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https://issues.apache.org/jira/browse/SOLR-11535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17770574#comment-17770574
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Michael Gibney edited comment on SOLR-11535 at 9/29/23 8:29 PM:
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This is still a problem! This issue was filed a long time ago, so it's possible
the exact mechanism was different at the time of initial reporting. But as of
now, basically new {{StateWatcher}} instances are created based on whether a
registered {{DocCollectionWatcher}} caused a new entry to be [added to
collectionWatches|https://github.com/apache/solr/blob/240ae14962a62192fedaea48d07590dd15ff1891/solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java#L1755-L1769].
When an entry is [removed from
collectionWatches|https://github.com/apache/solr/blob/240ae14962a62192fedaea48d07590dd15ff1891/solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java#L1987-L1992],
it leaves it's associated {{StateWatcher}} in place. The {{StateWatcher}} is
then supposed to find that its collection is [no longer represented in
collectionWatches|https://github.com/apache/solr/blob/240ae14962a62192fedaea48d07590dd15ff1891/solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java#L1332-L1336],
and exit (not resetting itself). But if another entry has been added to
collectionWatches in the meantime, the old StateWatcher will continue resetting
itself indefinitely, and a new StateWatcher will have been added for the entry
when it was created.
The essence of the fix is described
[here|https://github.com/apache/solr/pull/1964#discussion_r1341703136].
was (Author: mgibney):
This is still a problem! This issue was filed a long time ago, so it's possible
the exact mechanism was different at the time of initial reporting. But as of
now, basically new {{StateWatcher}} instances are created based on whether a
registered {{DocCollectionWatcher}} caused a new entry to be [added to
collectionWatches|https://github.com/apache/solr/blob/240ae14962a62192fedaea48d07590dd15ff1891/solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java#L1755-L1769].
When an entry is [removed from
collectionWatches|https://github.com/apache/solr/blob/240ae14962a62192fedaea48d07590dd15ff1891/solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java#L1987-L1992],
it leaves it's associated {{StateWatcher}} in place. The {{StateWatcher}} is
then supposed to find that its collection is [no longer represented in
collectionWatches|https://github.com/apache/solr/blob/240ae14962a62192fedaea48d07590dd15ff1891/solr/solrj-zookeeper/src/java/org/apache/solr/common/cloud/ZkStateReader.java#L1332-L1336],
and not exit (not resetting itself). But if another entry has been added to
collectionWatches in the meantime, the old StateWatcher will continue resetting
itself indefinitely, and a new StateWatcher will have been added for the entry
when it was created.
The essence of the fix is described
[here|https://github.com/apache/solr/pull/1964#discussion_r1341703136].
> 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
> Attachments: test.log
>
> Time Spent: 20m
> 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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