kevin-wu24 commented on code in PR #20859:
URL: https://github.com/apache/kafka/pull/20859#discussion_r2565350148
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raft/src/main/java/org/apache/kafka/raft/KafkaRaftClient.java:
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@@ -222,6 +222,8 @@ public final class KafkaRaftClient<T> implements
RaftClient<T> {
private volatile RemoveVoterHandler removeVoterHandler;
private volatile UpdateVoterHandler updateVoterHandler;
+ private volatile boolean hasJoined = false;
Review Comment:
I have an idea how we can model this state properly. We can use an
Optional<Boolean>. Now we can represent 3 distinct "hasJoined" states:
1. UNKNOWN: We'll use Optional.empty(), which applies to all nodes on
startup, including brokers
2. HAS_NOT_JOINED: We'll use Optional.of(false), which only controllers can
transition to.
3. HAS_JOINED: We'll use Optional.of(true), which only controllers can
transition to.
The possible state transitions for controllers who have auto-join enabled
are:
1. UNKNOWN -> HAS_JOINED: This occurs when the controller handles a fetch
response, is "up-to-date" with the HWM, and discovers it IS in the voter set,
or if the controller is the only member of the voter set when starting up.
2. UNKNOWN -> HAS_NOT_JOINED: This occurs when the controller handles a
fetch response, is "up-to-date" with the HWM, and discovers it is NOT in the
voter set.
3. HAS_NOT_JOINED -> HAS_JOINED: This also occurs when the controller
handles a fetch response, and discovers it is in the voter set. However, we do
not need to do the HWM check here because the transition from UNKNOWN ->
HAS_NOT_JOINED has already done it (i.e. we know we are up to date).
Importantly, state transition 2 can only occur once in the lifetime of the
controller, because we cannot transition to UNKNOWN during the lifetime of the
process. Additionally, the controller process cannot go from HAS_JOINED ->
HAS_NOT_JOINED ever.
`shouldSendAddOrRemoveVoter` will be conditioned on the state being
`HAS_NOT_JOINED`. Therefore, the two invariants above mean that, during its
process lifetime, a controller will not try to auto-join again after being
removed from the voter set.
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