[GitHub] [kafka] hachikuji commented on a change in pull request #9539: KAFKA-10634: Adding LeaderId to Voters list in LeaderChangeMessage

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hachikuji commented on a change in pull request #9539:
URL: https://github.com/apache/kafka/pull/9539#discussion_r516215666



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File path: raft/src/main/java/org/apache/kafka/raft/KafkaRaftClient.java
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@@ -317,6 +317,9 @@ private void appendLeaderChangeMessage(LeaderState state, long currentTimeMs) {
             .map(follower -> new Voter().setVoterId(follower))
             .collect(Collectors.toList());
 
+        // Adding the leader to the voters as the protocol ensures that leader always votes for itself.
+        voters.add(new Voter().setVoterId(state.election().leaderId()));

Review comment:
       I think the original KIP stated that the LeaderChange message would encode the set of voters that had voted for the leader. We thought this might be useful for debugging. Later on, we had a change of heart and decided it would just be the set of voters. Now I'm thinking it might be useful to have both. The log will always remember who the voters were at the time of the election and which voters had granted the leader's candidacy, which could be helpful in case of misconfigurations.
   
   For the set of voters which voted for the current leader, I think what we want is `CandidateState.grantingVoters`. However, by the time `onBecomeLeader` is fired, we have already dropped the `CandidateState`. One option is to carry `grantingVoters` over to `LeaderState`. We might also be able to pass it through `onBecomeLeader`. This will be easier if we get rid of the call to `onBecomeLeader` in `initialize()`. Following KAFKA-10527, it is not possible to initialize as a leader, so we could raise an exception instead.
   
   




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