omposite roles assisted caller, assisting registrar, and reg assister are bound to the PatientT, TeleSvcDsk, and ReceptionistT composite roles, respectively. The other is the consultation collaboration for which the composite roles consulter, consultant, and consult administrator are bound to the PatientT, Doctor T, and TeleSvcDsk composite roles, respectively. The collaboration overview diagram in Figure 5.2 describes the subcollabora- tions involved in the tele-consultation service, but not how their associated behav- ior should be composed in order to achieve the desired behavior for the composed service. We use global PESM diagrams and global composition policies to describe this, as explained in the next section. As will be specified later by the PESM dia- gram, only one of registration and aided Registration will be per- formed during a single run of the tele-consultation service. In the next two sections, we describe how composition policies and PESM dia- grams may be used to describe how the associated behavior should be composed in order to achieve the desired behavior for the composed service. Aided registration Assisting registrar Registrar Assisted caller registration Caller per : call Registeree Callee Reg assister Figure 5.4 Collaboration for the aided registration service used in the teleconsultation service. How to Model Dynamic Service Composition Using UML 2.x 5.4.2 Global Composition Policies A composition policy is a set of policy rules specifying how a given set of collabora- tions can be composed to obtain a given set of composite services. As such, a com- position policy applies to a family of related services, where the number of possible services in the family depends on the choices governed by the policy rules. Choices are made to decide which set of collaboration uses will be composed in order to deliver the selected service. In terms of the family of services defined by a composition policy, a UML 2.x collaboration overview diagram graphically represents a high-level view of the ser- v