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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 Sun, 08 Apr 2018 09:06:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16429690#comment-16429690
] 

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

GitHub user lavacat opened a pull request:

    https://github.com/apache/zookeeper/pull/500

    ZOOKEEPER-2959: ignore accepted epoch and LEADERINFO ack from observers

    https://issues.apache.org/jira/browse/ZOOKEEPER-2959
    - add getVotingView check for id in getEpochToPropose and waitForEpochAck
    - refactor waitForNewLeaderAck to use getVotingView
    - unit tests
    
    credit: Xiang Yongqiang (https://github.com/xyq000) for original PR and reporting the
issue

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/lavacat/zookeeper branch-3.4

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/zookeeper/pull/500.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #500
    
----
commit 98c40dac60951c61b3b1922d0038461d81b843a1
Author: Bogdan Kanivets <bkanivets@...>
Date:   2018-04-08T08:46:37Z

    ZOOKEEPER-2959: ignore accepted epoch and LEADERINFO ack from observers
    
    https://issues.apache.org/jira/browse/ZOOKEEPER-2959
    - add getVotingView check for id in getEpochToPropose and waitForEpochAck
    - refactor waitForNewLeaderAck to use getVotingView
    - unit tests
    
    credit: Xiang Yongqiang (https://github.com/xyq000) for original PR and reporting the
issue

----


> 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
>            Assignee: Bogdan Kanivets
>            Priority: Blocker
>
> 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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