Q. Presume that a distributed system is susceptible to server failure. What mechanisms would be needed to guarantee the exactly once semantics for execution of RPCs?
Answer: The server must keep track in stable storage (such as a disk log) information regarding what RPC operations were received whether they were successfully performed in addition to the results associated with the operations. When a server crash occurs and a RPC message is received the server can check whether the RPC had been before performed and therefore guarantee exactly once semanctics for the execution of RPCs.