In: Weber, Ehrig, Reisig (Eds.): Proc. of 2nd Int. Coll. on Petri Net Technologies for Modelling Communication Based Systems, pages 93-104. DFG Research Group "Petri Net Technology", September 2001.
Abstract: Petri Nets are widely used as workflow modeling tool.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 be able to discuss this claim, we propose a syntax and semantics for UML activity diagrams that remains within the UML spirit and that is motivated by our intention to model workflows.We compare our activity diagrams thus made precise,with both high-level and low-level Petri Net semantics,looking at semantics where workflow activities are represented by Petri net states as well as semantics where workflow activities are represented by Petri net transitions.Our comparison leads to the identification of trigger events as an important distinction between Petri nets and activity diagrams.We argue that trigger events are necessary in workflow modeling.Introduction of trigger events forces us to also introduce input event queues and a requirement in maximal steps in the execution semantics of activity diagrams.