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From "Erick Erickson (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SOLR-6691) REBALANCELEADERS needs to change the leader election queue.
Date Mon, 03 Nov 2014 17:30:35 GMT
Erick Erickson created SOLR-6691:
------------------------------------

             Summary: 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
node1 <- node2
           <- node3 <- node4

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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