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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