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The group coordinator is responsible for triggering a rebalance of the group when the metadata of the group changes in some cases. The group epoch represents the generation of the group metadata. It is incremented whenever the group metadata is updated. This happens in the following cases:
- A member joins or leaves the group.
- A member updates its subscriptions.
- A member is removed from the group by the group coordinator.
- The partition metadata is updated, such as when a new partition is added or a topic is created or deleted, or when the rack ID changes.
In all these cases, a new version of the group metadata is calculated by the group coordinator with an incremented group epoch. The new version of the group metadata signals that a new assignment is required for the group.
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