Alexandre Hardy commented on ZOOKEEPER-917:
At first pass this seems to indicate that we can't replace a failed zookeeper
server by a new one, but that statement is probably way too strong. If I
understand correctly, what you are saying is that the server can be replaced
only after a new leader has been elected? i.e. Any fresh server should only be
restarted once the quorum has been reestablished?
I'm not sure I understand exactly why the election went wrong. Were the old
election messages resent when the Fresh server was contactable? I would have
thought that election messages should be based on the current state, and never
send old state.
This will take some time to digest and think through properly. In the meantime,
can you suggest how we should deal with this situation, can we simply wait for
the two remaining nodes to establish a quorum, and then reintroduce the third
node? I suppose we could test if a quorum has been established by testing if we
can establish a new zookeeper session.
Thanks for the help
> Leader election selected incorrect leader
> Key: ZOOKEEPER-917
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-917
> Project: Zookeeper
> Issue Type: Bug
> Components: leaderElection, server
> Affects Versions: 3.2.2
> Environment: Cloudera distribution of zookeeper (patched to never
> cache DNS entries)
> Debian lenny
> Reporter: Alexandre Hardy
> Priority: Critical
> Fix For: 3.3.3, 3.4.0
> Attachments: zklogs-20101102144159SAST.tar.gz
> We had three nodes running zookeeper:
> * 192.168.130.10
> * 192.168.130.11
> * 192.168.130.14
> 192.168.130.11 failed, and was replaced by a new node 192.168.130.13
> (automated startup). The new node had not participated in any zookeeper
> quorum previously. The node 188.8.131.52 was permanently removed from
> service and could not contribute to the quorum any further (powered off).
> DNS entries were updated for the new node to allow all the zookeeper servers
> to find the new node.
> The new node 192.168.130.13 was selected as the LEADER, despite the fact that
> it had not seen the latest zxid.
> This particular problem has not been verified with later versions of
> zookeeper, and no attempt has been made to reproduce this problem as yet.
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