Guozhang Wang created KAFKA-8511:
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             Summary: Looking into the Future: Heartbeat Communicated Protocol
                 Key: KAFKA-8511
                 URL: https://issues.apache.org/jira/browse/KAFKA-8511
             Project: Kafka
          Issue Type: Sub-task
            Reporter: Guozhang Wang


Note that KIP-429 relies on the fact that COOPERATIVE and EAGER members can 
work together within the same generation as long as the leader recognize both; 
this however may not be true moving forward if we add a third rebalance 
protocol. One idea to resolve this in the future is that, instead of letting 
the members to decide which protocol to use "locally" before sending the 
join-group request, we will use Heartbeat request / response to piggy-back the 
communication of the group's supported protocols and let members to rely on 
that "global" information to make decisions. More specifically:

* On Heartbeat Request, we will add additional field as a list of protocols 
that this member supports.
* On Heartbeat Response, we will add additional field as a single protocol 
indicating which to use if the error code suggests re-joining the group.

The broker, upon receiving the heartbeat request, if the indicated supported 
protocols does not contain the one it has decided to use for the up-coming 
rebalance, then reply with an fatal error.



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