Karolos Antoniadis created ZOOKEEPER-3485:
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Summary: Measure reconfiguration time
Key: ZOOKEEPER-3485
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3485
Project: ZooKeeper
Issue Type: Improvement
Affects Versions: 3.5.5
Reporter: Karolos Antoniadis
This issue is created after some initial discussion in the _dev_ mailing list
(subject "Leader election logging during reconfiguration").
There does not seem to be a good way to measure reconfiguration time in
ZooKeeper. Additionally, reconfiguration time is mixed together with leader
election time*.* For instance, during reconfiguration, ZooKeeper logs a
{{LEADER ELECTION TOOK}} message even though no leader election might takes
place.
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, in the sense 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]).
*Goal* of this issue/improvement is to measure in a better and more accurate
way the time it takes for a reconfiguration to complete, as well as, to clearly
distinguish the measurement of reconfiguration versus leader election.
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