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From "Camille Fournier (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-1144) ZooKeeperServer not starting on leader due to a race condition
Date Thu, 11 Aug 2011 14:41:27 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-1144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13083137#comment-13083137

Camille Fournier commented on ZOOKEEPER-1144:

Vishal, can you just explain quickly why moving the ackSet.add to its new location (above
waitForEpochAck but below the LearnerCnxAcceptor start) will fix the bug as you described
it in the bug report? I'm not super familiar with this part of the code, but if you can just
point it out to me I'm happy to +1 this and check it in asap so our builds will stop failing.

> 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
>            Assignee: Vishal Kher
>            Priority: Blocker
>             Fix For: 3.4.0
>         Attachments: ZOOKEEPER-1144.patch
> 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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