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https://issues.apache.org/jira/browse/SOLR-11535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16440480#comment-16440480
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Tomás Fernández Löbbe commented on SOLR-11535:
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This may be related to the fact that we execute ZooKeeper watches in a
multi-thread executor, so if multiple threads in parallel could call the
{{StateWatcher.refreshAndWatch}} method concurrently (coming from two watches
fired on the same znode). Note that we re-set the watch when we call
{{fetchCollectionState}}. This is similar to what I saw while working on
SOLR-12172.
Maybe the solution is to not call {{constructState}} when
{{updateWatchedCollection}} returns {{false}}. Other option I think could be to
put
{code:java}
DocCollection newState = fetchCollectionState(coll, this);
updateWatchedCollection(coll, newState);
{code}
inside the synchronize. Other option is to document that the watchers could be
called more than once for the same data and make the watcher call handle it.
Maybe [~shalinmangar] also has some thoughts.
> Weird behavior of CollectionStateWatcher
> ----------------------------------------
>
> Key: SOLR-11535
> URL: https://issues.apache.org/jira/browse/SOLR-11535
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Affects Versions: 7.2, master (8.0)
> Reporter: Andrzej Bialecki
> Priority: Major
> Attachments: test.log
>
>
> 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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