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1. For every consumer: before sending the join-group request, change the behavior as follows based on the join-group triggering event:
   1. If subscription has changed: revoke all partitions who are not of subscription interest by calling onPartitionsRevoked, send join-group request with whatever left in the owned partitions in Subscription.
   2. If topic metadata has changed: call onPartitionsLost on those owned-but-no-longer-exist partitions; and if the consumer is the leader, send join-group request.
   3. If received REBALANCE_IN_PROGRESS from heartbeat response / commit response: re-join group with all the currently owned partitions as assigned partitions.
   4. If received UNKNOWN_MEMBER_ID or ILLEGAL_GENERATION from join-group / sync-group / commit / heartbeat response: reset generation / clear member-id correspondingly, call rebalance listener's onPartitiothosensLost onPartitionsLost for all the partition and then re-join group with empty assigned partition.
   5. If received MEMBER_ID_REQUIRED from join-group request: set the member id, and then re-send join-group (at this moment the owned partitions should be empty).
   6. If received assignment from previous rebalance's sync-group response contains error code NEEDS_REJOIN, call onPartitionsRevoked as required before sending the join-group request with newly formed assigned partitions (see 3.d below).
2. For the leader: after getting the received subscription topics, as well as the assigned-partitions, do the following:
   
   1. Collect the partitions that are claimed as currently owned from the subscriptions; let's call it owned-partitions.
   2. Call the registered assignor of the selected protocol, passing in the cluster metadata and get the returned assignment; let's call the returned assignment assigned-partitions. Note the this set could be different from owned-partitions.
   3. Compare the owned-partitions with assigned-partitions and generate three exclusive sub-sets:
      1. Intersection(owned-partitions, assigned-partitions). These are partitions that are still owned by some members, and some of them may be now allocated for new members. Let's call it maybe-revoking-partitions.
      2. Minus(assigned-partitions, owned-partitions). These are partitions that are not previously owned by any one. This set is non-empty when its previous owner is on older version and hence revoked them already before joining, or a partition is revoked in previous rebalance by the new versioned member and hence not in any assigned partitions, or it is a newly created partition due to add-partitions. Let's call it ready-to-migrate-partitions.
      3. Minus(owned-partitions, assigned-partitions). These are partitions that does not exist in assigned partitions, but are claimed to be owned by the members. It is non-empty if some topics are deleted, or if the leader's metadata is stale (and hence the generated assignment does not have those topics), or if the previous leader has created some topics in its assignor that are not in the cluster yet (consider the Streams case). Let's call it unknown-but-owned-partitions.
   4. For maybe-revoking-partitions, check if the owner has changed. If yes, exclude them from the assigned-partitions list to the new owner, instead set the error-code to NEED_REJOIN for all the members (this is for fast re-join and rebalance). The old owner will realize it does not own it any more, revoke it and then rejoin.
   5. For ready-to-migrate-partitions, it is safe to move them to the new member immediately since we know no one owns it before, and hence we can encode the owner from the newly-assigned-partitions directly.
   6. For unknown-but-owned-partitions, it is also safe to just give them back to whoever claimed to be their owners by encoding them directly as well. If this is due to topic metadata update, then a later rebalance will be triggered anyways.
3. For every consumer: after received the sync-group response, do the following:
   1. Calculate the newly-added-partitions as Minus(assigned-partitions, owned-partitions) and the revoked-partitions as Minus(owned-partitions, assigned-partitions).
   2. Update the assigned-partitions list.
   3. For those newly-added-partitions, call the rebalance listener's onPartitionsAssigned.
   4. For those revoked-partitions, call the rebalance listener's onPartitionsRevoked.
   5. Check the error code:
      1. If it is NONE, complete.
      2. If it is NEEDS_REJOIN, immediately send another join group request with the updated assigned partitions following step 1.e) above.

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From the user's perspective, the upgrade path of leveraging new protocols is just the same as switching to a new assignor. For example, assuming the current version of Kafka consumer is 2.2 and "range" assignor is specified in the config. The upgrade path would be:

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The key point behind this two rolling bounce is that, we want to avoid the situation where leader is on old byte-code and only recognize "eager", but due to compatibility would still be able to deserialize the new protocol data from newer versioned members, and hence just go ahead and do the assignment while new versioned members did not revoke their partitions before joining the group. Note the difference with KIP-415 here: since on consumer we do not have the luxury to leverage on list of built-in assignors since it is user-customizable and hence would be black box to the consumer coordinator, we'd need two rolling bounces instead of one rolling bounce to complete the upgrade, whereas Connect only need one rolling bounce.

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