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To help get access to consumer state for txn producer, consumer will expose a new API for some of its internal states as an opaque struct. This is already done by KIP-429, and we just take the details some high level information here for convenience.

ConsumerGroupMetadata {
  public String groupId();
 
  public int generationId();
 
  public String memberId();
 
  public Optional<String> groupInstanceId();
}

Thus producer could poll the metadata as it needs during normal processing. 

A new generation.id field shall be added to the `TxnOffsetCommitRequest` request. In the meantime we also suggest to add memberAs we see, the metadata exposed contains member.id, group.instance.id and generation.id, which are essentially the identifiers we use in the normal offset commit protocol. Inside `TxnOffsetCommitRequest`, member.id and group.instance.id generation.id to the request to field shall be added to make the txn offset commit fencing consistent with normal offset fencing. 

TxnOffsetCommitRequest => TransactionalId GroupId ProducerId ProducerEpoch Offsets GenerationId
  TransactionalId     => String
  GroupId             => String
  ProducerId  		  => int64		
  ProducerEpoch       => int16
  Offsets  	          => Map<TopicPartition, CommittedOffset>
  GenerationId        => int32 // NEW
  MemberId			  => String // NEW
  GroupInstanceId 	  => String // NEW

If one of the generation.id field is not matching group generationcorrectly on server side, the client will be fenced immediately. An edge case is defined as:

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