Comparing Petri Net and Activity Diagram Variants for Workﬂow Modelling – A Quest for Reactive Petri Nets Rik Eshuis  Roel Wieringa Department of Computer Science, University of Twente P.O.Box 217, NL-7500 AE, Enschede, The Netherlands, {eshuis,roelw}@cs.utwente.nl Abstract. Petri net variants are widely used as a workﬂow modelling technique. Recently, UML activity diagrams have been 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 or deny this claim, we need to formalise the intended semantics of activity diagrams and then compare this with various 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 un- derlie these two semantics and investigate whether these design choices can be met in low-level and high-level Petri net semantics. We argue that the main diﬀerence between the Petri net semantics and our seman- tics of UML activity diagrams is that the Petri net semantics models resource usage of 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 cannot model workﬂows accurately, unless they are extended with a syntax and semantics for reactivity. 1 Introduction Petri nets are a popular technique for modelling the control ﬂow dimension of workﬂows. When modelling workﬂows, people tend to draw nodes that represent tasks or activities, and arrows between the nodes that represent sequencing of activities. The resulting diagrams look like Petri nets, and so Petri nets seem a natural technique for modelling workﬂows [2,22]. The following arguments are of- ten used to support this: Petri nets are graphical, they have a formal semantics, they can express most of the desirable routing constructs, there is an abun- dance of analysis techniques for proving properties about them, and ﬁnally they are vendor-independent. Most of these arguments do not refer to the domain of workﬂow modelling (only the routing argument does) and point out advan- tages of Petri nets in general. Moreover, since Petri nets already existed before  Supported by NWO/SION, grant nr. 612-62-02 (DAEMON).