Design clarifies what is to be built in more precise terms, specifically, how the design meets the requirements. The goal of the Design phase is to show how the application will be realized in the implementation phase. You want to build an application that:

• Performs-in a specific implementation environment-the tasks and functions specified in the use-case descriptions.
  
• Fulfills all its requirements.
  
• Is structured to be robust (easy to change if and when its functional requirements change).
  
• Is simple, yet elegant.
  

The design model serves as an abstraction of the source code or a 'blueprint' of how the source code is structured and written.

Implementation

The purposes of implementation is as follows:

• To define the organization of the code, in terms of implementation subsystems organized in layers.
  
• To implement classes and objects in terms of components (source files, binaries, executables, and others).
  
• To test the developed components as units.
  
• To integrate the results produced by individual implementers (or teams), into an executable system.
  

Testing

The purposes of testing are as follows:

• To verify the interaction between objects.
  
• To verify the proper integration of all components of the software.
  
• To verify that all requirements have been correctly implemented.
  
• To identify and ensure defects are addressed prior to the deployment of the software.
  

The Process proposes an iterative approach, which means that you test throughout the project. This allows you to find defects as early as possible, which radically reduces the cost of fixing any defects.

Deployment

With each tested application, the development team makes a final build and installs the new application for the users. Any relevant installment and configuration notes and issues are properly documented for further releases.