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From "Scott Blum (JIRA)" <>
Subject [jira] [Commented] (SOLR-5872) Eliminate overseer queue
Date Wed, 05 Aug 2015 20:12:05 GMT


Scott Blum commented on SOLR-5872:

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:
>             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'ĂȘtre
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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