Cover two corner cases of leader election
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Key: ZOOKEEPER-159
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-159
Project: Zookeeper
Issue Type: Bug
Components: leaderElection
Reporter: Flavio Paiva Junqueira
Fix For: 3.0.0
Attachments: ZOOKEEPER-159.patch
In the default implementation of leader election, there are two undesirable
cases that need to be covered:
1- If there is a leader elected and this leader is supported by at least
quorum of peers, then it can happen that one peer disconnects from the leader,
and initiates a new leader election. As it is a new leader election, we
increment the logical clock of this peer, and according to the current
implementation, this peer won't accept any vote from a peer with a lower LE
turn (corresponds to the value of the logical clock of the voting peer). The
attached patch corrects this problem by allowing a peer to go back to a
previous epoch in the case a majority votes for a leader, and the peer also
receives a vote from the leader. This feature allows a peer to correct a false
suspicion of the current leader;
2- If a peer advances to a new turn before others, then it may end up voting
for a peer that either does not have the highest server id or the the highest
zxid. The attached patch fixes this problem by resetting the vote a peer when
it updates its logical clock upon receiving a notification with a higher turn
value.
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