Page History

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1. Producer: Determine which broker is the coordinator for its group.
2. - Producer: Send a BeginTransaction(producerId, generation, partitions... ) request to the coordinator and await response. We could provide a variation of this API that   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.
3. Broker: Generate a TxId.
4. Coordinator: Append a BEGIN(TxId, producerId, generation, partitions...) record to the journal log and send response.
5. Producer: Read back response (which will include the TxId).
6. Coordinator (and followers): Update in-memory state of in-flight transactions and data partitions of the transaction.

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