Assume that a distributed system is susceptible to server failure. What mechanisms would be required to guarantee the “exactly once” semantics for execution of RPCs?

The server should keep track in stable storage (such as a disk log) information regarding what RPC operations were received, whether they were successfully performed, and the results associated with the operations. When a server crash takes place and a RPC message is received, the server can check whether the RPC had been previously performed and therefore guarantee “exactly once” semanctics for the execution of RPCs.