Component Based Systems and Coordination. The concept of component based systems, especially component-based software, is a new phi- losophy or way of thinking to deal with the complexity in designing large scale computing systems[13, 135]. One of the main goals of this approach is to compose reusable compo- nents by some glue code. The model or the way in which these components are composed is called a coordination model. Thus, a component-based system has two main parts: a set of components and a coordinating subsystem. What are Components? A component is defined as a unit of computation that can be used independently and is subject to composition by a third party [135]. To be used indepen- dently, a clear distinction of the component from its environment and the other components is required. Thus, the component communicates with its environment through its interface. The interface of a component can be defined as a specification of its access points [135]. Therefore, a component is a black box computing system that communicates with the other components and its environment through its interface by which it offers its provided services. What is Coordination? A component based system needs connections and interactions among its components. The main goal of coordination is to manage the interaction among the components. Thus, coordination can be defined as the study of the dynamic topologies of interactions among components, and the construction of protocols to realize such topologies that ensure well-behavedness [15]. The task of coordination can be realized using different ways of communication, manage- ment and control strategies, methods of creation and termination of computational activities, and ways of synchronization of components. A set of definite choices for all of these different aspects is referred as a coordination model. Furthermore, a coordination language is the linguistic formalism that is used to specify a coordination model. Thus, a coordination language offers facilities for the specification of the ways of controlling synchronization, communication, creation and termination of compu- tational activities, and other aspects of the coordination model. There are many languages and models for coordination which provide formal description of the glue code for plugging components together. Coordination models and languages can be classified in different categories using (at least) three different aspects [15]:
