handles selector operations, see Fig. 7.59. These types of operations provide users with information about the actual state, i.e. information accessed from the SVS. Those operations cause no change to the resource state of the service object. 7 Concepts and Notations for Process Control Systems Modelling Confirm Request Fig. 7.59 Execution of a selector operation in the state machine 1. The external Request is detected by the PO, which accesses the operation speci- fication for the referred operation. PO checks the Request with respect to time and other constraints. Assuming that everything is in order, it checks the opera- tion property EFFECT and finds that the operation is of the selector type. 2. PO analyses the parameters of the Request and initiates one or several que- ryReq(query) to SO. 3. SO browses through the SVS and produces the requested answers in que- ryCnf(answer). 4. When PO has all necessary queryCnf(answer), it compiles them into the param- eters of the Confirm event. The next scenario describes how the machine handles internal initiating events. Normal scenario: ReportConfirm Forced state changes: Order Fig. 7.60 Cause and execution of internal initiating event in the state machine 7.6 The General State Machine Model Such events are always caused by a change in the resource state that is detected by the SO. SO then triggers EO to do something about it. In the normal scenario, changes to SVS are performed as a result of an external event initiated by an AO. There might, however, also exist special initiator operations that force the SO to change the value of SVS as a way of initiating the state machine in another resource state, see the example discussed in conjunction with Fig. 7.6 in sect. 7.2.1. However, since the SVS reflects the states of AOs, such operations must either be combined with internal initiating events that change the states of AOs accordingly, or the states of AOs must be set in some other way. The scenario for initiator oper- ations is the "forced state changes" scenario in F