A Comparison of Petri Net and Activity Diagram Variants

Petri net variants are widely used as a workflow modelling technique. Recently, UML activity diagrams are used for the same purpose, even though the syntax and semantics of activity diagrams has not been yet fully worked out. Nevertheless, activity diagrams seem very similar to Petri nets and on the surface, one may think that they are variants of each other. To substantiate this claim, we need to formalise the intended semantics of activity diagrams and then compare this with Petri Net semantics. In previous papers we have defined two formal semantics for UML activity diagrams that are intended for workflow modelling. In this paper, we discuss the design choices that underlie these two semantics and investigate whether these design choices are met in low-level and highlevel Petri net semantics. We argue that the main difference between the Petri net semantics and our semantics of UML activity diagrams is that the Petri net semantics models closed, active systems that are non-reactive, whereas our semantics of UML activity diagrams models open, reactive systems. Since workflow systems are open, reactive systems, we conclude that Petri nets are not entirely suitable for workflow modelling. We end with a discussion of the issues involved in defining a reactive Petri net semantics.