tation Fig. 7.3 The meaning of a service relation The operation concept is discussed in more detail in Sect. 7.4.3. Note that the fact that only the service-providing object in a service relation provides operations, does not mean that the service user object is the one that initiates all operations (as is normally assumed in request-response type of interfaces). The provider may ini- tiate an operation execution as well. In addition, the provider may execute opera- tions spontaneously and only notify the user what has been done. Thus, don't confuse the user-provider roles in a service relation with the interaction sequences between the user and the provider. As Fig. 7.2 shows, there are three classes of service objects: virtual object, agent and actor. Virtual objects capture the properties of a provider port and are key ele- ments for specifying primitive services. Given a virtual object, agents are identified and specified as units that are parts of the substructure of virtual objects, see the example in Fig. 7.4. Agents are key elements in specifying the abstraction hierar- chy of a service network. Virtual Abstraction level 0 object Agent Abstraction level 1 Virtual Virtual object object Fig. 7.4 Agents are parts of a substructure of a virtual object 7 Concepts and Notations for Process Control Systems Modelling Actors are service objects that have no provider port, but one or several user ports. This makes actors self-regulating objects, i.e. objects that maintain a steady state of something. Actors are therefore key elements in modelling, e.g. resource controllers. Figure 7.5 shows an actor that maintains a constant temperature of a fluid by using the operations temp, heaterOn and heaterOff provided by a sensor and an effector (modelled as virtual objects). Actor Sensor Heater model model heaterOnORD tempC_REP heaterOffORD Temp. Heater sensor Fig. 7.5 Example of an actor Although it is possible to imagine a function in a process control system that operates according to this model, we would probably be mor