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.
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)
