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From Karolos Antoniadis <karo...@gmail.com>
Subject Leader election logging during reconfiguration
Date Tue, 30 Jul 2019 00:28:20 GMT
Hi ZooKeeper developers,

ZooKeeper seems to be logging a "*LEADER ELECTION TOOK*" message even
though no leader election takes place during a reconfiguration.

This can be reproduced by following these steps:
1) start a ZooKeeper cluster (e.g., 3 participants)
2) start a client that connects to some follower
3) perform a *reconfig* operation that removes the leader from the cluster

After the reconfiguration takes place, we can see that the log files of the
remaining participants contain a "*LEADER ELECTION TOOK*" message. For
example, a line that contains

*2019-07-29 23:07:38,518 [myid:2] - INFO
 [QuorumPeer[myid=2](plain=0.0.0.0:2792)(secure=disabled):Follower@75] -
FOLLOWING - LEADER ELECTION TOOK - 57 MS*

However, no leader election took place. With that, I mean that no server
went *LOOKING *and then started voting and sending notifications to other
participants as would be in a normal leader election.
It seems, that before the *reconfig *is committed, the participant that is
going to be the next leader is already decided (see here:
https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/Leader.java#L865
).

I think the above issue raises the following questions:
- Should we avoid logging LEADER ELECTION messages altogether in such cases?
- Or, should there be some logging for the time it took for the
reconfiguration (e.g., the time between a participant gets a *reconfig*
operation till the operation is committed)? Would such a time value be
useful?

Best,
Karolos

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