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The PartitionRegistration also contains other changes like leader / elr. Every change will make the topic hash recomputation. It's no harm, because the hash only reflects change about uuid / name / number of partition / racks of partitions. If non of these fields change, the final hash will be the same, so it doesn't bump the group epoch or trigger a rebalance.

Remove a topic hash in the cache

In the coordinator, it maintains each topic is subscribed by which groups. When a group unsubscribes, the coordinator checks whether a topic is subscribed by any group. If it's not, the topic hash is removed from the cache.

A single hash of all subscribed topic in Group

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