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https://issues.apache.org/jira/browse/CASSANDRA-13910?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Aleksey Yeschenko updated CASSANDRA-13910:
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Resolution: Fixed
Status: Resolved (was: Ready to Commit)
> Remove 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
> Assignee: Aleksey Yeschenko
> Priority: Minor
> Fix For: 4.0
>
>
> 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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