[jira] [Created] (ZOOKEEPER-2746) Leader hand off during dynamic reconfig is a best effort, not a guarantee, when none trivial changes happen on current leader.

["Michael Han (JIRA)" <ji...@apache.org>]

Michael Han created ZOOKEEPER-2746:
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             Summary: Leader hand off during dynamic reconfig is a best effort, not a guarantee, when none trivial changes happen on current leader.
                 Key: ZOOKEEPER-2746
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2746
             Project: ZooKeeper
          Issue Type: Test
    Affects Versions: 3.5.2
            Reporter: Michael Han
            Assignee: Michael Han


When none trivial config change happens on the leader (e.g. port change, role change) to minimize disruption of quorum we do leader hand off by having the current leader nominate the next leader instead of kicking off a full leader election.

However this is a best effort and not a guarantee that the new leader of the quorum will be the nominated leader: it is possible that the nominated leader fail to establish the leadership during sync phase, which leads to new election rounds and a different leader elected. 

In ReconfigTest.testPortChange one check is the new leader after dynamic reconfig of the current leader has to be a different leader; based on what described earlier this is not always the case as the nominated leader might fail to get a quorum to ack its leadership during sync phase. We could either fixed the test by removing the check, or always guarantee that the new leader after dynamic reconfig is the nominated leader (which does not make much sense given the nominated leader also has a possibility to crash.).



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