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https://issues.apache.org/jira/browse/ZOOKEEPER-1075?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13039364#comment-13039364
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Vishal Kathuria commented on ZOOKEEPER-1075:
--------------------------------------------
The issue I think is this code below in FastLeaderElection.java
/**
* Before joining an established ensemble, verify that
* a majority are following the same leader.
*/
outofelection.put(n.sid, new Vote(n.leader, n.zxid,
n.epoch, n.state));
if (termPredicate(outofelection, new Vote(n.leader,
n.zxid, n.epoch, n.state))
&& checkLeader(outofelection, n.leader,
n.epoch)) {
In the case above, there is only one entry in outofelection that does not
constitute the majority. What we really need to check is whether
outofelection.size() + 1(this server) forms a majority because once this server
accepts the leader, the leader will have a majority of followers.
> Zookeeper Server cannot join an existing ensemble if the existing ensemble
> doesn't already have a quorum
> --------------------------------------------------------------------------------------------------------
>
> Key: ZOOKEEPER-1075
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1075
> Project: ZooKeeper
> Issue Type: Bug
> Components: leaderElection
> Affects Versions: 3.3.2
> Environment: Windows 7
> Reporter: Vishal Kathuria
> Original Estimate: 48h
> Remaining Estimate: 48h
>
> Here is the sequence of steps that reproduces the problem.
> On a 3 server ensemble,
> 1. Bring up two servers (say 1 and 2). Lets say 1 is leading.
> 2. Bring down 2
> 3. Bring up 2.
> 4. 2 gets a notification from 1 that it is leading but 2 doesn't accept it as
> a leader since it cannot find one other node that thinks 1 is the leader.
> So the ensemble gets stuck where 2 isn't following. If at this point, 3 comes
> up, then one of 2 & 3 will become a leader and 1 will keep thinking it is the
> leader.
> I am working on a patch to fix this issue.
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