On the computational complexity of the reachability problem in UML activity diagrams

The reachability problem, the problem of checking whether certain node can be reached from some initial settings such that all restrictions on flow of control are satisfied, arises frequently in various applications of Unified Modeling Language activity diagrams. However, the complexity of the problem, or in fact the complexity of any general problem in UML activity diagrams, has yet to be investigated. Towards this initiative, we specify a class of diagrams where the reachability problem is PSPACE-complete. However, if one further condition is applied, it can be shown that the problem is NP-complete.