Promise pipelining

The use of futures can dramatically reduce latency in distributed systems. For instance, futures enable promise pipelining, as implemented in the languages E and Joule, which was also called call-stream in the language Argus.
Consider an expression involving conventional remote procedure calls,
Each statement needs a message to be sent and a reply received before the next statement can proceed. Suppose, for example, that x, y, t1, and t2 are all located on the same remote machine. In this case, two complete network round-trips to that machine must take place before the third statement can begin to execute. The third statement will then cause yet another round-trip to the same remote machine.


The syntax used here is that of the language E, where x <- a() means to send the message a() asynchronously to x. All three variables are immediately assigned futures for their results, and execution proceeds to subsequent statements. Later attempts to resolve the value of t3 may cause a delay; however, pipelining can reduce the number of round-trips needed. If, as in the previous example, x, y, t1, and t2 are all located on the same remote machine, a pipelined implementation can compute t3 with one round-trip instead of three. Because all three messages are destined for objects which are on the same remote machine, only one request need be sent and only one response need be received containing the result. Note also that the send t1 <- c(t2) would not block even if t1 and t2 were on different machines to each other, or to x or y.
Promise pipelining should be distinguished from parallel asynchronous message passing. In a system supporting parallel message passing but not pipelining, the message sends x <- a() and y <- b() in the above example could proceed in parallel, but the send of t1 <- c(t2) would have to wait until both t1 and t2 had been received, even when x, y, t1, and t2 are on the same remote machine. The relative latency advantage of pipelining becomes even greater in more complicated situations involving many messages.
Promise pipelining also should not be confused with pipelined message processing in Actor systems, where it is possible for an actor to specify and begin executing a behaviour for the next message before having completed processing of the current message.