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https://issues.apache.org/jira/browse/SOLR-8323?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15283882#comment-15283882
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Alan Woodward commented on SOLR-8323:
-------------------------------------
OK, I think I see what's happening.
The test waits for a collection to be up, and then registers a watch to check
for subsequent changes. Once a wait has returned, then the watch is removed,
but the collection isn't actually removed from the 'interesting' list until the
next state update.
We have a race between state watchers being cleared after firing and the
subsequent removal of a collection from the 'interesting' list, and the new
watcher being added. On a fast machine, the new watcher is added before the
old one is cleared, and so the state is preserved between the calls; this means
that the new watcher isn't actually fired, because we check if the state has
changed before running notifications. On a slower machine, the watcher is
added afterwards, so there's no previous state to compare against, so the
watcher is fired immediately - resulting in a test failure, because the test is
expecting the state to have changed.
We can fix the test by calling registerCore() first, which puts the collection
permanently on the watched list, but I think there's a bigger question here
about how useful statewatchers are, as opposed to state predicate checks. All
the uses I've come up with so far have just been waitForState() calls.
[~dragonsinth] what do you think?
> Add CollectionWatcher API to ZkStateReader
> ------------------------------------------
>
> Key: SOLR-8323
> URL: https://issues.apache.org/jira/browse/SOLR-8323
> Project: Solr
> Issue Type: Improvement
> Affects Versions: 6.0
> Reporter: Alan Woodward
> Assignee: Alan Woodward
> Fix For: 6.1
>
> Attachments: SOLR-8323.patch, SOLR-8323.patch, SOLR-8323.patch,
> SOLR-8323.patch, SOLR-8323.patch
>
>
> An API to watch for changes to collection state would be a generally useful
> thing, both internally and for client use.
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