Architectural notes: a framework for distributed systems development

This thesis develops a framework of methods and techniques for distributed systems development. This framework consists of two related domains in which design concepts for distributed systems are defined: the entity domain and the behaviour domain. In the entity domain we consider structures of functional entities and their interconnection, while in the behaviour domain we consider behaviour definition and structuring. An interaction in which we abstract from the particular responsibilities of the participating functional entities is considered as an action. Behaviours consist of actions, interactions and their relationships. Relationships between actions and interactions are defined in terms of causality relations. In each causality relation the conditions and constraints for an action or interaction to occur are defined. Two important behaviour structuring techniques have been identified from the possible ways causality relations can be distributed: causality-oriented behaviour composition and constraint-oriented behaviour composition. Causality-oriented behaviour composition consists of placing some conditions of an action and the action itself in different sub-behaviours. Constraint-oriented behaviour composition consists of placing parts of the conditions and constraints of an action in different sub-behaviours, such that this action is shared by these sub-behaviours. This thesis identifies milestones in the design process of distributed systems, as well as the design steps to move from one milestone to another. These design steps are characterized using the concepts of the entity and the behaviour domain. We identified two crucial design operations of the behaviour domain that support these design steps: behaviour refinement and action refinement. Behaviour refinement consists of introducing (internal) structure in the causality relations of reference actions of an abstract behaviour, but preserving their causality and exclusion relationships and their attribute values. Action refinement consists of replacing abstract actions by activities, such that the completion of these activities correspond to the occurrence of the abstract actions. One important characteristic of action refinement is the possibility of distributing attribute values of the abstract actions over actions of the activities that replace them in the concrete behaviours. The area of research, scope and objectives of this thesis are discussed in Chapter 1. The concept of design culture and its elements is introduced in this chapter in order to provide an overview of the important aspects of the design process. Entity domain, behaviour domain, and design milestones are introduced and discussed in Chapter 2. This chapter also discusses the global objectives of design steps, and the abstraction obtained by considering interactions between cooperating functional entities as actions of the interaction system between these entities. Action, action attributes, causality and exclusion are discussed in Chapter 3. This chapter shows how a behaviour can be defined in terms of the causality relations of its actions in a monolithic form. Causality-oriented behaviour composition is discussed in Chapter 4. Entries and exits of a behaviour are the mechanisms that make it possible to assign parts of a condition of an action and the action itself to different sub-behaviours. Constraint-oriented behaviour composition is discussed in Chapter 5. Decomposition possibilities of monolithic behaviours are systematically studied in this chapter. Behaviour refinement is discussed in Chapter 6. This chapter defines a method to obtain an abstraction of a concrete behaviour. This method can be used to check whether the concrete behaviour corresponds to a certain abstract behaviour. Action refinement is discussed in Chapter 7. This chapter identifies some activity forms, and define the rules for considering these activities as implementations of an abstract action. These rules are used in a method to derive an abstraction of a concrete behaviour in which the abstract actions are implemented as activities. This method can be used to check whether the concrete behaviour corresponds to a certain abstract behaviour. Chapter 8 discusses a design example that is meant to illustrate the use of our design concepts. The example is an interaction server, which is a component that supports the interaction between multiple functional entities. Chapter 9 draws some conclusions and revisits the design milestones of Chapter 2, showing alternatives for the design trajectory which have been created with the use of actions and interactions in a single framework.