The SimpleProgressLog had a number of problems:
1. It polled for progress with no attempt to determine whether progress could realistically be made, so:
- as the number of pending transactions grew, the proportion of useful work dropped (as many would be unable to make progress without earlier transactions completing)
- each transaction in the chain could recover only on average 1/2 poll interval behind the last transaction to complete
2. It requested full transaction state from every replica on each attempt
3. It maintained a lot of in-memory state
4. Polling happened en-masse, allowing for little per-transaction control
We also separately maintained fairly expensive per-command listener state that negatively affected our command loading and caching.
The new DefaultProgressLog makes use of several new features: LocalListeners, RemoteListeners, Timers and Await messages.
- LocalListeners provide a memory-efficient collection for managing each CommandStore<E2><80><99>s transaction listeners, with dedicated record keeping for inter-transaction relationships.
- RemoteListeners provide a mechanism for request/response pairs that may be separated by longer than the normal Cassandra message timeout, and require minimal state on sender and recipient. This permits replicas to cheaply update their local state machine as soon as distributed information becomes available.
The DefaultProgressLog tracks each transaction with separate timers to handle per-transaction scheduling, backoff etc, and a succinct state machine. To reduce overhead correspondence is preferentially limited to a handful of replicas, and limited to the home shard where appropriate.
patch by Benedict; reviewed by Ariel Weisberg for CASSANDRA-19870
- Remove concept of non-participating home keys; home keys are required to be a participant in the transaction
- Remove covering/covers concept
- Various invalidation/truncation/erase behaviours
patch by Benedict; reviewed by Blake for CASSANDRA-19825
CommandsForKey periodically self-prunes, so as to continue functioning well in-between garbage collections. Once we prune we are left with potentially incomplete information, and have to sometimes load per-command information from disk. But the payoff is ensuring CommandsForKey objects - which drive the majority of the state machine - are kept to a reasonable size.
patch by Benedict; reviewed by Blake Eggleston and David Capwell
Introduce a special kind of non-durable read that provides only per-key linearizable isolation; i.e. strict-serializable isolation for single partition-key reads.
This read creates only a happens-before edge, by collecting dependencies for execution and ensuring that execution happens strictly after these dependencies
have executed, but at no precise time otherwise. So later writes may be witnessed, and if multiple keys are read they may represents different points in time.
patch by Benedict; reviewed by Ariel Weisberg for CASSANDRA-19305
Refactor CommandsForKey for efficiency, and to support transitive dependency elision
patch by Benedict; reviewed by Aleksey Yeshchenko for CASSANDRA-19310
David Capwell