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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2959) ignore accepted epoch and LEADERINFO ack from observers when a newly elected leader computes new epoch
Date Thu, 05 Apr 2018 06:49:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16426533#comment-16426533
] 

ASF GitHub Bot commented on ZOOKEEPER-2959:
-------------------------------------------

Github user shralex commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/438#discussion_r179363930
  
    --- Diff: src/java/main/org/apache/zookeeper/server/quorum/Leader.java ---
    @@ -1183,8 +1183,10 @@ public long getEpochToPropose(long sid, long lastAcceptedEpoch)
throws Interrupt
                 if (lastAcceptedEpoch >= epoch) {
                     epoch = lastAcceptedEpoch+1;
                 }
    -            connectingFollowers.add(sid);
                 QuorumVerifier verifier = self.getQuorumVerifier();
    +            if(verifier.getVotingMembers().containsKey(sid)) {
    --- End diff --
    
    If I recall correctly, the reason this wasn't done are concerns around the impact on performance
- containsQuorum is called every time an ACK is received for every operation proposal. So
if you need 3 asks to commit an operation, we'll be doing these checks (figuring out who's
a participant and who's not} for {ACK1}, for {ACK1, ACK2} and for {ACK1, ACK2, ACK3}. This
compared to comparing two ints as it stands now. So this is why it wasn't done...


> ignore accepted epoch and LEADERINFO 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: 3.4.10, 3.5.3
>            Reporter: xiangyq000
>            Priority: Major
>
> 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 final 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 call method "getEpochToPropose"
> # The number of all reporters is greater than half of participants.
> The problem is, an observer server will also send its accepted epoch to the leader, 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 accepted epochs 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. Then QuorumVerifier#containsQuorum will return
true, because it does not check whether the elements of the parameter are participants.
> The same flaw exists in org.apache.zookeeper.server.quorum.Leader#waitForEpochAck



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