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https://issues.apache.org/jira/browse/SOLR-6691?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14238998#comment-14238998
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Noble Paul commented on SOLR-6691:
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bq.When the sequence IDs are equal, as they are in the case of a node that has
rejoined at head, the secondary sort is the entire string, or I'm mis-reading
this code.
This second sort matters only for a brief amount of time , because we are going
to send one of these contenders back to the tail soon. Yes, if the leader
failed at this moment the wrong node becomes the leader. But the code is
written in such a way yo retry this if this occurs.
> 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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