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(Assuming no failures at each step.)

InitPhase

(Step 1 in the figure.)

1. Producer: Determine which broker is the coordinator for its group.- Producer: Send a BeginTransaction(producerId, generation, partitions... ) request to the coordinator and await response. We could provide a variation of this API that   also includes a time-out. If the producer needs to commit its consumer state as part of the transaction it will need to include the relevant partition of the __consumer_offsets topic in the BeginTransaction request.
2. Broker: Generate a TxId.
3. Coordinator: Append a BEGIN(TxId, producerId, generation, partitions...) record to the journal log and send response.
4. Producer: Read back response (which will include the TxId).
5. Coordinator (and followers): Update in-memory state of in-flight transactions and data partitions of the transaction.

SendPhase

(Step 2 in the figure.)

Producer: Send transaction payloads (i.e., records) to the leader brokers of the data partitions. Each record will contain the TxId and TxCtl fields. The TxCtl is really only required to mark the final commit (or abort). The producer request envelope will include the producer ID and generation as well, but these will not be appended to the data logs.

EndPhase (When the producer is ready to commit a transaction,)

(Steps 3, 4, 5 in the figure.)

1. Producer: Send an OffsetCommitRequest to commit the input state associated with the end of that transaction (i.e., input for start of next transaction).
2. Producer: Send a CommitTransaction(TxId, producerId, generation) request to the coordinator and await response. (A non-error response indicates to the producer that the transaction will be committed.)
3. Coordinator: Append the PREPARE_COMMIT(TxId) request to the journal log and then send a response to the producer.
4. Coordinator: Send a CommitTransaction(TxId, partitions...) request to every leader broker (for records in the transaction payload).
5. Leader brokers:
6. 1. If this is a leader for a partition of a topic other than __consumer_offsets: Upon receiving a CommitTransaction(TxId, partition1, partition2, ...) request it will    append an empty (null) record (i.e., no key/value) to the respective partition and set the TxId and TxCtl (to COMMITTED) fields of the record. The leader broker will then respond to the coordinator.
   2. If this is a leader for a partition of __consumer_offsets: append a record to the partition with key set to G-LAST-COMMIT, value set to TxId. It should also set the   TxId and TxCtl fields of the record. The broker will then respond to the coordinator.  
7. Coordinator: Append a COMMITTED(TxId) request to the journal log.
8. Coordinator (and followers): Advance HW if possible (see above for details of the HW).

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