As this evolutionary process begins, the software engineering team moves around the spiral in a clockwise direction,
beginning at the center. The first circuit around the spiral might result in the development of a product specification;
subsequent passes around the spiral might be used to develop a prototype and then progressively more sophisticated
versions of the software. Each pass through the planning region results in adjustments to the project plan. Cost and
schedule are adjusted based on feedback derived from customer evaluation. In addition, the project manager adjusts the
planned number of iterations required to complete the software.
Unlike classical process models that end when software is delivered, the spiral model can be adapted to apply throughout
the life of the computer software. An alternative view of the spiral model can be
considered by examining the project
entry point axis, also shown in Figure 2.8. Each cube placed along the axis can be used to represent the starting point for
different types of projects. A “concept development project” starts at the core of the spiral and will continue (multiple
iterations occur along the spiral path that bounds the central shaded region) until concept development is complete. If the
concept is to be developed into an actual product, the process proceeds through the next cube (new product development
project entry point) and a “new development project” is initiated. The new product will evolve through a number of
iterations around the spiral, following the path that bounds the region that has somewhat lighter shading than the core. In
essence, the spiral, when characterized in this way, remains operative until the software is retired. There are times when
the process is dormant, but whenever a change is initiated, the process starts at the appropriate entry point (e.g., product
enhancement).