Dynamic Composition of Services – a Model - Based Approach

This thesis addresses the problem of rapid development and dynamic deploymentof services in the distributed service environment. It is not enough to deploy ser-vices rapidly, as the dynamic service environment is changing continually, requiringservices to be adaptable also. By this we mean, that in addition to being able torapidly deploy new services, it must be possible to dynamically change existing ser-vices to adapt to conditions in the service environment that were not foreseen whenthe service was originally deployed. In this environment, service availability is animportant concern.Traditionally, availability has been seen as an atomic property asserting the aver-age time a system is "up" or "down". In order to model and analyse the availabilityof computerised systems in a world where the dependency on and complexity of suchsystems are increasing, this notion of availability is no longer sufficient. This thesisprovides a conceptual model for service availability designed to meet the challengesof ensuring availability in a rapidly changing telecommunications environment. Thecore of this model is a characterisation of service availability by means of accessibilityproperties and exclusivity properties, which is further specialised into measurableaspects of service availability.One of the core challenges of service engineering is to find practical ways to modelservices (partial functionalities) separately such that services may be composed intowell functioning systems while satisfying availability requirements. This is especiallychallenging for services being provided in a distributed manner requiring cooperationof several components and actors (users).This thesis presents a policy-driven methodology and approach for modellingdynamic composition of services. This involves a model-driven concept for enablingdynamic composition of separately specified service parts. We demonstrate this con-cept for dynamically adding authentication and authorization to existing services, asa means to provide and restrict access to the service for authorized users only. Theconcept of a policy enforcement state machine (PESM) diagram and compositionpolicies are used to model dynamic composition of services. The PESM diagramsand composition policies specify the coordination of separately specified UML2.xcollaborations.