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https://issues.apache.org/jira/browse/SOLR-13486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17005057#comment-17005057
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Erick Erickson commented on SOLR-13486:
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Thanks [~ab], that's what I thought too, I'll do so.
[~noble.paul] What Hoss saw _may_ be an artifact of the fact that the cluster
was being created/destroyed _between_ tests. So far when I only run a single
test at a time I'm not seeing failures, but that's not very conclusive at this
point.
I intend to run both tests on a couple of machines several thousand times
overnight, will post an update.
[~hossman] [~caomanhdat] Let's claim that I can't reproduce this overnight in
3,000-4,000 runs when both tests are enabled (and note that I was able to
fairly reliably make something bad happen within a few hundred runs up until
this change). I propose that I open a new JIRA for the test failures and push
my changes from there, then rename this one because what I'm doing does nothing
to address the analysis Hoss did. It's still possible that what Hoss saw was a
result of destroying/recreating the cluster between tests, but if so that's
something that seems less of a concern.
> race condition between leader's "replay on startup" and non-leader's "recover
> from leader" can leave replicas out of sync (TestCloudConsistency)
> ------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: SOLR-13486
> URL: https://issues.apache.org/jira/browse/SOLR-13486
> Project: Solr
> Issue Type: Bug
> Reporter: Chris M. Hostetter
> Assignee: Erick Erickson
> Priority: Major
> Attachments:
> apache_Lucene-Solr-BadApples-NightlyTests-master_61.log.txt.gz,
> apache_Lucene-Solr-BadApples-Tests-8.x_102.log.txt.gz
>
>
> I've been investigating some jenkins failures from TestCloudConsistency,
> which at first glance suggest a problem w/replica(s) recovering after a
> network partition from the leader - but in digging into the logs the root
> cause acturally seems to be a thread race conditions when a replica (the
> leader) is first registered...
> * The {{ZkContainer.registerInZk(...)}} method (which is called by
> {{CoreContainer.registerCore(...)}} & {{CoreContainer.load()}}) is typically
> run in a background thread (via the {{ZkContainer.coreZkRegister}}
> ExecutorService)
> * {{ZkContainer.registerInZk(...)}} delegates to
> {{ZKController.register(...)}} which is ultimately responsible for checking
> if there are any "old" tlogs on disk, and if so handling the "Replaying tlog
> for <URL> during startup" logic
> * Because this happens in a background thread, other logic/requests can be
> handled by this core/replica in the meantime - before it starts (or while in
> the middle of) replaying the tlogs
> ** Notably: *leader's that have not yet replayed tlogs on startup will
> erroneously respond to RTG / Fingerprint / PeerSync requests from other
> replicas w/incomplete data*
> ...In general, it seems scary / fishy to me that a replica can (aparently)
> become *ACTIVE* before it's finished it's {{registerInZk}} + "Replaying tlog
> ... during startup" logic ... particularly since this can happen even for
> replicas that are/become leaders. It seems like this could potentially cause
> a whole host of problems, only one of which manifests in this particular test
> failure:
> * *BEFORE* replicaX's "coreZkRegister" thread reaches the "Replaying tlog
> ... during startup" check:
> ** replicaX can recognize (via zk terms) that it should be the leader(X)
> ** this leaderX can then instruct some other replicaY to recover from it
> ** replicaY can send RTG / PeerSync / FetchIndex requests to the leaderX
> (either on it's own volition, or because it was instructed to by leaderX) in
> an attempt to recover
> *** the responses to these recovery requests will not include updates in the
> tlog files that existed on leaderX prior to startup that hvae not yet been
> replayed
> * *AFTER* replicaY has finished it's recovery, leaderX's "Replaying tlog ...
> during startup" can finish
> ** replicaY now thinks it is in sync with leaderX, but leaderX has
> (replayed) updates the other replicas know nothing about
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