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https://issues.apache.org/jira/browse/SOLR-11535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17866054#comment-17866054
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David Smiley commented on SOLR-11535:
-------------------------------------
{quote}(CHANGES.txt) Fix race condition in singleton-per-collection
StateWatcher creation
{quote}
Ideally, CHANGES.txt communicates information in ways a user has a chance of
understanding / how they would experience the change. This one is unclear to
me. It appears the change ultimately affects scalability maybe particularly
for high-core count?
> Weird behavior of CollectionStateWatcher
> ----------------------------------------
>
> Key: SOLR-11535
> URL: https://issues.apache.org/jira/browse/SOLR-11535
> Project: Solr
> Issue Type: Bug
> Affects Versions: 7.2, 8.0
> Reporter: Andrzej Bialecki
> Assignee: Michael Gibney
> Priority: Major
> Fix For: 9.6
>
> Attachments: test.log
>
> Time Spent: 1.5h
> Remaining Estimate: 0h
>
> While working on SOLR-11320 I noticed a strange behavior in
> {{ActiveReplicaWatcher}}, which is a subclass of {{CollectionStateWatcher}} -
> it appears that its {{onStateChanged}} method can be called from multiple
> threads with exactly the same {{DocCollection}} state, ie. unchanged between
> the calls.
> This seems to run contrary to the javadoc, which implies that this method is
> called only when the state actually changes, and it also doesn't mention
> anything about the need for thread-safety in the method implementation.
> I attached the log, which has a lot of additional debugging - but the most
> pertinent part being where a Watcher-s hashCode is printed together with the
> {{DocCollection}} - notice that these overlapping calls both submit an
> instance of {{DocCollection}} with the same zkVersion.
> [~dragonsinth], [~romseygeek] - could you please take a look at this? If this
> behavior is expected then the javadoc should be updated to state clearly that
> multiple calls can be made concurrently, with exactly the same state (which
> is kind of a weak guarantee for a method called {{onStateChanged}} ;) ).
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