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Sylvain Lebresne commented on CASSANDRA-13910:
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bq. +1 to creating a thread on @dev / @user for feed back
Not sure everybody saw it, but I did sent an email to the user list a few days
ago
[here|https://mail-archives.apache.org/mod_mbox/cassandra-user/201709.mbox/%3CCAKkz8Q3h9KENiBrsxzPLJfN9oLdgJago8BOmYMuTFkyOkyJm-Q%40mail.gmail.com%3E].
No feedback on that threads just yet but happy to leave it at least 1-2 more
weeks before making any move.
bq. I would believe that someone, somewhere is relying on it.
C* is successful enough that anything that has been in the product for some
amount of time is probably relied upon by someone, somewhere,, for some
definition of "rely". Guaranteeing that it's not the case as a bar for removing
anything would amount to remove nothing, and that would, imo, be dangerous for
the project. But don't get me wrong, I'm totally happy to wait more time to see
if we get at least a few genuine real-life reports of cases I haven't though
about where it's clear those options provide a good trade-off. Short of that
though, I suggest we move ahead with this rather than keep something we agree
is more harmful than helpful most of the time (and we seem to more or less
agree on that) on the off chance this may piss off somebody somewhere.
> 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
> Labels: CommunityFeedbackRequested
>
> 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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