Question

Where does the need for at-least-once and at-most-once semantics come from? Why can’t we have exactly-once semantics?

Answer 1

These terms are mostly used in cloud computing or on distributed systems where messages need to be sent. At-least-once means that many attempts can be made in sending a message and atleast one message will be delivered. But some messages can be lost in this process. At-most-once means that a message sent across a distributed system would be delivered one time or not delivered at all. So again messages can be lost in this process. No state needs to be maintained for the messages in this case and this method the cheapest of all.

Exactly-once means that message will be delivered for sure. However, exactly-once is not preferred due to many reasons. First, it is very costly. Second, distributed systems may not agree on a particular message in case one of the systems does not work. So this semantic won't be implemented on its own in a distributed system. Mostly, atleast-once is preferred.