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https://issues.apache.org/jira/browse/SOLR-13491?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16848250#comment-16848250
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Erick Erickson commented on SOLR-13491:
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We're routinely seeing many hundreds of replicas, so I should think anything we
can do to reduce the load on ZK would be A Good Thing.
[~gus_heck] Do you have any inkling of whether moving to ZK 3.5.5 would affect
either the problem or the solution? I'm fairly close to being able to commit
SOLR-8346.
> SolrZkClient can register the same watch twice.
> -----------------------------------------------
>
> Key: SOLR-13491
> URL: https://issues.apache.org/jira/browse/SOLR-13491
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: SolrCloud
> Affects Versions: 8.0
> Reporter: Gus Heck
> Priority: Major
>
> While working on SOLR-13420 and related tickets I noticed that we presently
> use the wrapWatch() method to wrap all zookeeper watches. This allows the
> watch to be processed asynchronously by an executor which is good, but it
> subtly changes the way zookeeper behaves. Normally zookeeper keeps a
> Map<String,Set<Watch>> that ensures that if the same watch is registered
> twice it's only notified once. However, our wrapper breaks this because now,
> if the same watch is added twice each one gets it's own wrapper with default
> Object.equals() and thus the two wrapped versions of the Watch aren not
> .equals() and may co-exist in the Set<Watch>
> I do not know of any current cases for which this would cause a problem, but
> changes I am contemplating for SOLR-13375 are likely to hit this case and at
> a minimum create wasted re-notifications of the same event and excess watch
> load in Zk.
> Moving the anonymous class wrapping to an explicit inner class and writing
> hashcode/equals carefully to act as a pass through such that any to wrappers
> with the same watch object are .equals() should fix this.
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