at-most-once and exactly-once
I am studying Distributed Systems and when it comes to the RPC part, I have heard about these two semantics (at-most-once and exactly-once). I understand that the at-most-once is used on databases for instances, when we don't want duplicate execution.
First question:
How is this achieved? How does the server know that it shouldnt execute the request again? It might be a duplicate but it might be a legitimate request as well.
The second question is:
What is the difference between the two semantics in the title? I can read :). I know that at-most-once might not be executed at all but, what does exactly-once do that guarantees the execution?
Answer
In cases of at most once semantics, request is sent again in case of failure, but request is filtered on the server for duplicates.
In exactly once semantics, request is sent again, request is filtered for duplicate and there is a guarantee for the server to restart after failure and start processing requests from where it crashed.
But exactly once is not realizable because what happens when client sends request, and before it reaches the server, server crashes. There is no way of tracking the request.
http://de.wikipedia.org/wiki/Remote_Procedure_Call#Fehlersemantik