he service- using object uses. This behaviour is therefore associated with the provider port and constrained by the user port. The basic behavioural element for describing the behaviour of objects is therefore the behaviour that is associated with primitive services. We will therefore make the following definitions: 7 Concepts and Notations for Process Control Systems Modelling A service behaviour defines the sequences of interactions with other objects that an object has in its provider port. An object behaviour defines the sequences of interactions an object has with other objects, considering all its service ports, i.e. both provider and use ports. Figure. 7.38 shows how service behaviour can be described for a small service network. Service behaviour: A Active state Numbered events: Sequence diagram: State diagram: P Pending state R Resource state user port behaviour iie internal initiating event provider port behaviour Fig. 7.38 Service behaviour - three methods The diagrams show a part of a service behaviour, consisting of a Request and a Confirm event. The figure also shows three ways in which the behaviour could be described: as numbered events in the service network model, as a sequence dia- gram or as state diagrams, consisting of different types of states and of transitions. In the latter case, separate state diagrams are constructed for the provider and user port. In Fig. 7.39 a substructure for the virtual object is introduced, and the object behaviour for the agent in the substructure is described. Object behaviour: Numbered events: Sequence diagram: State diagram: Cnf1 4 1 Req1 object behaviour S Synchronization state Fig. 7.39 Object behaviour The numbered event technique is frequently used in this book. It is a good tech- nique for tutorial purposes because it combines the structure of the service net- 7.5 Behaviour Descriptions work with its behaviour. However, from a specification point of view, it is not usable, since different sequences of events are not easily separated and states cannot be 