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https://issues.apache.org/jira/browse/ZOOKEEPER-1144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13079095#comment-13079095
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Eugene Koontz commented on ZOOKEEPER-1144:
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Thanks Vishal, hopefully this is the same problem with the same solution.
> ZooKeeperServer not starting on leader due to a race condition
> --------------------------------------------------------------
>
> Key: ZOOKEEPER-1144
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1144
> Project: ZooKeeper
> Issue Type: Bug
> Affects Versions: 3.4.0
> Reporter: Vishal Kher
> Priority: Blocker
> Fix For: 3.4.0
>
>
> I have found one problem that is causing QuorumPeerMainTest:testQuorum to
> fail. This test uses 2 ZK servers.
> The test is failing because leader is not starting ZooKeeperServer after
> leader election. so everything halts.
> With the new changes, the server is now started in Leader.processAck() which
> is called from LeaderHandler. processAck() starts ZooKeeperServer if majority
> have acked NEWLEADER. The leader puts its ack in the the ackSet in
> Leader.lead(). Since processAck() is called from LearnerHandler it can happen
> that the learner's ack is processed before the leader is able to put its ack
> in the ackSet. When LearnerHandler invokes processAck(), the ackSet for
> newLeaderProposal will not have quorum (in this case 2). As a result, the
> ZooKeeperServer is never started on the Leader.
> The leader needs to ensure that its ack is put in ackSet before starting
> LearnerCnxAcceptor or invoke processAck() itself after adding to ackSet. I
> haven't had time to go through the ZAB2 changes so I am not too familiar with
> the code. Can Ben/Flavio fix this?
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