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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ZOOKEEPER-569) Failure of elected leader can lead to never-ending leader election
Date Tue, 02 Feb 2010 18:44:18 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-569?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12828708#action_12828708
] 

Hadoop QA commented on ZOOKEEPER-569:
-------------------------------------

-1 overall.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12434553/zookeeper-569.patch
  against trunk revision 903483.

    +1 @author.  The patch does not contain any @author tags.

    +1 tests included.  The patch appears to include 3 new or modified tests.

    -1 patch.  The patch command could not apply the patch.

Console output: http://hudson.zones.apache.org/hudson/job/Zookeeper-Patch-h7.grid.sp2.yahoo.net/61/console

This message is automatically generated.

> Failure of elected leader can lead to never-ending leader election
> ------------------------------------------------------------------
>
>                 Key: ZOOKEEPER-569
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-569
>             Project: Zookeeper
>          Issue Type: Bug
>            Reporter: Henry Robinson
>            Assignee: Henry Robinson
>         Attachments: zookeeper-569.patch
>
>
> It is possible for basic LeaderElection to enter a situation where it never terminates.

> As an example, consider a three node cluster A, B and C.
> 1. In the first round, A votes for A, B votes for B and C votes for C
> 2. Since C > B > A, all nodes resolve to vote for C in the second round as there
is no first round winner
> 3. A, B vote for C, but C fails.
> 4. C is not elected because neither A nor B hear from it, and so votes for it are discarded
> 5. A and B never reset their votes, despite not hearing from C, so continue to vote for
it ad infinitum. 
> Step 5 is the bug. If A and B reset their votes to themselves in the case where the heard-from
vote set is empty, leader election will continue.
> I do not know if this affects running ZK clusters, as it is possible that the out-of-band
failure detection protocols may cause leader election to be restarted anyhow, but I've certainly
seen this in tests. 
> I have a trivial patch which fixes it, but it needs a test (and tests for race conditions
are hard to write!)

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