Problem definition and specification. The hardest, but most important, part of the task of creating a software system is defining what the problem really is and then specifying what is needed to solve it. Usually, problem definition begins by analyzing the users’ requirements, but these requirements are often imprecise and hard to write down. The system architect may have to consult with the future users of the system and iterate the specification, until both the specifier and the users are satisfied that the specification defines and solves the problem at hand. In the Prototyping specification stage, it may be helpful to build a simple prototype or model of the final system, to gain insight into its behavior and intended use. Data modeling is also an important tool in the problem-definition phase. Design. Once the specification is complete, a high-level design of the system is created, with the major components identified. A document outlining the high-level design is prepared, and performance requirements for the system may be included. More detailed specifications of some of the major components may also be included Software reuse during this phase. A cost-effective design often calls for the reuse or modification of previously constructed components. Various software methodologies such as object- oriented technology facilitate the reuse of components.
Implementation. Once the design is fixed, implementation of the components can proceed. Many of the algorithms discussed in this book are useful in the implementation of new components. Once a component has been implemented, it is subject to a series of tests to make sure that it behaves as specified. Integration and system testing. When the components have been implemented and individually tested, the entire system is assembled and tested. Installation and field testing. Once the developer is satisfied that the system will work to the customer’s satisfaction, the system is installed on the customer’s premises and the final field testing takes place. Maintenance. At this point, we might think that the bulk of the work has been done. Maintenance remains, however, and in many situations maintenance can account for more than half the cost of system development.
Maintenance may involve modifying components to eliminate unforeseen side-effects, to correct or improve system performance, or to add features. Because maintenance is such an important part of software systems design, it is important to write programs that are correct, rugged, efficient, modifiable, and — whenever possible — portable from one computer to another. It is very important to catch errors as early as possible, preferably during the problem-definition phase. At each successive phase, the cost of fixing a design error or programming bug rises greatly. Independent reviews of requirements and designs are beneficial in reducing downstream errors.