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https://issues.apache.org/jira/browse/SOLR-5872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13939413#comment-13939413
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Mark Miller commented on SOLR-5872:
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bq. is to move the replica states out into their own ZK nodes.
That is also how I first implemented the clusterstate - it was super slow to
read the state and required a ridiculous number of watchers. Now that they have
some options to read multiple nodes in one call, it may be that you can work
around some of the issues we had, but it was really only good for the case
where you had small changes in state to read - users had real issues with
performance otherwise and that is why we moved to clusterstate.json.
It's a similar issue - we have been there before, we moved because of tough
issues, it's should be a high bar to go back.
> 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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