Question

In an asynchronous messaging system, which delivery semantic guarantees that a message is processed exactly once, even if the producer retries due to a network timeout?

Accepted Answer

The delivery semantic that guarantees a message is processed exactly once is called exactly-once processing. In an asynchronous messaging system, this is achieved by combining two mechanisms: idempotent producers and idempotent consumers. An idempotent producer assigns a unique identifier to every message it sends, allowing the messaging broker to detect and discard duplicate messages resulting from producer retries after a network timeout. An idempotent consumer uses a unique identifier to track which messages have already been processed, ensuring that even if a message is delivered multiple times by the broker, the system state is updated only once. To guarantee exactly-once processing across both the messaging system and the storage of the resulting state, the system must often use atomic transactions or transactional commits, which bundle the consumption of the message and the writing of the outcome into a single, indivisible operation. Without these combined safeguards, a system might default to at-least-once delivery, where network issues lead to message duplication, or at-most-once delivery, where messages can be lost during processing failures.

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In an asynchronous messaging system, which delivery semantic guarantees that a message is processed exactly once, even if the producer retries due to a network timeout?

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