Comment by mlyle
A 4 year old piece laying out the exact difference as it's understood and how people use the terms:
https://blog.bulloak.io/post/20200917-the-impossibility-of-e...
I read similar content at least 2 decades ago...
> While exactly-once-delivery is not possible, we have a way out: Exactly-once processing. Exactly-once processing is the guarantee that even though we may receive a message multiple times, in the end, we observe the effects of a single processing operation. This can be achieved in two ways:
> Deduplication: dropping messages if they are received more than once
> Idempotent processing: applying messages more than once has precisely the same effect as applying it exactly once
(I view deduplication as a special case of idempotency).
> A 4 year old piece laying out the exact difference...
"Exactly-once delivery guarantee is the guarantee that a message can be delivered to a recipient once, and only once."
That seems circular to me.
Also, the author's proof is flawed. The 2GP requires more than exactly-once delivery, it requires common knowledge. It is not enough for the first general to know that the message will be received, it is required that the first general knows that the message has been received, and that the second general knows that the first general knows this, and that the first general knows that the second general knows... and so on.