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https://issues.apache.org/jira/browse/SOLR-5872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14658807#comment-14658807
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Scott Blum commented on SOLR-5872:
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Now that SOLR-5756 is close to landed, I want to take a serious stab at making
updates to format2 collections not go through overseer. IE, anything that
modifies clusterstate.json goes through overseer, but anything that modifies a
/collection/foo/state.json would be handled by the local node with a CAS loop.
I realize that for a collection with a huge number of shards+replicas, there
could be contention on that single node. Worth nothing that the current
implementation doesn't batch format2 updates anyway, it ends up doing a
(non-contended) write for every individual mutation.
> 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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