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Ëoft w areengineers have deriv eda progressively better understanding of the characteristics of complexity in software. It is now widely recognised that inter action is probably the most important single characteristic of complex software. Ëoft w are arc hitectures thatcontain many dynamically interacting components, each with their own thread of con trol, and engaging in complex coordination protocols, are typically orders of magnitude more complex to correctly and efficiently engineer than those that simply compute a function of some input through a single thread of control.

Unfortunately, it turns out that many (if not most) realw orld applicationsha ve precisely thesecharacteristics. As a consequence, a major research topic in computer science over at least the past tw o decades has been the development of tools and techniques to model, understand, and implement systems in which interaction is the norm. Indeed …