Draw a diagram containing a source system, a destination system, and an unreliable communication channel between them. The destination system also has an output with no unreliable communications channel.
Exactly-once delivery means that a message sent from the source system reaches the destination system exactly once, and its result reaches the output channel exactly once as a consequence.
Excactly-once processing means that a message sent from the source system produces the expected output from the destination system once, even though it may be received by the destination system more than once.
(That's a little sloppy because it could use more discussion of the conditions in which it won't be received zero times, and how those are different between exactly-once and at-most-once delivery, but that's mostly beside the point because it isn't part of the distinction between exactly-once delivery and exactly-once processing. And, while definitely technical, they always involved a somewhat idealized view of the destination system, because all communications channels, including those internal to a single device, have some degree of unreliability.)
Yes, that is exactly my point. The only way you can make it non-sloppy is to define "delivery" as being something that happens exclusively upstream of deduping.
No, I'm saying its "sloppy" as a definition because while it addresses the distinction you ask about it, it doesn't fully cover what distinguishes exactly-once from at-most-once.
> The only way you can make it non-sloppy is to define "delivery" as being something that happens exclusively upstream of deduping.
"Deduping" can happen in many places. If it happens anywhere before the destination system end of the unreliable connection it is part of delivery (but also can't get you to exactly-once delivery). If it happens on the destination side of the unreliable communication channel, then yes, it's not part of the delivery guarantee, it is how you get exactly-once processing from at-least-once delivery. This has been well-known for a very long time. (I don't think it was new when I first encountered it in 1999.)
Exactly-once delivery means that a message sent from the source system reaches the destination system exactly once, and its result reaches the output channel exactly once as a consequence.
Excactly-once processing means that a message sent from the source system produces the expected output from the destination system once, even though it may be received by the destination system more than once.
(That's a little sloppy because it could use more discussion of the conditions in which it won't be received zero times, and how those are different between exactly-once and at-most-once delivery, but that's mostly beside the point because it isn't part of the distinction between exactly-once delivery and exactly-once processing. And, while definitely technical, they always involved a somewhat idealized view of the destination system, because all communications channels, including those internal to a single device, have some degree of unreliability.)