As a consequence of the growing interest in active databases the need for abstract activity modelling concepts arises. The object-oriented approach provides a natural view of evolving systems as collections of objects having both structure as well as behaviour. Based on this view we dicuss \em event calling structures as an abstract model for synchronous communication of objects. This basic model defines a powerful activity concept on a higher description level abstracting from operational procedure calling. Event calling structures have to be analyzed for consistency in terms of conflicting updates. We discuss possible restrictions that make such analyzation feasible. Since event calling structures are used to model triggering of a set of events, finiteness of calling sets is a desirable feature. Again we identify restrictions that make it possible to guarantee finiteness.