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  1. ZooKeeper
  2. ZOOKEEPER-975

new peer goes in LEADING state even if ensemble is online

    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: 3.3.2
    • Fix Version/s: 3.4.0
    • Component/s: None
    • Labels:
      None

      Description

      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?

        Attachments

        1. ZOOKEEPER-975.patch
          24 kB
          Vishal Kher
        2. ZOOKEEPER-975.patch
          6 kB
          Vishal Kher
        3. ZOOKEEPER-975.patch2
          24 kB
          Vishal Kher
        4. ZOOKEEPER-975.patch3
          29 kB
          Vishal Kher
        5. ZOOKEEPER-975.patch4
          32 kB
          Vishal Kher
        6. ZOOKEEPER-975.patch5
          32 kB
          Vishal Kher
        7. ZOOKEEPER-975.patch6
          32 kB
          Vishal Kher

          Activity

            People

            • Assignee:
              vishalmlst Vishal Kher
              Reporter:
              vishalmlst Vishal Kher
            • Votes:
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              Dates

              • Created:
                Updated:
                Resolved: