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Transient Replication 4.0 status update


Hi all,

I wanted to give everyone an update on how development of Transient Replication is going and where we are going to be as of 9/1. Blake Eggleston, Alex Petrov, Benedict Elliott Smith, and myself have been working to get TR implemented for 4.0. Up to now we have avoided merging anything related to TR to trunk because we weren't 100% sure we were going to make the 9/1 deadline and even minimal TR functionality requires significant changes (see 14405).

We focused on getting a minimal set of deployable functionality working, and want to avoid overselling what's going to work in the first version. The feature is marked explicitly as experimental and has to be enabled via a feature flag in cassandra.yaml. The expected audience for TR in 4.0 is more experienced users who are ready to tackle deploying experimental functionality. As it is deployed by experienced users and we gain more confidence in it and remove caveats the # of users it will be appropriate for will expand.

For 4.0 it looks like we will be able to merge TR with support for normal reads and writes without monotonic reads. Monotonic reads require blocking read repair and blocking read repair with TR requires further changes that aren't feasible by 9/1.

Future TR support would look something like

4.0.next:
    * vnodes (https://issues.apache.org/jira/browse/CASSANDRA-14404)

4.next:
    * Monotonic reads (https://issues.apache.org/jira/browse/CASSANDRA-14665)
    * LWT (https://issues.apache.org/jira/browse/CASSANDRA-14547)
    * Batch log (https://issues.apache.org/jira/browse/CASSANDRA-14549)
    * Counters (https://issues.apache.org/jira/browse/CASSANDRA-14548)

Possibly never:
    * Materialized views
 
Probably never:
    * Secondary indexes

The most difficult changes to support Transient Replication should be behind us. LWT, Batch log, and counters shouldn't be that hard to make transient replication aware. Monotonic reads require some changes to the read path, but are at least conceptually not that hard to support. I am confident that by 4.next TR will have fewer tradeoffs.

If you want to take a peek the current feature branch is https://github.com/aweisberg/cassandra/tree/14409-7 although we will be moving to 14409-8 to rebase on to trunk.

Regards,
Ariel

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