Fangmin Lv created ZOOKEEPER-3109:
-------------------------------------
Summary: Avoid long unavailable time due to voter changed mind
when activating the leader during election
Key: ZOOKEEPER-3109
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3109
Project: ZooKeeper
Issue Type: Improvement
Components: quorum, server
Affects Versions: 3.6.0
Reporter: Fangmin Lv
Assignee: Fangmin Lv
Fix For: 3.6.0
Occasionally, we'll find it takes long time to elect a leader, might longer
then 1 minute, depends on how big the initLimit and tickTime are set.
This exposes an issue in leader election protocol. During leader election,
before the voter goes to the LEADING/FOLLOWING state, it will wait for a
finalizeWait time before changing its state. Depends on the order of
notifications, some voter might change mind just after it voting for a server.
If the server it was previous voting for has majority of votes after
considering this one, then that server will goto LEADING state. In some corner
cases, the leader may end up with timeout waiting for epoch ACK from majority,
because of the changed mind voter. This usually happen when there are even
number of servers in the ensemble (either because one of the server is down or
being restarted and it takes long time to restart). If there are 5 servers in
the ensemble, then we'll find two of them in LEADING/FOLLOWING state, another
two in LOOKING state, but the LOOKING servers cannot join the quorum since
they're waiting for majority servers FOLLOWING the current leader before
changing to FOLLOWING as well.
As far as we know, this voter will change mind if it received a vote from
another host which just started and start to vote itself, or there is a server
takes long time to shutdown it's previous ZK server and start to vote itself
when starting the leader election process.
Also the follower may abandon the leader if the leader is not ready for
accepting learner connection when the follower tried to connect to it.
To solve this issue, there are multiple options: #
increase the finalizeWait time
#
smartly detect this state on leader and quit earlier
The 1st option is straightforward and easier to change, but it will cause
longer leader election time in common cases.
The 2nd option is more complexity, but it can efficiently solve the problem
without sacrificing the performance in common cases. It remembers the first
majority servers voting for it, checking if there is anyone changed mind while
it's waiting for epoch ACK. The leader will wait for sometime before quitting
LEADING state, since one voter changed may not be a problem if there are still
majority voters voting for it.
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
