[jira] Commented: (ZOOKEEPER-975) new peer goes in LEADING state even if ensemble is online

["Flavio Junqueira (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-975?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12983331#action_12983331 ] 

Flavio Junqueira commented on ZOOKEEPER-975:
--------------------------------------------

Thanks for bringing this up, Vishal. This is not a new observation, although I can't remember if we discussed it in a jira or not. In general, I'm lukewarm about this change. It is certainly not an issue to avoid the server going into LEADING before it goes correctly into LOOKING, but I'm not entirely comfortable with manipulating the queues of notifications. Being able to have two servers concurrently thinking they are leading is a situation supported by our protocols, and such a modification would be an optimization to avoid the unnecessary LEADING step. 

Regarding application recovery time, we don't have a load balance scheme at this point, which could be quite useful, so bringing a new follower up does not guarantee that clients will move their sessions to the new follower. Note that this situation occurs only if there is an ensemble running and a server joins or recovers.

> new peer goes in LEADING state even if ensemble is online
> ---------------------------------------------------------
>
>                 Key: ZOOKEEPER-975
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-975
>             Project: ZooKeeper
>          Issue Type: Bug
>    Affects Versions: 3.3.2
>            Reporter: Vishal K
>             Fix For: 3.4.0
>
>
> Scenario:
> 1. 2 of the 3 ZK nodes are online
> 2. Third node is attempting to join
> 3. Third node unnecessarily goes in "LEADING" state
> 4. Then third goes back to LOOKING (no majority of followers) and finally goes to FOLLOWING state.
> While going through the logs I noticed that a peer C that is trying to
> join an already formed cluster goes in LEADING state. This is because
> QuorumCnxManager of A and B sends the entire history of notification
> messages to C. C receives the notification messages that were
> exchanged between A and B when they were forming the cluster.
> In FastLeaderElection.lookForLeader(), due to the following piece of
> code, C quits lookForLeader assuming that it is supposed to lead.
> 740                             //If have received from all nodes, then terminate
> 741                             if ((self.getVotingView().size() == recvset.size()) &&
> 742                                     (self.getQuorumVerifier().getWeight(proposedLeader) != 0)){
> 743                                 self.setPeerState((proposedLeader == self.getId()) ?
> 744                                         ServerState.LEADING: learningState());
> 745                                 leaveInstance();
> 746                                 return new Vote(proposedLeader, proposedZxid);
> 747
> 748                             } else if (termPredicate(recvset,
> This can cause:
> 1.  C to unnecessarily go in LEADING state and wait for tickTime * initLimit and then restart the FLE.
> 2. C waits for 200 ms (finalizeWait) and then considers whatever
> notifications it has received to make a decision. C could potentially
> decide to follow an old leader, fail to connect to the leader, and
> then restart FLE. See code below.
> 752                             if (termPredicate(recvset,
> 753                                     new Vote(proposedLeader, proposedZxid,
> 754                                             logicalclock))) {
> 755 
> 756                                 // Verify if there is any change in the proposed leader
> 757                                 while((n = recvqueue.poll(finalizeWait,
> 758                                         TimeUnit.MILLISECONDS)) != null){
> 759                                     if(totalOrderPredicate(n.leader, n.zxid,
> 760                                             proposedLeader, proposedZxid)){
> 761                                         recvqueue.put(n);
> 762                                         break;
> 763                                     }
> 764                                 }
> In general, this does not affect correctness of FLE since C will
> eventually go back to FOLLOWING state (A and B won't vote for
> C). However, this delays C from joining the cluster. This can in turn
> affect recovery time of an application.
> Proposal: A and B should send only the latest notification (most
> recent) instead of the entire history. Does this sound reasonable?

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