Comment by lisper
> 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.
The piece makes a very simple distinction.
Delivery is the property of a message showing up at a receiver, irrespective of the receiver making state changes.
Processing is making state changes.
You can't dedupe messages without some kind of state change. Your guards, writing down that a given message has been here before, have been delivered the message. An endpoint on a lossy medium has to cope with either (0 or 1) or (1 or many) messages.
Now, can the guards deliver it to you at most once? Well, if there's no lossy medium between, sure. But we already know that we can deliver exactly once when the medium is perfectly reliable. The guards have "processed" the message for this purpose, and the fact that they can deliver it to you over a perfectly reliable channel is moot.
The distinction is between the characteristics of the channel (delivery) versus what the receiver must do to achieve appropriate processing properties.
These might come from some combination of intrinsic idempotency, timers, persisting past messages to disks, establishing a message ordering, etc, etc, etc. These are the mechanisms that you need to cope with "one or many" delivery, and they all shape the state model of your system with respect to messages.