ition Trigger Action Policy Condition Trigger Action rule ID PDP1_Arl P1, x, B.y Do CUI PDP2_Arl P2, t, C.r Do CU2 PDP2_Br2 P3, f,C.g Do CU3 Model transformation Realization-dynamic composition Management/monitoring Service platforms Enablers Figure 5.1 Overview of the policy-enabled approach to service composition. modeled structurally using UML 2.x collaboration diagrams [1], and policy rules stored in a table are used for governing dynamic choices among collaborations, that is, dynamic composition. The PESM diagram graphically depicts the choreography of the collaboration uses involved in the composed service. While the service model layer provides a description of the service as a whole, the design model layer describes the service behavior as it is distributed to each separate component. The individual role behaviors are given by UML 2.x state machines, and the coordination of these is specified by local policy rules and local PESM diagrams, derived by transforma- tions from the global models and rules at the service model layer. From the design models, implementations can be generated, which in combination with a policy- management system in the runtime system, enable the actual runtime realization of the dynamic composition. In Figure 5.1, we have illustrated deployment of the policy-based approach delivered over next-generation networks (NGN) which con- sist of common IP-based networks and services in a distributed environment [6]. The policy-management system for enabling dynamic composition in the runtime system is illustrated on the left, to emphasize that the policy management is a sepa- rate management layer in the deployed system. Services are dynamically composed across user terminal equipment (TE) and service platforms which make use of enablers in the NGN. As shown in Figure 5.1, the policy management is a separate management layer in the deployed system. How to Model Dynamic Service Composition Using UML 2.x A policy-based dynamic composition approach requires a policy-specification model to exp