Joseph Lynch created CASSANDRA-13924:
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Summary: Continuous/Infectious Repair
Key: CASSANDRA-13924
URL: https://issues.apache.org/jira/browse/CASSANDRA-13924
Project: Cassandra
Issue Type: Improvement
Components: Repair
Reporter: Joseph Lynch
Priority: Minor
I've been working on a way to keep data consistent without
scheduled/external/manual repair, because for large datasets repair is
extremely expensive. The basic gist is to introduce a new kind of hint that
keeps just the primary key of the mutation (indicating that PK needs repair)
and is recorded on replicas instead of coordinators during write time. Then a
periodic background task can issue read repairs to just the PKs that were
mutated. The initial performance degradation of this approach is non trivial,
but I believe that I can optimize it so that we are doing very little
additional work (see below in the design doc for some proposed optimizations).
My extremely rough proof of concept (uses a local table instead of HintStorage,
etc) so far is [in a
branch|https://github.com/apache/cassandra/compare/cassandra-3.11...jolynch:continuous_repair]
and has a rough [design
document|https://github.com/jolynch/cassandra/blob/c597c0fc6415e00fa8db180be5034214d148822d/doc/source/architecture/continuous_repair.rst].
I'm working on getting benchmarks of the various optimizations, but I figured
I should start this ticket before I got too deep into it.
I believe this approach is particularly good for high read rate clusters
requiring consistent low latency, and for clusters that mutate a relatively
small proportion of their data (since you never have to read the whole dataset,
just what's being mutated). I view this as something that works _with_
incremental repair to reduce work required because with this technique we could
potentially flush repaired + unrepaired sstables directly from the memtable. I
also see this as something that would be enabled or disabled per table since it
is so use case specific (e.g. some tables don't need repair at all). I think
this is somewhat of a hybrid approach based on incremental repair, ticklers
(read all partitions @ ALL), mutation based repair (CASSANDRA-8911), and hinted
handoff. There are lots of tradeoffs, but I think it's worth talking about.
If anyone has feedback on the idea, I'd love to chat about it. [~bdeggleston],
[~aweisberg] I chatted with you guys a bit about this at NGCC; if you have time
I'd love to continue that conversation here.
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