Dynamic Composition of Services– a Model-Based Approach
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This thesis addresses the problem of rapid development and dynamic deployment of services in the distributed service environment. It is not enough to deploy services rapidly, as the dynamic service environment is changing continually, requiring services to be adaptable also. By this we mean, that in addition to being able to rapidly deploy new services, it must be possible to dynamically change existing services to adapt to conditions in the service environment that were not foreseen when the service was originally deployed. In this environment, service availability is an important concern. Traditionally, availability has been seen as an atomic property asserting the average time a system is "up" or "down". In order to model and analyse the availability of computerised systems in a world where the dependency on and complexity of such systems are increasing, this notion of availability is no longer sufficient. This thesis provides a conceptual model for service availability designed to meet the challenges of ensuring availability in a rapidly changing telecommunications environment. The core of this model is a characterisation of service availability by means of accessibility properties and exclusivity properties, which is further specialised into measurable aspects of service availability. One of the core challenges of service engineering is to find practical ways to model services (partial functionalities) separately such that services may be composed into well functioning systems while satisfying availability requirements. This is especially challenging for services being provided in a distributed manner requiring cooperation of several components and actors (users). This thesis presents a policy-driven methodology and approach for modelling dynamic composition of services. This involves a model-driven concept for enabling dynamic composition of separately specified service parts. We demonstrate this concept for dynamically adding authentication and authorization to existing services, as a means to provide and restrict access to the service for authorized users only. The concept of a policy enforcement state machine (PESM) diagram and composition policies are used to model dynamic composition of services. The PESM diagrams and composition policies specify the coordination of separately specified UML2.x collaborations.