69. Process Networks - PN

In the process networks (PN) domain, created by Mudit Goel, processes communicate by sending messages through channels that can buffer the messages. The sender of the message need not wait for the receiver to be ready to receive the message. This style of communication is often called asynchronous message passing.

There are several variants of this technique, but the PN domain specifically implements one that ensures determinate computation, namely Kahn process networks. In the PN model of computation, the arcs represent sequences of data values (tokens), and the entities represent functions that map input sequences into output sequences. Certain technical restrictions on these functions are necessary to ensure determinacy, meaning that the sequences are fully specified.

In particular, the function implemented by an entity must be prefix monotonic. The PN domain realizes a subclass of such functions, first described by Kahn and MacQueen, where blocking reads ensure monotonicity. PN models are loosely coupled, and hence relatively easy to parallelize or distribute. They can be implemented efficiently in both software and hardware, and hence leave implementation options open.

A key weakness of PN models is that they are awkward for specifying control logic, although much of this awkwardness may be ameliorated by combining them with FSM. The PN domain in Ptolemy II has a highly experimental timed extension. This adds to the blocking reads a method for stalling processes until time advances. We anticipate that this timed extension will make interoperation with timed domains much more practical.