Visual formalisms revisited

The development of an interactive application is a complex task that has to consider data, behavior, intercommunication, architecture and distribution aspects of the modeled system. In particular it presupposes the successful communication between the customer and the software expert. To enhance this communication most modern software engineering methods recommend specification of the different aspects of a system by visual formalisms. In essence, visual specifications are directed graphs that are interpreted in a particular way for each aspect of the system. They are also intended to be compositional. This means that each node can itself be a graph with a separate meaning. However, the lack of a denotational model for hierarchical graphs often leads to the loss of compositionality. This has severe negative consequences in the development of realistic applications. We present a simple denotational model (which is by definition compositional) for the architecture and behavior aspects of a system. This model is then used to give a semantics to almost all the concepts occurring in ROOM. Our model also provides a compositional semantics for or-states an statecharts