Vishal K commented on ZOOKEEPER-917:

Hi Flavio,

Sorry for not making much progress on 
(http://wiki.apache.org/hadoop/ZooKeeper/ClusterMembership). I have spent some 
time to understand the code. But It is a bit difficult to focus on development 
without dedicated development time. I am pushing to get dedicated development 
time at work for this so that I don't have to rely on my spare time. 

Few questions related to your comments:
1. Can you please elaborate on : "At the same time, a server A decides to 
follow another server B if it receives a message from B saying that B is 
leading and from a quorum saying that they are following, even if A is in a 
later election epoch. This mechanism is there to avoid A being locked out of 
the ensemble in the case it partitions away and comes back later."

2. Why is it not OK for B to give up leadership when it sees that its 
<epoch,zxid> is lower than others?


> 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:
>   *
>   *
>   *
> failed, and was replaced by a new node 
> (automated startup). The new node had not participated in any zookeeper 
> quorum previously. The node 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 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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