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https://issues.apache.org/jira/browse/ZOOKEEPER-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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xiangyq000 updated ZOOKEEPER-2959:
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Summary: ignore epoch proposal and ack from observers when a newly elected
leader computes new epoch (was: ignore epoch proposal and ack from observer
when a newly elected leader computes new epoch)
> ignore epoch proposal and ack from observers when a newly elected leader
> computes new epoch
> -------------------------------------------------------------------------------------------
>
> 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 learners
> report their accepted epoch to the leader for the computation of new cluster
> epoch.
> org.apache.zookeeper.server.quorum.Leader#getEpochToPropose
> {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 :
> # The leader has reported its epoch.
> # 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.
> # The connectingFollowers set can contain elements of SID of observers. (In
> fact, at least it must contain the SID of the leader).
> # 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.
> There are 2 candidate solutions.
> # Ignore epoch from observers.
> # require (number_of_reported_peers > number_of_all_peers / 2) , instead of
> existing (number_of_reported_peers > number_of_all_participants / 2).
> The similar confusion exists in the following procedure when the leader
> counts the ACKs for the new epoch from learners.
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