[
https://issues.apache.org/jira/browse/ZOOKEEPER-975?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12993428#comment-12993428
]
Vishal K commented on ZOOKEEPER-975:
------------------------------------
Hi Flavio,
Do you agree that sending the current state is a better approach?
I as looking at the code to add a method createNotificationMessage() that will
return a notification message based on the current state of the server. QCM can
call this method and send the notification message at the place where we send
lastMessage. However, adding it to FastLeaderElection only wont be enough since
there are other leader election algorithms as well. The Election interface is
very minimal. Should we add this method to this interface? Which leader
election algorithms do we support and is it ok to make this change only for the
supported algorithms?
Thanks
-Vishal
> 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
>
> Attachments: ZOOKEEPER-975.patch
>
>
> 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?
--
This message is automatically generated by JIRA.
-
For more information on JIRA, see: http://www.atlassian.com/software/jira
