Page History

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• Timeout or error response when producer sends BeginTransaction(TxId): producer simply retries (with same TxId).
• Broker-side error while the producer is sending the data: The producer should abort (and subsequently redo) the transaction (with a new TxId). If the producer does not  abort the transaction, the coordinator will abort the transaction after the transaction's timeout period. Redoing the transaction is required only in the case of an error for which the request data may have been appended and replicated to the follower. For example, a producer request timeout would require a redo while a NotLeaderForPartitionException does not require a redo.
• Timeout or error response when producer sends CommitTransaction(TxId): producer simply retries the transaction (with same TxId). (However, see section on idempotence further down.)
• Producer failure while a transaction is pending: if the coordinator is in a position to detect a closed socket (when it sends the response to the BeginTransactionRequest) then it can proactively abort the transaction.  Otherwise the transaction will be aborted after its timeout period.
• Coordinator failure: i.e., when the coordinator moves to another broker (i.e., leadership of a journal partition moves). The coordinator will scan the log from the last checkpointed HW. If there are any transactions that were in PREPARE_COMMIT or PREPARE_ABORT, the new coordinator will redo the COMMIT and ABORT. Note that transactions that are in-flight when a coordinator goes down don't necessarily need to be aborted - i.e., the producer can just send its CommitTransactionRequest to the new coordinator.

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