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Once the initial ConsumerGroupHeartbeatResponse is received successfully, the CGMSM will update its local MemberId and MemberEpoch based on the returned data. It will then transition to the JOINED state.

JOINED

When the CGMSM has joined the group, we enter into its namesake state. However, this The JOINED state simply indicates that the Consumer instance is known to the coordinator as a member of the group. It does not necessarily imply that it has been assigned any partitions. While in the JOINED state the CGMSM will periodically send requests to the coordinator at the needed cadence in order to maintain membership.

ASSIGNING

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The CGMSM should transition back to the JOINING state if the ConsumerGroupHeartbeatResponse has an error of UNKNOWN_MEMBER_ID or FENCED_MEMBER_EPOCH. If either of those errors occur, the CGMSM will clear its "assigned" partition set (without any revocation), and transition to the JOINING set so that it rejoins the group with the same MemberId and the MemberEpoch of 0.

The CGMSM will transition into the ASSIGNING state when the ConsumerGroupHeartbeatResponse contains a non-null value for Assignment.

ASSIGNING

The ASSIGNING state is entered with the intention that the CGMSM will need to perform the assignment reconciliation process. As is done in the JOINED state, the CGMSM will continue to communicate with the coordinator via the heartbeat mechanism to maintain its membership.

The first action that is performed in this state is to update the CGMSM's value for the member epoch as provided in the ConsumerGroupHeartbeatResponse.

Next, the CGMSM performs a comparison between its current the assignment and the value of Assignment contained in the ConsumerGroupHeartbeatResponse. If the two assignments are equal, the CGMSM has reconciled the assignment successful and will transition back to the JOINED state. If they are not equal, the reconciliation process begins.

KIP-848 states that during reconciliation, partitions are revoked first and then assigned second, as two distinct steps.

Partition revocation involves:

1. Removing the partitions from the CGMSM's "assigned" set
2. Commits the offsets for the revoked partitions
3. Invokes ConsumerRebalanceListener.onPartitionsRevoked()

QUESTION: do we need to heartbeat between revocation and assignment?

Partition assignment includes:

1. Adding the partitions to the CGMSM's "assigned" set
2. Invokes ConsumerPartitionAssignor.onAssignment(), if one is set
3. Invokes ConsumerRebalanceListener.onPartitionsAssigned()

TERMINATING

TBD

TERMINATED

TBD

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