Q. Once more considering the RPC mechanism consider the exactly once semantic. Does the algorithm for implementing this semantic implement correctly even if the ACK message back to the client is lost due to a network problem? Explain the sequence of messages and whether exactly once is still preserved.
Answer: The exactly once semantics make sure that a more procedure will be executed exactly once and only once. The universal algorithm for ensuring this combines an acknowledgment (ACK) scheme combined with timestamps (or some other incremental counter that allows the server to distinguish between duplicate messages).
The universal strategy is for the client to send the RPC to the server along with a timestamp. The client will as well start a timeout clock. The client will after that wait for one of two occurrences: (1) it will receive an ACK as of the server indicating that the remote procedure was performed or (2) it will time out If the client times out it presume the server was unable to perform the remote procedure so the client invokes the RPC a second time sending a later timestamp. The client mayn't receive the ACK for one of two reasons (1) the original RPC was never received by the server or (2) the RPC was correctly received-and performed-by the server however the ACK was lost. In situation (1) the use of ACKs permits the server ultimately to receive and perform the RPC. In situation (2) the server will obtain a duplicate RPC and it will use the timestamp to identify it as a duplicate so as not to perform the RPC a second time. It is significant to note that the server must send a second ACK back to the client to inform the client the RPC has been performed.