(D,...) SRid-userServintOrd interrupt state activity state pre: problems iie: interrupt pre: noProblem SRid-servCnf(...) post: AandDinCall iie: relFromOtherParty EFSM state (AandDinCall) SRid-userServRelOrd Fig. 6.17 An alternative service behaviour: the interrupt state Most state diagrams for service behaviour contain a mix of reactive states and active states, as was shown in the previous diagrams. However, the definition of transitions and states differ: A reactive state takes time (the time it takes for the other object to react). A transition from a reactive state is normally initiated by an external initiating event, but may also be initiated by an internal initiating event. A transition between reactive states is associated with an implied activity. The transition therefore takes time (the time it takes to perform the activity that is implied by the transition). 6 Process Control System Modelling An active state takes time (the time it takes to perform the activity, or less time if the activity is interrupted or prematurely stopped). A transition from an active state is normally caused by a certain precondition becoming true while the activity is performed, but may also be caused by an interrupt. Transitions to and from active states are not associated with any activity, and therefore take no time. It is important to use different types of state symbols for reactive states and active states in state diagrams. Note that most state machine modelling methods do not support the distinction between reactive states and active states. 6.2.5 Object Resource Model Service resource models are defined only from an interface point of view. That is sufficient for specifying operations and service behaviour (virtual objects), but not enough for describing the behaviour of other types of objects in the service net- work, e.g. the behaviour of agents. In our PCS, the TEPC and the CNPC are agents. An agent is an object that provides a single service, but depends on a number of other service objects for providing that servi