Github user lvfangmin commented on the issue:
https://github.com/apache/zookeeper/pull/703
@anmolnar following are my understanding about the acceptedEpoch,
currentEpoch and electionEpoch:
* acceptedEpoch : the previous epoch we accepted so far, usually is the
epoch is the highest zxid on that server.
* currentEpoch : the current epoch after syncing with the new leader, it's
based on the maximum acceptedEpoch in the quorum, and usually it's the
max(acceptedEpoch) + 1. The currentEpoch is used as the peerEpoch in the leader
election, as we know (sid, zxid, peerEpoch) are the set used to decide a leader.
* electionEpoch : not part of the factors to decide leader, but it's used
as a logical clock to avoid considering a vote delayed from a while ago.
Basically, we know there is a corner case where the learner may not update
it's zxid, peerEpoch, and electionEpoch after leader election (check the new
comment I added Leader.updateElectionVote), peerEpoch is fixed with a hack
solution, but we cannot easily update the zxid and electionEpoch, so we try to
ignore it. But IGNOREVALUE introduced will have compatible issue when rolling
upgrade ensemble, that's why we introduced version in notification, and only
compare id or peerEpoch based on version.
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