jsancio commented on code in PR #19416:
URL: https://github.com/apache/kafka/pull/19416#discussion_r2054309528
##########
raft/src/main/java/org/apache/kafka/raft/VoterSet.java:
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@@ -250,6 +251,26 @@ public Optional<VoterSet> updateVoter(VoterNode voter) {
return Optional.empty();
}
+ /**
+ * Update a voter by only comparing the node id.
+ *
+ * This update voter operation doesn't compare the directory id. This is
useful when upgrading
+ * from a voter set that doesn't support directory id to one that supports
directory ids.
Review Comment:
Yeah. The different is a bit subtle. In kraft.version 1 all of the voters in
the voter set have a directory id. To update a voter in kraft.version 1 the
node id and directory id must match.
In kraft.version 0 none of the voters in the voter set have a directory id
but the voter set stored KRaftVersionUpgrade may have the directory id set even
though the kraft.version is 0. This is needed so that the leader can discover
the directory id of all of the voters before upgrading the kraft.version.
Now lets assume that we have a cluster in kraft.version 0 with the voter id
1, 2 and 3. All of the voter are upgraded to Kafka 4.1 and they send an update
voter request with the directory ids d1, d2 and d3 respectively. Lets say that
the leader is 1 and the leader updates the in-memory voter set to include all
of the directories. Finally, voter 3 crashes and replaces its disk. The new
directory id for voter 3 is d3'. For the upgrade to be correct voter 3 needs to
send an update voter request and the leader (1) needs to update the voter set
to be d3' even though it already has d3 in its in-memory voter set.
updateVoterIgnoringDirectoryId is allowing the leader to set the voter set
to the latest directory id even if it changes while in kraft version 0.
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