How does idempotency differ from Exactly-once delivery?

Idempotency and Exactly-once delivery are different concepts related to reliability and repeatability of operations.

• Idempotency is a property of an operation where multiple executions with the same input do not change the result compared to a single execution. That is, a repeated call does not lead to additional side effects.

  Example: HTTP PUT method for updating a resource — if the same request is sent multiple times, the resource's state remains the same.

• Exactly-once delivery is a guarantee that a message will be delivered or an operation will be performed exactly once, without losses or duplicates.

  In message transmission systems, this means that the message will be processed exactly once, despite possible failures or retries.

Differences:

• Idempotency is a property of an operation that allows it to be safely repeated without changing the result.
• Exactly-once delivery is a guarantee of the delivery system that the operation will be performed exactly once.

In unreliable delivery systems, these approaches are often combined: making operations idempotent so that repeated delivery (at-least-once) does not cause unwanted effects, or implementing complex mechanisms for exactly-once.

In embedded/IoT systems, where connectivity can be unreliable, idempotency simplifies the processing of repeated messages, while exactly-once delivery requires additional protocols and acknowledgments.