSpec.RangeIndexMode.journal_sai;
+import static org.apache.cassandra.config.AccordConfig.RangeIndexMode.journal_sai;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccord;
 import static org.apache.cassandra.config.DatabaseDescriptor.getAccordJournalDirectory;
 import static org.apache.cassandra.service.accord.JournalKey.Type.COMMAND_DIFF;
@@ -230,14 +232,14 @@ public class AccordJournal implements accord.api.Journal, RangeSearcher.Supplier
     }
 
     @Override
-    public Command.Minimal loadMinimal(int commandStoreId, TxnId txnId, RedundantBefore redundantBefore, DurableBefore durableBefore)
+    public MinimalCommand loadMinimal(int commandStoreId, TxnId txnId, RedundantBefore redundantBefore, DurableBefore durableBefore)
     {
         CommandChanges builder = CommandChanges.cleanupAndFilter(loadDiffs(commandStoreId, txnId, MINIMAL), redundantBefore, durableBefore);
         return builder == null ? null : builder.asMinimal();
     }
 
     @Override
-    public Command.MinimalWithDeps loadMinimalWithDeps(int commandStoreId, TxnId txnId, RedundantBefore redundantBefore, DurableBefore durableBefore)
+    public MinimalWithDeps loadMinimalWithDeps(int commandStoreId, TxnId txnId, RedundantBefore redundantBefore, DurableBefore durableBefore)
     {
         CommandChanges builder = CommandChanges.cleanupAndFilter(loadDiffs(commandStoreId, txnId, MINIMAL_WITH_DEPS), redundantBefore, durableBefore);
         return builder == null ? null : builder.asMinimalWithDeps();
@@ -596,8 +598,8 @@ public class AccordJournal implements accord.api.Journal, RangeSearcher.Supplier
     @Override
     public boolean replay(CommandStores commandStores, Object param)
     {
-        ReplayMode mode = params instanceof JournalSpec ? ((JournalSpec)params).replay
-                                                        : getAccord().journal.replay;
+        ReplayMode mode = params instanceof JournalConfig ? ((JournalConfig)params).replay
+                                                          : getAccord().journal.replay;
         return Replay.replay(this, mode, commandStores.all(), param);
     }
 

src/java/org/apache/cassandra/service/accord/journal/Replay.java

@@ -36,7 +36,7 @@ import accord.primitives.TxnId;
 import accord.utils.Invariants;
 import accord.utils.UnhandledEnum;
 
-import org.apache.cassandra.config.AccordSpec.JournalSpec.ReplayMode;
+import org.apache.cassandra.config.AccordConfig.JournalConfig.ReplayMode;
 import org.apache.cassandra.io.util.FileUtils;
 import org.apache.cassandra.journal.Journal;
 import org.apache.cassandra.service.accord.AccordCommandStore;

src/java/org/apache/cassandra/service/accord/serializers/CommandStoreSerializers.java

@@ -39,7 +39,6 @@ import accord.local.DurableBefore;
 import accord.local.MaxConflicts;
 import accord.local.MaxDecidedRX;
 import accord.local.RedundantBefore;
-import accord.local.RejectBefore;
 import accord.primitives.Range;
 import accord.primitives.Ranges;
 import accord.primitives.SaveStatus;
@@ -77,7 +76,7 @@ public class CommandStoreSerializers
     public static final UnversionedSerializer<MaxDecidedRX> maxDecidedRX = new ReducingRangeMapSerializer<>(new DecidedRXSerializer(), MaxDecidedRX.DecidedRX[]::new, MaxDecidedRX.SerializerSupport::create, MaxDecidedRX.EMPTY);
     public static final UnversionedSerializer<RedundantBefore.Bounds> redundantBeforeShortBounds = new RedundantBeforeShortBoundsSerializer();
     public static final UnversionedSerializer<RedundantBefore> redundantBefore = new ReducingRangeMapSerializer<>(redundantBeforeShortBounds, RedundantBefore.Bounds[]::new, RedundantBefore.SerializerSupport::create, RedundantBefore.EMPTY);
-    public static final UnversionedSerializer<RejectBefore> rejectBefore = new ReducingRangeMapSerializer<>(CommandSerializers.timestamp, Timestamp[]::new, RejectBefore.SerializerSupport::create, RejectBefore.EMPTY);
+    public static final UnversionedSerializer<MaxConflicts> rejectBefore = new RejectBeforeSerializer();
     public static final UnversionedSerializer<NavigableMap<TxnId, Ranges>> bootstrapBeganAt = new TimestampToRangesMapSerializer<>(CommandSerializers.txnId);
     public static final UnversionedSerializer<NavigableMap<Timestamp, Ranges>> safeToRead = new TimestampToRangesMapSerializer<>(CommandSerializers.timestamp);
     public static final UnversionedSerializer<TxnListener> txnListener = new TxnListenerSerializer();
@@ -578,14 +577,15 @@ public class CommandStoreSerializers
     private static final class MaxConflictsSerializer extends BTreeReducingRangeMapSerializer<MaxConflicts.Entry, MaxConflicts>
     {
         // use top bits of a single byte vint, to leave room for base impl to fill other way
-        private static final int SEPARATE_WRITES = 0x40;
+        private static final int INCLUDES_WRITE = 0x40;
+        private static final int INCLUDES_REJECT = 0x20;
 
         private MaxConflictsSerializer() {}
 
         @Override
         protected int mapFlags()
         {
-            return SEPARATE_WRITES;
+            return INCLUDES_WRITE | INCLUDES_REJECT;
         }
 
         @Override
@@ -605,30 +605,78 @@ public class CommandStoreSerializers
         {
             CommandSerializers.timestamp.serialize(entry.any, out);
             CommandSerializers.timestamp.serialize(entry.write, out);
+            CommandSerializers.timestamp.serialize(entry.reject, out);
         }
 
         @Override
         long serializedSizeWithoutRange(MaxConflicts.Entry entry)
         {
             return CommandSerializers.timestamp.serializedSize(entry.any)
-                    + CommandSerializers.timestamp.serializedSize(entry.write);
+                    + CommandSerializers.timestamp.serializedSize(entry.write)
+                    + CommandSerializers.timestamp.serializedSize(entry.reject);
         }
 
         @Override
         MaxConflicts.Entry deserialize(RoutingKey start, RoutingKey end, DataInputPlus in, int mapFlags) throws IOException
         {
-            Timestamp all = CommandSerializers.timestamp.deserialize(in);
-            Timestamp writes = all;
-            if ((mapFlags & SEPARATE_WRITES) != 0)
-                writes = CommandSerializers.timestamp.deserialize(in);
-            return new MaxConflicts.Entry(start, end, all, writes);
+            Timestamp any = CommandSerializers.timestamp.deserialize(in);
+            Timestamp write = any;
+            if ((mapFlags & INCLUDES_WRITE) != 0)
+                write = CommandSerializers.timestamp.deserialize(in);
+            Timestamp reject = Timestamp.NONE;
+            if ((mapFlags & INCLUDES_REJECT) != 0)
+                reject = CommandSerializers.timestamp.deserialize(in);
+            return MaxConflicts.Entry.create(start, end, any, write, reject);
         }
 
         @Override
         MaxConflicts.Entry deserializeArrayModeWithoutRange(DataInputPlus in) throws IOException
         {
             Timestamp all = CommandSerializers.timestamp.deserialize(in);
-            return new MaxConflicts.Entry(all, all);
+            return MaxConflicts.Entry.create(all, all, Timestamp.NONE);
+        }
+    }
+
+    // legacy version that deserializes RejectsBefore into MaxConflicts
+    private static final class RejectBeforeSerializer extends BTreeReducingRangeMapSerializer<MaxConflicts.Entry, MaxConflicts>
+    {
+        private RejectBeforeSerializer() {}
+
+        @Override
+        MaxConflicts empty()
+        {
+            return MaxConflicts.EMPTY;
+        }
+
+        @Override
+        BTreeReducingRangeMap.Builder<MaxConflicts.Entry, MaxConflicts> builder()
+        {
+            return new MaxConflicts.Builder();
+        }
+
+        @Override
+        void serializeWithoutRange(MaxConflicts.Entry entry, DataOutputPlus out)
+        {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        long serializedSizeWithoutRange(MaxConflicts.Entry entry)
+        {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        MaxConflicts.Entry deserialize(RoutingKey start, RoutingKey end, DataInputPlus in, int mapFlags) throws IOException
+        {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        MaxConflicts.Entry deserializeArrayModeWithoutRange(DataInputPlus in) throws IOException
+        {
+            Timestamp reject = CommandSerializers.timestamp.deserialize(in);
+            return MaxConflicts.Entry.create(Timestamp.NONE, Timestamp.NONE, reject);
         }
     }
 

src/java/org/apache/cassandra/service/accord/serializers/RemoteSuccessSerializers.java

@@ -0,0 +1,56 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.cassandra.service.accord.serializers;
+
+import java.io.IOException;
+
+import accord.messages.RemoteSuccess;
+import accord.primitives.TxnId;
+
+import org.apache.cassandra.io.VersionedSerializer;
+import org.apache.cassandra.io.util.DataInputPlus;
+import org.apache.cassandra.io.util.DataOutputPlus;
+import org.apache.cassandra.service.accord.txn.TxnData;
+
+public class RemoteSuccessSerializers
+{
+    public static final VersionedSerializer<RemoteSuccess, Version> remoteSuccess = new VersionedSerializer<>()
+    {
+        @Override
+        public void serialize(RemoteSuccess msg, DataOutputPlus out, Version version) throws IOException
+        {
+            CommandSerializers.txnId.serialize(msg.txnId, out);
+            TxnData.serializer.serialize((TxnData) msg.result, out, version);
+        }
+
+        @Override
+        public RemoteSuccess deserialize(DataInputPlus in, Version version) throws IOException
+        {
+            TxnId txnId = CommandSerializers.txnId.deserialize(in);
+            TxnData data = TxnData.serializer.deserialize(in, version);
+            return new RemoteSuccess(txnId, data);
+        }
+
+        @Override
+        public long serializedSize(RemoteSuccess msg, Version version)
+        {
+            return CommandSerializers.txnId.serializedSize(msg.txnId)
+                   + TxnData.serializer.serializedSize((TxnData) msg.result, version);
+        }
+    };
+}

src/java/org/apache/cassandra/service/accord/topology/AccordTopologyService.java

@@ -20,6 +20,7 @@ package org.apache.cassandra.service.accord.topology;
 
 import java.util.HashSet;
 import java.util.Set;
+import java.util.concurrent.TimeUnit;
 import java.util.function.BiConsumer;
 
 import javax.annotation.concurrent.GuardedBy;
@@ -42,6 +43,8 @@ import accord.utils.async.AsyncResults.SettableByCallback;
 import org.apache.cassandra.concurrent.ScheduledExecutorPlus;
 import org.apache.cassandra.concurrent.ScheduledExecutors;
 import org.apache.cassandra.concurrent.Stage;
+import org.apache.cassandra.config.AccordConfig;
+import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.locator.InetAddressAndPort;
 import org.apache.cassandra.net.MessageDelivery;
 import org.apache.cassandra.net.MessagingService;
@@ -94,10 +97,40 @@ public class AccordTopologyService implements TopologyService, TopologyListener
             previouslyRemovedIds = removed;
         }
         node.topology().addListener(watermarkCollector);
-        node.topology().addListener(syncPropagator);
-        syncPropagator.onNodesRemoved(removed);
+        if (DatabaseDescriptor.getAccord().topology_sync_propagator_enabled_pre_start)
+        {
+            node.topology().addListener(syncPropagator);
+            syncPropagator.onNodesRemoved(removed);
+        }
     }
 
+    public void afterStartup(Node node)
+    {
+        AccordConfig config = DatabaseDescriptor.getAccord();
+        if (config.topology_sync_propagator_enabled_post_startup && !config.topology_sync_propagator_enabled_pre_start)
+        {
+            node.topology().addListener(syncPropagator);
+            syncPropagator.onNodesRemoved(previouslyRemovedIds);
+        }
+
+        long watermarkIntervalNanos = config.topology_watermark_interval.toNanoseconds();
+        if (watermarkIntervalNanos > 0)
+            fetchAndReportWatermarksRecurring(node, watermarkIntervalNanos);
+    }
+
+    public void fetchAndReportWatermarksRecurring(Node node, long intervalNanos)
+    {
+        if (state == State.SHUTDOWN)
+            return;
+
+        ScheduledExecutors.scheduledFastTasks.scheduleSelfRecurring(() -> {
+            WatermarkCollector.fetchAndReportWatermarksAsync(node.topology()).addCallback((success, fail) -> {
+                fetchAndReportWatermarksRecurring(node, intervalNanos);
+            });
+        }, intervalNanos, TimeUnit.NANOSECONDS);
+    }
+
+
     public synchronized void shutdown()
     {
         state = State.SHUTDOWN;

src/java/org/apache/cassandra/service/accord/topology/FastPathStrategy.java

@@ -44,7 +44,7 @@ public interface FastPathStrategy
 {
     enum Kind
     {
-        SIMPLE, PARAMETERIZED, INHERIT_KEYSPACE;
+        SIMPLE, PARAMETERIZED, INHERIT_KEYSPACE, UP;
 
         static final String KEY = "kind";
         private static final Map<String, Kind> LOOKUP;
@@ -54,6 +54,7 @@ public interface FastPathStrategy
             builder.put(SIMPLE.name(), SIMPLE);
             builder.put(PARAMETERIZED.name(), PARAMETERIZED);
             builder.put(INHERIT_KEYSPACE.name(), INHERIT_KEYSPACE);
+            builder.put(UP.name(), UP);
             LOOKUP = builder.build();
         }
 
@@ -115,6 +116,8 @@ public interface FastPathStrategy
                 return ParameterizedFastPathStrategy.fromMap(map);
             case INHERIT_KEYSPACE:
                 return inheritKeyspace();
+            case UP:
+                return up();
             default:
                 throw new IllegalArgumentException("Unhandled strategy kind: " + kind);
         }
@@ -127,8 +130,10 @@ public interface FastPathStrategy
             return InheritKeyspaceFastPathStrategy.instance;
         if (s.equals("simple"))
             return SimpleFastPathStrategy.instance;
+        if (s.equals("up"))
+            return UpFastPathStrategy.instance;
 
-        throw new ConfigurationException("Fast path strategy must either be 'keyspace', `default` or a map size and optional dcs {'size':n, 'dcs': dc0,dc1...");
+        throw new ConfigurationException("Fast path strategy must either be 'keyspace', 'simple', 'up' or a map size and optional dcs {'size':n, 'dcs': dc0,dc1...");
     }
 
     static FastPathStrategy keyspaceStrategyFromString(String s)
@@ -136,8 +141,10 @@ public interface FastPathStrategy
         s = toLowerCaseLocalized(s).trim();
         if (s.equals("simple"))
             return SimpleFastPathStrategy.instance;
+        if (s.equals("up"))
+            return UpFastPathStrategy.instance;
 
-        throw new ConfigurationException("Fast path strategy must either be `default` or a map size and optional dcs {'size':n, 'dcs': dc0,dc1...");
+        throw new ConfigurationException("Fast path strategy must either be 'simple', 'up' or a map size and optional dcs {'size':n, 'dcs': dc0,dc1...");
     }
 
     static FastPathStrategy simple()
@@ -145,6 +152,11 @@ public interface FastPathStrategy
         return SimpleFastPathStrategy.instance;
     }
 
+    static FastPathStrategy up()
+    {
+        return UpFastPathStrategy.instance;
+    }
+
     static FastPathStrategy inheritKeyspace()
     {
         return InheritKeyspaceFastPathStrategy.instance;
@@ -171,6 +183,8 @@ public interface FastPathStrategy
                     return ParameterizedFastPathStrategy.serializer.deserialize(in, version);
                 case INHERIT_KEYSPACE:
                     return inheritKeyspace();
+                case UP:
+                    return up();
                 default:
                     throw new IllegalArgumentException("Unhandled type: " + type);
             }

src/java/org/apache/cassandra/service/accord/topology/SimpleFastPathStrategy.java

@@ -24,9 +24,6 @@ import java.util.Set;
 import com.google.common.collect.ImmutableMap;
 
 import accord.local.Node;
-import accord.topology.Shard;
-import accord.utils.ArrayBuffers;
-import accord.utils.Invariants;
 import accord.utils.SortedArrays.SortedArrayList;
 
 public class SimpleFastPathStrategy implements FastPathStrategy
@@ -40,27 +37,7 @@ public class SimpleFastPathStrategy implements FastPathStrategy
     @Override
     public SortedArrayList<Node.Id> calculateFastPath(SortedArrayList<Node.Id> nodes, Set<Node.Id> unavailable, Map<Node.Id, String> dcMap)
     {
-        int maxFailures = Shard.maxToleratedFailures(nodes.size());
-        int discarded = 0;
-
-        if (unavailable.isEmpty())
-            return nodes;
-
-        Object[] tmp = ArrayBuffers.cachedAny().get(nodes.size());
-        for (int i=0,mi=nodes.size(); i<mi; i++)
-        {
-            Node.Id node = nodes.get(i);
-            if (unavailable.contains(node) && discarded < maxFailures)
-                discarded++;
-            else
-                tmp[i - discarded] = node;
-        }
-
-        Node.Id[] array = new Node.Id[nodes.size() - discarded];
-        System.arraycopy(tmp, 0, array, 0, nodes.size() - discarded);
-        SortedArrayList<Node.Id> fastPath = new SortedArrayList<>(array);
-        Invariants.require(fastPath.size() >= Shard.slowQuorumSize(nodes.size()));
-        return fastPath;
+        return nodes;
     }
 
     @Override

src/java/org/apache/cassandra/service/accord/topology/UpFastPathStrategy.java

@@ -0,0 +1,89 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.service.accord.topology;
+
+import java.util.Map;
+import java.util.Set;
+
+import com.google.common.collect.ImmutableMap;
+
+import accord.local.Node;
+import accord.topology.Shard;
+import accord.utils.ArrayBuffers;
+import accord.utils.Invariants;
+import accord.utils.SortedArrays.SortedArrayList;
+
+public class UpFastPathStrategy implements FastPathStrategy
+{
+    public static final UpFastPathStrategy instance = new UpFastPathStrategy();
+
+    private static final Map<String, String> SCHEMA_PARAMS = ImmutableMap.of(Kind.KEY, Kind.UP.name());
+
+    private UpFastPathStrategy() {}
+
+    @Override
+    public SortedArrayList<Node.Id> calculateFastPath(SortedArrayList<Node.Id> nodes, Set<Node.Id> unavailable, Map<Node.Id, String> dcMap)
+    {
+        int maxFailures = Shard.maxToleratedFailures(nodes.size());
+        int discarded = 0;
+
+        if (unavailable.isEmpty())
+            return nodes;
+
+        Object[] tmp = ArrayBuffers.cachedAny().get(nodes.size());
+        for (int i=0,mi=nodes.size(); i<mi; i++)
+        {
+            Node.Id node = nodes.get(i);
+            if (unavailable.contains(node) && discarded < maxFailures)
+                discarded++;
+            else
+                tmp[i - discarded] = node;
+        }
+
+        Node.Id[] array = new Node.Id[nodes.size() - discarded];
+        System.arraycopy(tmp, 0, array, 0, nodes.size() - discarded);
+        ArrayBuffers.cachedAny().forceDiscard(tmp, array.length);
+        SortedArrayList<Node.Id> fastPath = new SortedArrayList<>(array);
+        Invariants.require(fastPath.size() >= Shard.slowQuorumSize(nodes.size()));
+        return fastPath;
+    }
+
+    @Override
+    public Kind kind()
+    {
+        return Kind.UP;
+    }
+
+    @Override
+    public String toString()
+    {
+        return "up";
+    }
+
+    public Map<String, String> asMap()
+    {
+        return SCHEMA_PARAMS;
+    }
+
+    @Override
+    public String asCQL()
+    {
+        return "'up'";
+    }
+}

src/java/org/apache/cassandra/service/accord/topology/WatermarkCollector.java

@@ -20,12 +20,11 @@ package org.apache.cassandra.service.accord.topology;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Comparator;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.Set;
 import java.util.function.BiConsumer;
 
 import javax.annotation.Nullable;
@@ -62,6 +61,7 @@ import org.apache.cassandra.service.accord.AccordService;
 import org.apache.cassandra.service.accord.TokenRange;
 import org.apache.cassandra.tcm.ClusterMetadata;
 import org.apache.cassandra.utils.FBUtilities;
+import org.apache.cassandra.utils.concurrent.Future;
 
 import static org.apache.cassandra.service.accord.api.AccordWaitStrategies.retryFetchWatermarks;
 
@@ -124,14 +124,18 @@ public class WatermarkCollector implements TopologyListener
     };
 
     @VisibleForTesting
-    public static void fetchAndReportWatermarksAsync(TopologyManager topologyManager)
+    public static Future<?> fetchAndReportWatermarksAsync(TopologyManager topologyManager)
     {
         SharedContext context = SharedContext.Global.instance;
-        Set<InetAddressAndPort> peers = new HashSet<>();
-        peers.addAll(ClusterMetadata.current().directory.allAddresses());
-        peers.remove(FBUtilities.getBroadcastAddressAndPort());
+        List<InetAddressAndPort> peers = new ArrayList<>();
+        for (InetAddressAndPort peer : ClusterMetadata.current().directory.allAddresses())
+        {
+            if (!peer.equals(FBUtilities.getBroadcastAddressAndPort()))
+                peers.add(peer);
+        }
+        Collections.shuffle(peers);
 
-        context.messaging().<NoPayload, Snapshot>sendWithRetries(retryFetchWatermarks(),
+        return context.messaging().<NoPayload, Snapshot>sendWithRetries(retryFetchWatermarks(),
                                                                  context.optionalTasks()::schedule,
                                                                  Verb.ACCORD_FETCH_WATERMARKS_REQ,
                                                                  NoPayload.noPayload,

src/java/org/apache/cassandra/service/accord/txn/TxnData.java

@@ -222,13 +222,13 @@ public class TxnData extends Int2ObjectHashMap<TxnDataValue> implements TxnResul
     }
 
     @Override
-    public boolean validateReply(TxnId txnId, Timestamp executeAt, boolean futureReadPossible)
+    public boolean validateReply(TxnId txnId, Timestamp executeAt, long safeToReadHlc)
     {
-        if (futureReadPossible)
+        if (safeToReadHlc > 0)
         {
             for (TxnDataValue value : values())
             {
-                if (value.maxTimestamp() >= executeAt.hlc())
+                if (value.maxTimestamp() >= safeToReadHlc)
                     return false;
             }
         }
@@ -238,7 +238,7 @@ public class TxnData extends Int2ObjectHashMap<TxnDataValue> implements TxnResul
     @Override
     public long estimatedSizeOnHeap()
     {
-        long size = EMPTY_SIZE + (size() * TypeSizes.INT_SIZE);
+        long size = EMPTY_SIZE + ObjectSizes.sizeOfReferenceArray(capacity()) + (capacity() * TypeSizes.INT_SIZE);
         for (TxnDataValue value : values())
             size += value.estimatedSizeOnHeap();
         return size;

src/java/org/apache/cassandra/service/accord/txn/TxnUpdate.java

@@ -759,7 +759,7 @@ public final class TxnUpdate extends AccordUpdate
         public void serialize(TxnUpdate update, TableMetadatasAndKeys tablesAndKeys, DataOutputPlus out, Version version) throws IOException
         {
             // Serializing it with the condition result set shouldn't be needed
-            checkState(update.anyConditionResult == null, "Can't serialize if conditionResult is set without adding it to serialization");
+//            checkState(update.anyConditionResult == null, "Can't serialize if conditionResult is set without adding it to serialization");
             // Once in accord "mixedTimeSource" and "yes" are the same, so only care about the side effect: that the timestamp is preserved or not
             out.writeByte(update.preserveTimestamps.preserve ? FLAG_PRESERVE_TIMESTAMPS : 0);
             tablesAndKeys.serializeKeys(update.keys, out);

src/java/org/apache/cassandra/service/accord/txn/TxnWrite.java

@@ -111,7 +111,9 @@ public class TxnWrite extends AbstractKeySorted<TxnWrite.Update> implements Writ
         public long estimatedSizeOnHeap()
         {
             // we don't measure the key, as this is shared
-            return EMPTY_SIZE + ByteBufferUtil.estimatedSizeOnHeap(unsafeBytes());
+            if (latestVersionBytes == null)
+                return EMPTY_SIZE;
+            return EMPTY_SIZE + ByteBufferUtil.estimatedSizeOnHeap(latestVersionBytes);
         }
 
         @Override
@@ -518,8 +520,8 @@ public class TxnWrite extends AbstractKeySorted<TxnWrite.Update> implements Writ
     public long estimatedSizeOnHeap()
     {
         long size = EMPTY_SIZE;
-        for (Update update : this)
-            size += update.estimatedSizeOnHeap();
+        for (int i = 0 ; i < size() ; ++i)
+            size += get(i).estimatedSizeOnHeap();
         return size;
     }
 

src/java/org/apache/cassandra/tcm/transformations/AlterSchema.java

@@ -36,7 +36,7 @@ import com.google.common.collect.Streams;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import org.apache.cassandra.config.AccordSpec;
+import org.apache.cassandra.config.AccordConfig;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.cql3.statements.schema.AlterSchemaStatement;
 import org.apache.cassandra.dht.Range;
@@ -312,7 +312,7 @@ public class AlterSchema implements Transformation
         if (!started.isEmpty())
         {
             List<Range<Token>> ranges;
-            AccordSpec.TransactionalRangeMigration migration = DatabaseDescriptor.getTransactionalRangeMigration();
+            AccordConfig.TransactionalRangeMigration migration = DatabaseDescriptor.getTransactionalRangeMigration();
             switch (migration)
             {
                 default: throw new IllegalStateException("Unhandled transactional range migration: " + migration);

src/java/org/apache/cassandra/tools/StandaloneJournalUtil.java

@@ -35,7 +35,7 @@ import accord.primitives.Timestamp;
 import accord.primitives.TxnId;
 import accord.utils.Invariants;
 
-import org.apache.cassandra.config.AccordSpec;
+import org.apache.cassandra.config.AccordConfig;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.config.DurationSpec;
 import org.apache.cassandra.db.ColumnFamilyStore;
@@ -266,7 +266,7 @@ public class StandaloneJournalUtil implements Runnable
 
             setAccordJournalDirectory(journalSegments);
             Keyspace.setInitialized();
-            AccordJournal journal = new AccordJournal(new AccordSpec.JournalSpec().setFlushPeriod(new DurationSpec.IntMillisecondsBound("1500ms")), new File(journalSegments).parent(), Keyspace.open(SchemaConstants.ACCORD_KEYSPACE_NAME).getColumnFamilyStore(AccordKeyspace.JOURNAL));
+            AccordJournal journal = new AccordJournal(new AccordConfig.JournalConfig().setFlushPeriod(new DurationSpec.IntMillisecondsBound("1500ms")), new File(journalSegments).parent(), Keyspace.open(SchemaConstants.ACCORD_KEYSPACE_NAME).getColumnFamilyStore(AccordKeyspace.JOURNAL));
 
             Keyspace ks = Schema.instance.getKeyspaceInstance("system_accord");
             ColumnFamilyStore cfs = ks.getColumnFamilyStore("journal");
@@ -388,7 +388,7 @@ public class StandaloneJournalUtil implements Runnable
 
             setAccordJournalDirectory(journalSegments);
             Keyspace.setInitialized();
-            AccordJournal journal = new AccordJournal(new AccordSpec.JournalSpec().setFlushPeriod(new DurationSpec.IntMillisecondsBound("1500ms")), new File(journalSegments).parent(), Keyspace.open(SchemaConstants.ACCORD_KEYSPACE_NAME).getColumnFamilyStore(AccordKeyspace.JOURNAL));
+            AccordJournal journal = new AccordJournal(new AccordConfig.JournalConfig().setFlushPeriod(new DurationSpec.IntMillisecondsBound("1500ms")), new File(journalSegments).parent(), Keyspace.open(SchemaConstants.ACCORD_KEYSPACE_NAME).getColumnFamilyStore(AccordKeyspace.JOURNAL));
 
             Keyspace ks = Schema.instance.getKeyspaceInstance("system_accord");
             ColumnFamilyStore cfs = ks.getColumnFamilyStore("journal");

src/java/org/apache/cassandra/tracing/Tracing.java

@@ -52,7 +52,7 @@ import static org.apache.cassandra.utils.TimeUUID.Generator.nextTimeUUID;
  */
 public abstract class Tracing extends ExecutorLocals.Impl
 {
-    public static final IVersionedSerializer<TraceType> traceTypeSerializer = new IVersionedSerializer<TraceType>()
+    public static final IVersionedSerializer<TraceType> traceTypeSerializer = new IVersionedSerializer<>()
     {
         public void serialize(TraceType traceType, DataOutputPlus out, int version) throws IOException
         {
@@ -258,7 +258,7 @@ public abstract class Tracing extends ExecutorLocals.Impl
 
         TraceType traceType = header.traceType();
 
-        if (header.verb.isResponse())
+        if (header.verb.isManagedResponse())
         {
             // received a message for a session we've already closed out.  see CASSANDRA-5668
             return new ExpiredTraceState(newTraceState(header.from, sessionId, traceType));
@@ -294,7 +294,7 @@ public abstract class Tracing extends ExecutorLocals.Impl
             else
             {
                 state.trace(logMessage);
-                if (message.verb().isResponse())
+                if (message.verb().isManagedResponse())
                     doneWithNonLocalSession(state);
             }
         }

src/java/org/apache/cassandra/utils/Clock.java

@@ -17,10 +17,13 @@
  */
 package org.apache.cassandra.utils;
 
+import java.time.Instant;
 import java.util.concurrent.TimeUnit;
 
 import org.slf4j.Logger;
 
+import accord.utils.Invariants;
+
 import static org.apache.cassandra.config.CassandraRelevantProperties.CLOCK_GLOBAL;
 import static org.apache.cassandra.utils.Shared.Scope.SIMULATION;
 
@@ -108,6 +111,15 @@ public interface Clock
             return instance.currentTimeMillis();
         }
 
+        /**
+         * Semantically equivalent to {@link System#currentTimeMillis()},
+         * but yields a time with microseconds granularity
+         */
+        public static long currentTimeMicros()
+        {
+            return instance.currentTimeMicros();
+        }
+
         /**
          * Semantically equivalent to {@link FBUtilities#nowInSeconds()}
          */
@@ -134,6 +146,19 @@ public interface Clock
         {
             return System.currentTimeMillis(); // checkstyle: permit system clock
         }
+
+        @Override
+        public long currentTimeMicros()
+        {
+            Instant now = Instant.now(); // checkstyle: permit this invocation
+            long seconds = now.getEpochSecond();
+            long nanos = now.getNano();
+            Invariants.require(seconds >= 0);
+            long micros = Math.multiplyExact(seconds, 1000_000);
+            micros = Math.addExact(micros, nanos/1000);
+            return micros;
+
+        }
     }
 
     /**
@@ -146,6 +171,11 @@ public interface Clock
      */
     public long currentTimeMillis();
 
+    public default long currentTimeMicros()
+    {
+        return currentTimeMillis() * 1000L;
+    }
+
     public default long nowInSeconds()
     {
         return currentTimeMillis() / 1000L;

src/java/org/apache/cassandra/utils/MonotonicClock.java

@@ -29,6 +29,7 @@ import org.slf4j.LoggerFactory;
 
 import org.apache.cassandra.concurrent.ScheduledExecutors;
 
+import static java.util.concurrent.TimeUnit.MICROSECONDS;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static org.apache.cassandra.config.CassandraRelevantProperties.APPROXIMATE_TIME_PRECISION_MS;
 import static org.apache.cassandra.config.CassandraRelevantProperties.CLOCK_MONOTONIC_APPROX;
@@ -146,21 +147,28 @@ public interface MonotonicClock
         @VisibleForTesting
         public static class AlmostSameTime implements MonotonicClockTranslation
         {
+            final long microsSinceEpoch;
             final long millisSinceEpoch;
             final long monotonicNanos;
             final long error; // maximum error of millis measurement (in nanos)
 
             @VisibleForTesting
-            public AlmostSameTime(long millisSinceEpoch, long monotonicNanos, long errorNanos)
+            public AlmostSameTime(long sinceEpoch, TimeUnit unitsSinceEpoch, long monotonicNanos, long errorNanos)
             {
-                this.millisSinceEpoch = millisSinceEpoch;
+                this.microsSinceEpoch = unitsSinceEpoch.toMicros(sinceEpoch);
+                this.millisSinceEpoch = unitsSinceEpoch.toMillis(sinceEpoch);
                 this.monotonicNanos = monotonicNanos;
                 this.error = errorNanos;
             }
 
-            public long fromMillisSinceEpoch(long currentTimeMillis)
+            public long fromMillisSinceEpoch(long millisSinceEpoch)
             {
-                return monotonicNanos + MILLISECONDS.toNanos(currentTimeMillis - millisSinceEpoch);
+                return monotonicNanos + MILLISECONDS.toNanos(millisSinceEpoch - this.millisSinceEpoch);
+            }
+
+            public long fromMicrosSinceEpoch(long microsSinceEpoch)
+            {
+                return monotonicNanos + MICROSECONDS.toNanos(microsSinceEpoch - this.microsSinceEpoch);
             }
 
             public long toMillisSinceEpoch(long nanoTime)
@@ -174,15 +182,17 @@ public interface MonotonicClock
             }
         }
 
-        final LongSupplier millisSinceEpoch;
+        final LongSupplier sinceEpoch;
+        final TimeUnit sinceEpochUnits;
 
-        private volatile AlmostSameTime almostSameTime = new AlmostSameTime(0L, 0L, Long.MAX_VALUE);
+        private volatile AlmostSameTime almostSameTime = new AlmostSameTime(0L, MILLISECONDS, 0L, Long.MAX_VALUE);
         private Future<?> almostSameTimeUpdater;
         private static double failedAlmostSameTimeUpdateModifier = 1.0;
 
-        AbstractEpochSamplingClock(LongSupplier millisSinceEpoch)
+        AbstractEpochSamplingClock(LongSupplier sinceEpoch, TimeUnit sinceEpochUnits)
         {
-            this.millisSinceEpoch = millisSinceEpoch;
+            this.sinceEpoch = sinceEpoch;
+            this.sinceEpochUnits = sinceEpochUnits;
             resumeEpochSampling();
         }
 
@@ -217,7 +227,7 @@ public interface MonotonicClock
             samples[0] = nanoTime();
             for (int i = 1 ; i < samples.length ; i += 2)
             {
-                samples[i] = millisSinceEpoch.getAsLong();
+                samples[i] = sinceEpoch.getAsLong();
                 samples[i + 1] = now();
             }
 
@@ -229,12 +239,12 @@ public interface MonotonicClock
                     best = i;
             }
 
-            long millis = samples[best];
+            long since = samples[best];
             long nanos = (samples[best+1] / 2) + (samples[best-1] / 2);
             long error = (samples[best+1] / 2) - (samples[best-1] / 2);
 
             AlmostSameTime prev = almostSameTime;
-            AlmostSameTime next = new AlmostSameTime(millis, nanos, error);
+            AlmostSameTime next = new AlmostSameTime(since, sinceEpochUnits, nanos, error);
 
             if (next.error > prev.error && next.error > prev.error * failedAlmostSameTimeUpdateModifier)
             {
@@ -258,7 +268,7 @@ public interface MonotonicClock
         // class to ACC_PUBLIC, and ensured proper testing relationship from both the surrounding and nested class.
         public SystemClock()
         {
-            super(Clock.Global::currentTimeMillis);
+            super(Clock.Global::currentTimeMicros, MICROSECONDS);
         }
 
         @Override