...

JoinGroupRequest => GroupId SessionTimeout RebalanceTimeout MemberId GroupInstanceId ProtocolType GroupProtocols JoinReason
  GroupId             => String
  SessionTimeout      => int32
  RebalanceTimeout	  => int32
  MemberId            => String
  GroupInstanceId     => String // new
  ProtocolType        => String
  GroupProtocols      => [Protocol MemberMetadata]
  Protocol            => String
  MemberMetadata      => bytes
  JoinReason		  => Enum

public static Schema[] schemaVersions() {

Proposed Changes

    return new Schema[] {JOIN_GROUP_REQUEST_V0, JOIN_GROUP_REQUEST_V1, JOIN_GROUP_REQUEST_V2, JOIN_GROUP_REQUEST_V3, JOIN_GROUP_REQUEST_V5, JOIN_GROUP_REQUEST_V6};
}

public static Schema[] schemaVersions() {
    return new Schema[] {JOIN_GROUP_RESPONSE_V0, JOIN_GROUP_RESPONSE_V1, JOIN_GROUP_RESPONSE_V2, JOIN_GROUP_RESPONSE_V3, JOIN_GROUP_RESPONSE_V4, JOIN_GROUP_REQUEST_V5, JOIN_GROUP_REQUEST_V6};
}

For the version one, we will just have two JoinReason to handle the leader rejoin case:

public enum JoinReason {
  BLIND("blind"), // Join request from a start-up consumer 
  TOPIC_METADATA_CHANGE("topic_metadata_change"); // The topic metadata has changed (must be from the leader)
}

Proposed Changes

When leader consumer detects a change of topic metadata, it should currently be the only case when it wants to trigger a group rebalance. We will explicitly set the JoinReason to `topic_metadata_change` so that group coordinator will proceed to rebalance stage. If there is no member protocol change and JoinReason is `Blind`, then we shall not rebalance on known member rejoin. Be aware that JoinReason takes lower priority than the protocol change monitoringDescribe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Rejected Alternatives

• This change doesn't involve backward incompatible change. Broker will still be able to read existing join group request, and for request < v6 the JoinReason field will be interpreted as "unknown" which shouldn't confuse broker's judgement on whether to trigger rebalance.

Rejected Alternatives

Not applicable so farIf 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.