...

• Change the default setting of partition.assignment.strategy in implementation and tests
  
• When changing the default assignor we need to make sure this is clearly documented in the upgrade guide for 3.0
• After this KIP, the client will have:

  • New applications will enable cooperative rebalancing by default

  • 
    

    Existing applications which don’t set an assignor can safely upgrade using a single rolling bounce with no extra steps, and will automatically transition to cooperative rebalancing

  • Existing applications which do set an assignor that uses EAGER can likewise upgrade their applications to COOPERATIVE with a single rolling bounce

  • Once on new version (ex: V3.0), applications can safely go back and forth between EAGER and COOPERATIVE

  • Applications can safely downgrade

• This change will also propagate to Connect, when consumer groups we bring up for sink tasks

Compatibility, Upgrade path

No special upgrade path is necessary.

Note: After this will require users to follow the upgrade path laid out in KIP-429 to safely perform a rolling upgrade. Please note: KAFKA-12477 is doing introduced, we'll have the upgrade experience improvement to reduce the safe upgrade path to just a single rolling bounce. Also, KAFKA-12477 will We also make sure it will fall back to "RangeAssignor" if user doesn't follow the recommended upgrade path. It's still in progress so far. Before it completed, we still need to follow the following upgrade path:

<Copy from KIP-429>

From the user's perspective, the upgrade path of leveraging new protocols is similar to 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 (or no assignor is configured, which is identical as the RangeAssignor is the default below 3.0). The upgrade path would be:

1. The first rolling bounce is to replace the byte code (i.e. swap the jars) and introduce the cooperative assignor: set the assignors to "cooperative-sticky, range" (or round-robin/sticky/etc if you are using a different assignor already). At this stage, the new versioned byte code sends both assignors in their join-group request, but will still choose EAGER as the protocol since it's still configured with the "range" assignor, and the selected rebalancing protocol must be supported by all assignors. in the list. The "range" assignor will be selected to assign partitions while everyone is following the EAGER protocol. This rolling bounce is safe.
2. The second rolling bounce is to remove the "range" (or round-robin/sticky/etc) assignor, i.e. only leave the "cooperative-sticky" assignor in the config. At this stage, whoever has been bounced will then choose the COOPERATIVE protocol and not revoke partitions while others not-yet-bounced will still go with EAGER and revoke everything. However the "cooperative-sticky" assignor will be chosen since at least one member who's already bounced will not have "range" any more. The "cooperative-sticky" assignor works even when there are some members in EAGER and some members in COOPERATIVE: it is fine as long as the leader can recognize them and make assignment choice accordingly, and for EAGER members, they've revoked everything and hence did not have any pre-assigned-partitions anymore in the subscription information, hence it is safe just to move those partitions to other members immediately based on the assignor's output.

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 bouncethere's "short circuit" when there's mixing the cooperative and eager rebalancing protocols and the group leader is in old byte code. Detailed implementation can be found in KAFKA-12477.

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.