Flavio Junqueira commented on ZOOKEEPER-917:

Hi Alexandre, It is an key premise of important replication algorithms, like 
Paxos, that there is a portion of the state that persists across crashes (and 
recoveries). By replacing server 2 with a fresh server, you simply got rid of 
the persistent state. In general, making that replacement you've made may lead 
you to trouble due to the problem I exposed a few postings up. Of course, if 
you wait for a successful election, the problem is supposed to go away because 
you have reestablished a quorum and this quorum does not contain the faulty 
server, but then you have to make sure the election happens before you 
introduce the fresh server perhaps through jmx or by inspecting the logs. 
Simply setting a reasonable timeout will work in most cases, but the leader 
election is not guaranteed to succeed, and there is a chance, likely to be 
small, that you'll end up with a corrupt state. 

> 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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