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A new metadata image is computed by controller. An epoch can represent the version of TopicImage. If there is number of partitions or partition racks change, the controller bumps the epoch. When coordinator receives a new metadata image and there is a new topic epoch, it triggers a rebalance for a group. The group also can store topic epoch map in records, so it can know the difference if the coordinator restarts. This approach save cpu and storage resources and avoid the downside of "Check Topic Delta to Trigger a Rebalance". However, this approach mixes the group coordinator logic within the controller.

Bump Group Version

Increase the group.version feature level to 2 to enable rack-aware rebalancing. However, there are two drawbacks to implementing this:

1. It requires maintaining lot of if-else block to determine whether to use subscription metadata or group hash.
2. When changing the group.version to 2, it triggers an epoch bump for all groups.