Modelling the Interaction of Distributed Systems as Protocols

In descriptions of loosely coupled process-like interactions of computational systems and especially in the literature on electronic business processes, the protocol notion is often used only informally. However, its complete expressive power becomes effective only with a precise formalization. Based on Holzmann's protocol concept, a formal protocol definition is introduced, providing an inductive definition of the protocol transition relation. The protocol transition relation describes all possible interactions between system components which are formally specified as nondeterministic extended finite input output automata. The formal approach is illustrated by means of a buying selling business interaction. Additionally, it is used to illustrate some semantic shortcomings of the "transaction pattern" approach, which tries to partition process-like interactions into one- and two-way interactions. In the discussion, the "loose" aspect of a protocol based interaction is treated. Motivated by the non-functional system relation defined by a protocol, it is proposed to classify the exchanged information between systems according to their logical relation in documents for (nondeterministic) protocols and I/O parameters for remote function calls.