Vendor: "Exactly once is possible. We do it!" No, as a vendor you're either deliberately lying, or completely incompentent. Let's insert the words they don't w

Vendor: "Exactly once is possible. We do it!" No, as a vendor you're either deliberately lying, or completely incompentent. Let's insert the words they don't want to. "We do exactly once good enough for your application!" How could you possibly know that? "We do exactly once good enough for all practical purposes!" So are my purposes practical? Am I a true scotsman? "We do exactly once good enough for applications we're good enough for!" Finally, a truthful claim. I'm not holding my breath. Edit: More directly address parent: It's not possible to even guarentee at-least-once in a finite amount of time, let alone exactly-once. For example, a high-frequency trading system wants to deliver exactly one copy of that 10-million-share order. And do it in under a microsecond. If you've got a 1.5 microsecond round trip, it's not possible to even get confirmation an order was received in that time limit, much less attempt another delivery. Your options are a) send once and hope. b) send lots of copies, with some kind of unique identifier so the receiver can do process-at-most-once, and hope you don't wind up sending to two different receiving servers.