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From "Bogdan Kanivets (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 Wed, 09 May 2018 21:14:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16469516#comment-16469516
] 

Bogdan Kanivets commented on ZOOKEEPER-2959:
--------------------------------------------

I think this is ready to merge. There are 3 PRs for 3.4, 3.5 and master.

Steps to reproduce the bug:

Start with 3 servers. Config:

 
{code:java}
clientPort=2181
leaderServes=yes
server.1=<server.1-ip>:2888:3888
server.2=<server.2-ip>:2888:3888
server.3=<server.3-ip>:2888:3888:observer
{code}
 

On server.2 block follower port from server.1 to server.2:
{code:java}
sudo iptables -A INPUT -s <server.1-ip> -p tcp --destination-port 2888 -j DROP{code}
Start server.1, server.2 and server.3
Wait for server.2 to declare itself a leader and then fail in waitForNewLeaderAck

 
{code:java}
2018-04-16 20:56:25,990 [myid:2] - INFO [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:Leader@361]
- LEADING - LEADER ELECTION TOOK - 3903
2018-04-16 20:56:27,275 [myid:2] - INFO [LearnerHandler-/<server.3-ip>:29223:LearnerHandler@329]
- Follower sid: 3 : info : org.apache.zookeeper.server.quorum.QuorumPeer$QuorumServer@136ca5bc
2018-04-16 20:56:27,281 [myid:2] - INFO [LearnerHandler-/<server.3-ip>:29223:LearnerHandler@384]
- Synchronizing with Follower sid: 3 maxCommittedLog=0x0 minCommittedLog=0x0 peerLastZxid=0x0
2018-04-16 20:56:27,281 [myid:2] - INFO [LearnerHandler-/<server.3-ip>:29223:LearnerHandler@393]
- leader and follower are in sync, zxid=0x0
2018-04-16 20:56:27,282 [myid:2] - INFO [LearnerHandler-/<server.3-ip>:29223:LearnerHandler@458]
- Sending DIFF
2018-04-16 20:56:27,291 [myid:2] - INFO [LearnerHandler-/<server.3-ip>:29223:LearnerHandler@518]
- Received NEWLEADER-ACK message from 3
2018-04-16 20:56:47,283 [myid:2] - INFO [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:Leader@502]
- Shutting down
2018-04-16 20:56:47,284 [myid:2] - INFO [QuorumPeer[myid=2]/0:0:0:0:0:0:0:0:2181:Leader@508]
- Shutdown called
java.lang.Exception: shutdown Leader! reason: Waiting for a quorum of followers, only synced
with sids: [ 2 ]
at org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:508)
at org.apache.zookeeper.server.quorum.Leader.lead(Leader.java:406)
at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:859){code}

On server.2 check that currentEpoch is incremented in currentEpoch file. This is the bug.
Epoch is incremented in getEpochToPropose because server.3 is counted in connectingFollowers.

> 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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