Flavio Junqueira commented on ZOOKEEPER-917:
Hi Vishal, It is certainly understand not having dedicated development time
being an issue. I actually didn't know you're interested in the cluster
membership... I'm glad to hear, though.
On your questions:
# Suppose we have an ensemble comprising 3 servers: A, B, and C. Now suppose
that C is the leader, and both A and B follow C. If A disconnects from C for
whatever reason (e.g., network partition) and it tries to elect a leader, it
won't get any other process in the LOOKING state. It will actually receive a
notification from C saying that it is leading and one from B saying that it is
following C, both with an earlier leader election epoch. To avoid having A
locked out (not able to elect C as leader), we implemented this exception: a
process accepts going back to an earlier leader election only if it receives a
notification from the leader saying that it is leading and from a quorum saying
that it is following;
# I'm not sure if you referring to specific problem of this jira or if you are
asking about my hypothetical example. Assuming it is the former, the follower
(Follower:followLeader()) checks if the leader is proposing an earlier epoch,
and if not, it accepts the leader snapshot. Because the epoch is the same, all
followers will accept the leader snapshot follow it.
> 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 18.104.22.168 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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