Sylvain Lebresne created CASSANDRA-13910:
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Summary: Consider deprecating (then removing)
read_repair_chance/dclocal_read_repair_chance
Key: CASSANDRA-13910
URL: https://issues.apache.org/jira/browse/CASSANDRA-13910
Project: Cassandra
Issue Type: Improvement
Reporter: Sylvain Lebresne
Priority: Minor
First, let me clarify so this is not misunderstood that I'm not *at all*
suggesting to remove the read-repair mechanism of detecting and repairing
inconsistencies between read responses: that mechanism is imo fine and useful.
But the {{read_repair_chance}} and {{dclocal_read_repair_chance}} have never
been about _enabling_ that mechanism, they are about querying all replicas
(even when this is not required by the consistency level) for the sole purpose
of maybe read-repairing some of the replica that wouldn't have been queried
otherwise. Which btw, bring me to reason 1 for considering their removal: their
naming/behavior is super confusing. Over the years, I've seen countless users
(and not only newbies) misunderstanding what those options do, and as a
consequence misunderstand when read-repair itself was happening.
But my 2nd reason for suggesting this is that I suspect
{{read_repair_chance}}/{{dclocal_read_repair_chance}} are, especially nowadays,
more harmful than anything else when enabled. When those option kick in, what
you trade-off is additional resources consumption (all nodes have to execute
the read) for a _fairly remote chance_ of having some inconsistencies repaired
on _some_ replica _a bit faster_ than they would otherwise be. To justify that
last part, let's recall that:
# most inconsistencies are actually fixed by hints in practice; and in the case
where a node stay dead for a long time so that hints ends up timing-out, you
really should repair the node when it comes back (if not simply
re-bootstrapping it). Read-repair probably don't fix _that_ much stuff in the
first place.
# again, read-repair do happen without those options kicking in. If you do
reads at {{QUORUM}}, inconsistencies will eventually get read-repaired all the
same. Just a tiny bit less quickly.
# I suspect almost everyone use a low "chance" for those options at best
(because the extra resources consumption is real), so at the end of the day,
it's up to chance how much faster this fixes inconsistencies.
Overall, I'm having a hard time imagining real cases where that trade-off
really make sense. Don't get me wrong, those options had their places a long
time ago when hints weren't working all that well, but I think they bring more
confusion than benefits now.
And I think it's sane to reconsider stuffs every once in a while, and to clean
up anything that may not make all that much sense anymore, which I think is the
case here.
Tl;dr, I feel the benefits brought by those options are very slim at best and
well overshadowed by the confusion they bring, and not worth maintaining the
code that supports them (which, to be fair, isn't huge, but getting rid of
{{ReadCallback.AsyncRepairRunner) wouldn't hurt for instance).
Lastly, if the consensus here ends up being that they can have their use in
weird case and that we fill supporting those cases is worth confusing everyone
else and maintaining that code, I would still suggest disabling them totally by
default.
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