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From "Scott Blum (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-5872) Eliminate overseer queue
Date Mon, 17 Aug 2015 20:08:46 GMT

    [ https://issues.apache.org/jira/browse/SOLR-5872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14700144#comment-14700144
] 

Scott Blum commented on SOLR-5872:
----------------------------------

At the risk of creating two code paths, here's an idea.

1) We could improve batching *significantly* at the Overseer level, to be able to batch even
when the same collection isn't updated twice in a row.  We just need something like a dirty
list instead of only tracking the last one and the shared clusterStateModified.  This could
be an independent improvement.

2) When performing updates on format=2, we could use a size heuristic to decide whether or
not to go through the queue.  For collections with less than N shards, we could just do a
local CAS loop for state update ops.  For collections with more than N shares we'd just always
go through the queue.

> 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'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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