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From "Vishal Kher (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-1144) ZooKeeperServer not starting on leader due to a race condition
Date Wed, 03 Aug 2011 22:37:27 GMT
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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