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https://issues.apache.org/jira/browse/SOLR-5872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16106025#comment-16106025
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David Smiley commented on SOLR-5872:
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Is it really true that every Solr node subscribes/watches to ZK state changes
to all collections all the time, _even to Collections that have no replicas on
the current node_? Albert's comment
https://issues.apache.org/jira/browse/SOLR-5872?focusedCommentId=15890021&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15890021
indicate this is so. I could understand doing this to query collections on
different nodes but I think such watches should expire if not continuously
utilized. Is there another JIRA issue about this?
> Eliminate overseer queue
> -------------------------
>
> Key: SOLR-5872
> URL: https://issues.apache.org/jira/browse/SOLR-5872
> Project: Solr
> Issue Type: Improvement
> Components: SolrCloud
> Reporter: Noble Paul
> Assignee: Noble Paul
>
> The overseer queue is one of the busiest points in the entire system. The
> raison d'être of the queue is
> * Provide batching of operations for the main clusterstate,json so that
> state updates are minimized
> * Avoid race conditions and ensure order
> Now , as we move the individual collection states out of the main
> clusterstate.json, the batching is not useful anymore.
> Race conditions can easily be solved by using a compare and set in Zookeeper.
> The proposed solution is , whenever an operation is required to be performed
> on the clusterstate, the same thread (and of course the same JVM)
> # read the fresh state and version of zk node
> # construct the new state
> # perform a compare and set
> # if compare and set fails go to step 1
> This should be limited to all operations performed on external collections
> because batching would be required for others
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