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From "Flavio Paiva Junqueira (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-335) zookeeper servers should commit the new leader txn to their logs.
Date Tue, 22 Jun 2010 11:14:58 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12881168#action_12881168
] 

Flavio Paiva Junqueira commented on ZOOKEEPER-335:
--------------------------------------------------

Thanks for detailed assessment, Vishal. In Step b, the fact that the process believes it is
the leader is not a problem, and it happens because we queue notification messages during
leader election. 

The real issue is that leader code is setting the last processed zxid to the first of the
new epoch even before connecting to a quorum of followers. Because the leader code sets this
value before connecting to a quorum of followers (Leader.java:281) and the follower code throws
an IOException (Follower.java:73) if the leader epoch is smaller, we have that when the false
leader drops leadership and becomes a follower, it finds a smaller epoch and kills itself.

I noticed that this follower check was not there before (not present in 3.0 branch), and it
might have been introduced when we did the observer reorganization. For now I propose that
we move line Leader.java:281 to Leader.java:470. It simply changes the point in which we set
the last processed zxid to one in which we know that  a quorum of followers supports the leader.
I reasoned a bit about it and verified that tests pass.

A patch for the change I'm proposing is trivial, but a unit test will require some work, so
I'd rather hear opinions first. Also, please note that this problem is not related to the
topic of this jira, so we might consider working on a different jira from this point on. 



> zookeeper servers should commit the new leader txn to their logs.
> -----------------------------------------------------------------
>
>                 Key: ZOOKEEPER-335
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-335
>             Project: Zookeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.1.0
>            Reporter: Mahadev konar
>            Assignee: Mahadev konar
>            Priority: Blocker
>             Fix For: 3.4.0
>
>         Attachments: faultynode-vishal.txt, zk.log.gz, zklogs.tar.gz
>
>
> currently the zookeeper followers do not commit the new leader election. This will cause
problems in a failure scenarios with a follower acking to the same leader txn id twice, which
might be two different intermittent leaders and allowing them to propose two different txn's
of the same zxid.

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