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https://issues.apache.org/jira/browse/ZOOKEEPER-569?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Flavio Paiva Junqueira updated ZOOKEEPER-569:
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Resolution: Fixed
Status: Resolved (was: Patch Available)
+1, thanks Henry! I have just committed it (Committed revision 912119).
> 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
> Fix For: 3.3.0
>
> Attachments: zookeeper-569.patch, ZOOKEEPER-569.patch,
> zookeeper-569.patch, zookeeper-569.patch, zookeeper-569.patch,
> 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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