FrankYang0529 commented on code in PR #17444:
URL: https://github.com/apache/kafka/pull/17444#discussion_r1820813089
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group-coordinator/src/main/java/org/apache/kafka/coordinator/group/Utils.java:
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@@ -235,4 +238,20 @@ public static ApiMessage
messageOrNull(ApiMessageAndVersion apiMessageAndVersion
return apiMessageAndVersion.message();
}
}
+
+ public static long hashSubscriptionMetadata(Map<String, TopicMetadata>
subscriptionMetadata) {
Review Comment:
> Why not using a single hash?
If just using a single hash for subscribed topics in a group, that means
each group needs to recalculate the hash value. If using a hash for a topic,
the coordinator can calculate a new hash when there is topic update. each group
can get the latest hash and compare with current hash to know whether there is
any update.
> The cache will be updated whenever a group needs to calculate the hash for
an uncached topic or when new metadata brings a topic change.
Like I mentioned in previous comment
(https://github.com/apache/kafka/pull/17444#discussion_r1820720961). When a
coordinator starts, it replays all records. At that time, the coordinator
calculate initial hash in `GroupMetadataManager#subscribeGroupToTopic`. When
there is new metadata image, we calculate a new hash for each changed topic and
request metadata refresh for related groups. In next group heartbeat, the
coordinator constructs a new hash map from the cache and compares with current
hash map in the group.
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