mirror of https://github.com/apache/cassandra
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156 lines
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.. Licensed to the Apache Software Foundation (ASF) under one
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.. or more contributor license agreements. See the NOTICE file
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.. distributed with this work for additional information
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.. regarding copyright ownership. The ASF licenses this file
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.. to you under the Apache License, Version 2.0 (the
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.. "License"); you may not use this file except in compliance
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.. with the License. You may obtain a copy of the License at
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..
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.. http://www.apache.org/licenses/LICENSE-2.0
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..
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.. Unless required by applicable law or agreed to in writing, software
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.. distributed under the License is distributed on an "AS IS" BASIS,
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.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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.. See the License for the specific language governing permissions and
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.. limitations under the License.
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Transient Replication
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---------------------
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**Note**:
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Transient Replication (`CASSANDRA-14404
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<https://issues.apache.org/jira/browse/CASSANDRA-14404>`_) is an experimental feature designed for expert Apache Cassandra users who are able to validate every aspect of the database for their application and deployment.
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That means being able to check that operations like reads, writes, decommission, remove, rebuild, repair, and replace all work with your queries, data, configuration, operational practices, and availability requirements.
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Apache Cassandra 4.0 has the initial implementation of transient replication. Future releases of Cassandra will make this feature suitable for a wider audience.
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It is anticipated that a future version will support monotonic reads with transient replication as well as LWT, logged batches, and counters. Being experimental, Transient replication is **not** recommended for production use.
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Objective
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^^^^^^^^^
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The objective of transient replication is to decouple storage requirements from data redundancy (or consensus group size) using incremental repair, in order to reduce storage overhead.
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Certain nodes act as full replicas (storing all the data for a given token range), and some nodes act as transient replicas, storing only unrepaired data for the same token ranges.
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The optimization that is made possible with transient replication is called "Cheap quorums", which implies that data redundancy is increased without corresponding increase in storage usage.
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Transient replication is useful when sufficient full replicas are available to receive and store all the data.
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Transient replication allows you to configure a subset of replicas to only replicate data that hasn't been incrementally repaired.
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As an optimization, we can avoid writing data to a transient replica if we have successfully written data to the full replicas.
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After incremental repair, transient data stored on transient replicas can be discarded.
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Enabling Transient Replication
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Transient replication is not enabled by default. Transient replication must be enabled on each node in a cluster separately by setting the following configuration property in ``cassandra.yaml``.
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::
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enable_transient_replication: true
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Transient replication may be configured with both ``SimpleStrategy`` and ``NetworkTopologyStrategy``. Transient replication is configured by setting replication factor as ``<total_replicas>/<transient_replicas>``.
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As an example, create a keyspace with replication factor (RF) 3.
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::
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CREATE KEYSPACE CassandraKeyspaceSimple WITH replication = {'class': 'SimpleStrategy',
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'replication_factor' : 3/1};
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As another example, ``some_keysopace keyspace`` will have 3 replicas in DC1, 1 of which is transient, and 5 replicas in DC2, 2 of which are transient:
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::
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CREATE KEYSPACE some_keysopace WITH replication = {'class': 'NetworkTopologyStrategy',
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'DC1' : '3/1'', 'DC2' : '5/2'};
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Transiently replicated keyspaces only support tables with ``read_repair`` set to ``NONE``.
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Important Restrictions:
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- RF cannot be altered while some endpoints are not in a normal state (no range movements).
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- You can't add full replicas if there are any transient replicas. You must first remove all transient replicas, then change the # of full replicas, then add back the transient replicas.
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- You can only safely increase number of transients one at a time with incremental repair run in between each time.
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Additionally, transient replication cannot be used for:
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- Monotonic Reads
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- Lightweight Transactions (LWTs)
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- Logged Batches
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- Counters
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- Keyspaces using materialized views
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- Secondary indexes (2i)
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Cheap Quorums
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^^^^^^^^^^^^^
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Cheap quorums are a set of optimizations on the write path to avoid writing to transient replicas unless sufficient full replicas are not available to satisfy the requested consistency level.
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Hints are never written for transient replicas. Optimizations on the read path prefer reading from transient replicas.
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When writing at quorum to a table configured to use transient replication the quorum will always prefer available full
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replicas over transient replicas so that transient replicas don't have to process writes. Tail latency is reduced by
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rapid write protection (similar to rapid read protection) when full replicas are slow or unavailable by sending writes
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to transient replicas. Transient replicas can serve reads faster as they don't have to do anything beyond bloom filter
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checks if they have no data. With vnodes and large cluster sizes they will not have a large quantity of data
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even for failure of one or more full replicas where transient replicas start to serve a steady amount of write traffic
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for some of their transiently replicated ranges.
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Speculative Write Option
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^^^^^^^^^^^^^^^^^^^^^^^^
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The ``CREATE TABLE`` adds an option ``speculative_write_threshold`` for use with transient replicas. The option is of type ``simple`` with default value as ``99PERCENTILE``. When replicas are slow or unresponsive ``speculative_write_threshold`` specifies the threshold at which a cheap quorum write will be upgraded to include transient replicas.
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Pending Ranges and Transient Replicas
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Pending ranges refers to the movement of token ranges between transient replicas. When a transient range is moved, there
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will be a period of time where both transient replicas would need to receive any write intended for the logical
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transient replica so that after the movement takes effect a read quorum is able to return a response. Nodes are *not*
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temporarily transient replicas during expansion. They stream data like a full replica for the transient range before they
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can serve reads. A pending state is incurred similar to how there is a pending state for full replicas. Transient replicas
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also always receive writes when they are pending. Pending transient ranges are sent a bit more data and reading from
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them is avoided.
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Read Repair and Transient Replicas
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Read repair never attempts to repair a transient replica. Reads will always include at least one full replica.
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They should also prefer transient replicas where possible. Range scans ensure the entire scanned range performs
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replica selection that satisfies the requirement that every range scanned includes one full replica. During incremental
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& validation repair handling, at transient replicas anti-compaction does not output any data for transient ranges as the
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data will be dropped after repair, and transient replicas never have data streamed to them.
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Transitioning between Full Replicas and Transient Replicas
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The additional state transitions that transient replication introduces requires streaming and ``nodetool cleanup`` to
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behave differently. When data is streamed it is ensured that it is streamed from a full replica and not a transient replica.
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Transitioning from not replicated to transiently replicated means that a node must stay pending until the next incremental
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repair completes at which point the data for that range is known to be available at full replicas.
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Transitioning from transiently replicated to fully replicated requires streaming from a full replica and is identical
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to how data is streamed when transitioning from not replicated to replicated. The transition is managed so the transient
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replica is not read from as a full replica until streaming completes. It can be used immediately for a write quorum.
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Transitioning from fully replicated to transiently replicated requires cleanup to remove repaired data from the transiently
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replicated range to reclaim space. It can be used immediately for a write quorum.
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Transitioning from transiently replicated to not replicated requires cleanup to be run to remove the formerly transiently replicated data.
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When transient replication is in use ring changes are supported including add/remove node, change RF, add/remove DC.
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Transient Replication supports EACH_QUORUM
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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(`CASSANDRA-14727
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<https://issues.apache.org/jira/browse/CASSANDRA-14727>`_) adds support for Transient Replication support for ``EACH_QUORUM``. Per (`CASSANDRA-14768
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<https://issues.apache.org/jira/browse/CASSANDRA-14768>`_), we ensure we write to at least a ``QUORUM`` of nodes in every DC,
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regardless of how many responses we need to wait for and our requested consistency level. This is to minimally surprise
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users with transient replication; with normal writes, we soft-ensure that we reach ``QUORUM`` in all DCs we are able to,
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by writing to every node; even if we don't wait for ACK, we have in both cases sent sufficient messages.
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