[jira] [Created] (ZOOKEEPER-2959) New epoch computation logic confusion

["xiangyq000 (JIRA)" <ji...@apache.org>]

xiangyq000 created ZOOKEEPER-2959:
-------------------------------------

             Summary: New epoch computation logic confusion
                 Key: ZOOKEEPER-2959
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2959
             Project: ZooKeeper
          Issue Type: Bug
    Affects Versions: 3.4.11
            Reporter: xiangyq000


Once the ZooKeeper cluster finishes the election for new leader, all leaners report their accepted epoch to the leader for the computation of new cluster epoch.

{code:java}

    private HashSet<Long> connectingFollowers = new HashSet<Long>();
    public long getEpochToPropose(long sid, long lastAcceptedEpoch) throws InterruptedException, IOException {
        synchronized(connectingFollowers) {
            if (!waitingForNewEpoch) {
                return epoch;
            }
            if (lastAcceptedEpoch >= epoch) {
                epoch = lastAcceptedEpoch+1;
            }
            connectingFollowers.add(sid);
            QuorumVerifier verifier = self.getQuorumVerifier();
            if (connectingFollowers.contains(self.getId()) && 
                                            verifier.containsQuorum(connectingFollowers)) {
                waitingForNewEpoch = false;
                self.setAcceptedEpoch(epoch);
                connectingFollowers.notifyAll();
            } else {
                long start = Time.currentElapsedTime();
                long cur = start;
                long end = start + self.getInitLimit()*self.getTickTime();
                while(waitingForNewEpoch && cur < end) {
                    connectingFollowers.wait(end - cur);
                    cur = Time.currentElapsedTime();
                }
                if (waitingForNewEpoch) {
                    throw new InterruptedException("Timeout while waiting for epoch from quorum");        
                }
            }
            return epoch;
        }
    }
{code}

The computation will get an outcome once :
1. The leader has reported its epoch.
2. The number of all reporters is greater than half quorum, i.e., half of PARTICIPANTS.

The problem is, an observer server is not a PARTICIPANT, while this procedure treat observers as participants.

Supposed that the cluster consists of 1 leader, 2 followers and 1 observer, and now the leader and the observer have reported their epoch while neither of the followers has. Thus, the connectingFollowers set consists of two elements, resulting in a size of 2, which is greater than half quorum, namely, 2. So the if condition is met.

This procedure can be confusing. 
1. The  connectingFollowers set can contain elements of SID of observers. (In fact, at least it must contain the SID of the leader).
2. The intent of QuorumVerifier#containsQuorum is to check whether a set of PARTICIPANTS makes a quorum. However, here it just regards a set of peers as a set of participants.

The similar confusion exists in the afterwards procedure when the leader counts the ACKs for new epoch from learners.



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