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• If the `member.id` uses UNKNOWN_MEMBER_ID, we shall always generate a new member id and replace the one within current map
  • if `group.instance.id` was found on the static map, we shall reuse the current member.id to reply to the member rejoin request immediately when the group is doing stable. We also expect that after KIP-394, all the join group requests are requiring `member.id` to physically enter the consumer group, so the behavior of static member is consistent with that proposal.
  • if not found, we shall generate a new member id and add the new key-value pair into static member map. 
• we are requiring member.id (if not unknown) to match the value stored in cache, otherwise reply MEMBER_ID_MISMATCH. The edge case is that if we could have members with the same `group.instance.id` (for example mis-configured instances with a valid `member.id` but added a used `group.instance.id` on runtime). When `group.instance.id` has duplicates, we could refuse join request from members with an outdated `member.id`, since we update the mapping upon each join group request. In an edge case where the client hits this exception in the response, it is suggesting that some other consumer takes its spot. The client should immediately fail itself to inform end user that there is a configuration bug which is generating duplicate consumers with same identity. For first version of this KIP, we just want to have straightforward handling to expose the error in early stage and reproduce bug cases easily.

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