A software life cycle is a phased approach to developing software, with specific deliverables and milestones within each phase. A software life cycle model is an abstraction of the software development process, which is convenient to use for planning purposes. This chapter takes a software life cycle perspective on software development. Different software life cycle models (also referred to as software process models), including the spiral model and the Unified Software Development Process, are briefly described and compared. The roles of design verification and validation and of software testing are discussed.

SOFTWARE LIFE CYCLE MODELS

The waterfall model was the earliest software life cycle model to be widely used. This section starts with an overview of the waterfall model. It then outlines alternative software life cycle models that have since been developed to overcome some of the limitations of the waterfall model. These are the throwaway prototyping life cycle model, the incremental development life cycle model (also referred to as evolutionary prototyping), the spiral model, and the Unified Software Development Process.

Waterfall Life Cycle Model

Since the 1960s, the cost of developing software has grown steadily and the cost of developing and purchasing hardware has rapidly decreased. Furthermore, software now typically costs eighty percent of a total project's budget, whereas in the early days of software development, the hardware was by far the largest project cost (Boehm 2006).