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Let's recapitulate how the classic rebalance protocol works in Streams. First, the consumers in a Streams client join or re-join the group with the JoinGroup API. The JoinGroup request contains the subscribed topics, the owned partitions, the generation ID, the Streams-specific subscription info, and some other fields. The Streams-specific subscription info contains the process ID of the Streams client, the owned active and standby tasks, the task offset sums, the user endpoints for Interactive Query, and some more fields. When all the consumers of the Streams clients of the Streams application have joined, the group coordinator picks a leader for the group out of the consumers and sends back the JoinGroup response to all the members of the group (i.e., consumers that joined). The JoinGroup response contains the member ID, the generation ID, and the member ID of the leader (to make the leader aware of leadership) as well as all the consumer and Streams-specific metadata about subscribed topics, owned partitions, tasks, etc. The leader uses the data in the JoinGroup response for computing the assignment. Second, all the members in the Streams clients collect their assignment—computed by the leader—by using the SyncGroup API. The leader sends the computed assignment with the SyncGroup request to the group coordinator, and the group coordinator distributes the assignment to the members through the SyncGroup response. In parallel, the members heartbeat with the Heartbeat API in order to maintain their session. The Heartbeat API is also used by the group coordinator to inform the members about an ongoing rebalance. All those interactions are synchronized on the generation of the group. It is important to note that the consumer does not make any assumption about the generation ID. It basically uses what it receives from the group coordinator. The classic rebalance protocol used in Streams supports two modes: Eager and Cooperative. In the eager mode, the consumer revokes all its partitions before rejoining the group during a rebalance. In the cooperative mode, the consumer does not revoke any partitions before rejoining the group. However, it revokes the partitions that it does not own anymore when it receives its new assignment and rejoins immediately if he it had to revoke any partitions.

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