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https://issues.apache.org/jira/browse/SOLR-6691?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14249455#comment-14249455
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Noble Paul commented on SOLR-6691:
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bq.So I have a couple of options here:
5.0 is veru near .This touches the guts of SolrCloud and any bug will have huge
impact on the stability of the system. I would like this feature to be off from
5.0 and just keep in trunk till we have enough confidence
> 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
> Attachments: BalanceLeaderTester.java, SOLR-6691.patch
>
>
> 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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