ahuang98 commented on code in PR #18240:
URL: https://github.com/apache/kafka/pull/18240#discussion_r1899341945
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raft/src/main/java/org/apache/kafka/raft/KafkaRaftClient.java:
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@@ -2935,14 +3014,18 @@ private long pollResigned(long currentTimeMs) {
// until either the shutdown expires or an election bumps the epoch
stateTimeoutMs = shutdown.remainingTimeMs();
} else if (state.hasElectionTimeoutExpired(currentTimeMs)) {
- if (quorum.isVoter()) {
- transitionToCandidate(currentTimeMs);
- } else {
+// if (quorum.isVoter()) {
+ // canElectNewLeaderAfterOldLeaderPartitioned fails if we do
not bump epoch since it is possible
+ // that the replica ends up as follower in the same epoch.
+ // resigned(leaderId=local) -> prospective(leaderId=local) ->
follower(leaderId=local) which is illegal
+// transitionToProspective(quorum.epoch() + 1, currentTimeMs);
+// transitionToCandidate(currentTimeMs);
+// } else {
Review Comment:
the existing raft event simulation tests picked up on a new bug in
pollResigned - if we simply replace the transitionToCandidate(currentTimeMs)
with transitionToProspective(currentTimeMs), a cordoned leader in epoch 5 could
resign in epoch 5, transition to prospective in epoch 5 (with
leaderId=localId), fail election and then attempt to become follower of itself
in epoch 5.
there are a few alternatives which have their pros/cons
- resigned voter in epoch X should transition to prospective in epoch X+1
- cons: need to create a special code path just for this case to allow
becoming prospective in epoch+1 (would also add trivial complexity for
determining if votedKey or leaderId should be kept from prior transition).
transitioning to prospective in epoch + 1 is almost as disruptive as
transitioning directly to candidate since it involves an epoch bump
- pro: probably the option which follows intentions of past logic most
closely
- resigned voter in epoch X should simply transition to unattached in epoch
X+1 (current version)
- con: resigned replica has to wait two election timeouts after
resignation to become prospective
- pro: simplified logic. unless this is the only replica eligible for
leadership in the quorum (e.g. due to network partitioning), the impact of
waiting two election timeouts after resignation is small - all other replicas
should be starting their own elections within a single fetch timeout/election
timeout
- resigned voter in epoch X instead waits a smaller backoffTimeMs before
transitioning to unattached in epoch X+1
- con: scope creep, additional changes to resignedState
- pro: resigned voter waits less time before becoming eligible to start
a new election.
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