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From "Noble Paul (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-6691) REBALANCELEADERS needs to change the leader election queue.
Date Tue, 04 Nov 2014 09:43:34 GMT

    [ https://issues.apache.org/jira/browse/SOLR-6691?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14195960#comment-14195960
] 

Noble Paul commented on SOLR-6691:
----------------------------------

yeah, this is pretty much what should be done. In the overseer roles feature, momentarily,
2 nodes watch the leader. 

But if the wrong node wins because of a failure , the whole thing is repeated 

> REBALANCELEADERS needs to change the leader election queue.
> -----------------------------------------------------------
>
>                 Key: SOLR-6691
>                 URL: https://issues.apache.org/jira/browse/SOLR-6691
>             Project: Solr
>          Issue Type: Bug
>            Reporter: Erick Erickson
>            Assignee: Erick Erickson
>
> The original code (SOLR-6517) assumed that changes in the clusterstate after issuing
a command to the overseer to change the leader indicated that the leader was successfully
changed. Fortunately, Noble clued me in that this isn't the case and that the potential leader
needs to insert itself in the leader election queue before trigging the change leader command.
> Inserting themselves in the front of the queue should probably happen in BALANCESHARDUNIQUE
when the preferredLeader property is assigned as well.
> [~noble.paul] Do evil things happen if a node joins at the head but it's _already_ in
the queue? These ephemeral nodes in the queue are watching each other. So if node1 is the
leader you have
> node1 <- node2 <- node3 <- node4
> where <- means "watches".
> Now, if node3 puts itself at the head of the list, you have
> {code}
> node1 <- node2
>       <- node3 <- node4
> {code}
> I _think_ when I was looking at this it all "just worked". 
> 1> node 1 goes down. Nodes 2 and 3 duke it out but there's code to insure that node3
becomes the leader and node2 inserts itself at then end so it's watching node 4.
> 2> node 2 goes down, nobody gets notified and it doesn't matter.
> 3> node 3 goes down, node 4 gets notified and starts watching node 2 by inserting
itself at the end of the list.
> 4> node 4 goes down, nobody gets notified and it doesn't matter.



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