3. Use Component-Based Architecture

Use cases drive the Rational Unified Process throughout the entire lifecycle, but design activities center on architecture -- either system architecture or, for software-intensive systems, software architecture. The main focus of early iterations is to produce and validate a software architecture. In the initial development cycle, this takes the form of an executable architectural prototype that gradually evolves, through subsequent iterations, into the final system.

The Rational Unified Process provides a methodical, systematic way to design, develop, and validate an architecture. It offers templates for describing an architecture based on the concept of multiple architectural views. It provides for the capture of architectural style, design rules, and constraints. The design process component contains specific activities aimed at identifying architectural constraints and architecturally significant elements, as well as guidelines on how to make architectural choices. The management process shows how planning the early iterations takes into account the design of an architecture and the resolution of major technical risks.

A component can be defined as a nontrivial piece of software: a module, package, or subsystem that fulfills a clear function, has a clear boundary, and can be integrated into a well-defined architecture. It is the physical realization of an abstraction in your design. Component-based development can proceed in several ways:

• In defining a modular architecture, you identify, isolate, design, develop, and test well-formed components. These components can be individually tested and gradually integrated to form the whole system.
•Furthermore, some of these components can be developed to be reusable, especially components that provide solutions to a wide range of common problems. Reusable components are typically larger than mere collections of utilities or class libraries. They form the basis of reuse within an organization, increasing overall software productivity and quality.
• More recently, the advent of commercially successful infrastructures supporting the concept of software components -- such as Common Object Request Broker Architecture (CORBA), the Internet, ActiveX, and JavaBeans -- has launched a whole industry of off-the-shelf components for various domains, allowing developers to buy and integrate components rather than develop them in-house. The first point above exploits the old concepts of modularity and encapsulation, bringing the concepts underlying object-oriented technology a step further. The final two points shift software development from programming software (one line at a time) to composing software (by assembling components).

The Rational Unified Process supports component-based development in several ways.

• The iterative approach allows developers to progressively identify components and decide which ones to develop, which ones to reuse, and which ones to buy.
• The focus on software architecture allows you to articulate the structure. The architecture enumerates the components and the ways they integrate, as well as the fundamental mechanisms and patterns by which they interact.
• Concepts such as packages, subsystems, and layers are used during analysis and design to organize components and specify interfaces.
• Testing is organized around single components first and then is gradually expanded to include larger sets of integrated components.