Akihiro Suda created ZOOKEEPER-2189:
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Summary: multiple leaders can be elected when configs conflict
Key: ZOOKEEPER-2189
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2189
Project: ZooKeeper
Issue Type: Bug
Components: leaderElection
Affects Versions: 3.5.0
Reporter: Akihiro Suda
This sequence leads the ensemble to a split-brain state:
* Start server 1 (config=1:participant, 2:participant, 3:participant)
* Start server 2 (config=1:participant, 2:participant, 3:participant)
* 1 and 2 believe 2 is the leader
* Start server 3 (config=1:observer, 2:observer, 3:participant)
* 3 believes 3 is the leader, although 1 and 2 still believe 2 is the leader
Such a split-brain ensemble is very unstable.
Znodes can be lost easily:
* Create some znodes on 2
* Restart 1 and 2
* 1, 2 and 3 can think 3 is the leader
* znodes created on 2 are lost, as 1 and 2 sync with 3
I consider this behavior as a bug and that ZK should fail gracefully if a
participant is listed as an observer in the config.
In current implementation, ZK cannot detect such an invalid config, as
FastLeaderElection.sendNotification() sends notifications to only voting
members and hence there is no message from observers(1 and 2) to the new voter
(3).
I think FastLeaderElection.sendNotification() should send notifications to all
the members and FastLeaderElection.Messenger.WorkerReceiver.run() should verify
acks.
Any thoughts?
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