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https://issues.apache.org/jira/browse/ZOOKEEPER-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16306144#comment-16306144
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ASF GitHub Bot commented on ZOOKEEPER-2959:
-------------------------------------------
GitHub user xyq000 opened a pull request:
https://github.com/apache/zookeeper/pull/438
ZOOKEEPER-2959: ignore accepted epoch and ack from observers
https://issues.apache.org/jira/browse/ZOOKEEPER-2959
After a round of elections completes, followers and observers send their
accepted epochs to the leader to determine a final epoch.
Since `QuorumVerifier#containsQuorum(Set set)` does not check whether the
elements of argument `set` exactly represent participants, this pull request is
intended to ignore reported epochs and acks from observers for logical
consistency.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/xyq000/zookeeper ZOOKEEPER-2959
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/zookeeper/pull/438.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 #438
----
commit 647061aa7ba1182b83b44b7f2671508012a30b4c
Author: Yongqiang Xiang <xiangyongqiang@...>
Date: 2017-12-29T08:20:06Z
ignore accepted epoch and ack from observers
----
> 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: 3.4.10, 3.5.3
> 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 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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