Exactly-Once Delivery Impossibility
The cluster discusses the challenges and impossibility of achieving exactly-once message delivery in distributed systems, often citing the Two Generals' Problem, and explores alternatives like at-least-once delivery with idempotency or exactly-once processing.
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exactly once delivery is not satisfied if the same message is delivered to the recipient multiple times
Even aliens haven't solved the problem of exactly-once message delivery
You left out exactly-once delivery and exactly-once delivery.
What if the "acknowledge receipt to sender" message gets lost?
Is guaranteed at-most-once delivery impossible?
Note that this also means that messages can be delivered more than once and/or that the clients need to remember the messages that they processed. In some setups that can be a showstopper.
Exactly once processing, not exactly once delivery. Because that's impossible. Provably so.https://en.wikipedia.org/wiki/Two_Generals%27_Problem
You can't guarantee delivery of any message. This is the basis of the 2 generals problem.
I've noticed that a lot of message queuing systems have an "delivery at least once" property. What exactly does the possibility that a message could be delivered more than once buy you?
That's probably by design. Would you prefer one unimportant message lost here and there, or be able to handle 1/1000 of the current traffic to make sure every single message is delivered?