The Waterfall Model^[1] as shown in Figure 1.13, “The Waterfall Model” is a linear sequential model that begins with definition and ends with system operation and maintenance. It is the most common software development life cycle model and is particularly useful when specifying overview project plans as it fits neatly into a Gantt chart format^[2].

Figure 1.13. The Waterfall Model

The seven phases in the process as shown in Figure 1.13, “The Waterfall Model” are:

The Waterfall Model is a general model, where in small projects some of the phases can be dropped, whereas in large scale software development projects some of these phases may be split into further phases. At the end of each phase the outcome is evaluated and if approved development can progress to the next phase. If the evaluation is rejected then the last phase must be revisited and in some cases earlier phases may need to be examined. In Figure 1.13, “The Waterfall Model” the thicker line shows the likely path if all phases are going well. The thinner lines show a retrace of steps to the same phase or previous phases.

The Spiral Model^[3] was suggested by Boehm (1988) as a methodology for overseeing large scale software development projects that show high prospects for failure. It is an iterative model that builds in risk analysis and formal client participation into prototype development. This model can be demonstrated as in Figure 1.14, “The Spiral Model”.

Figure 1.14. The Spiral Model

The spiral, as shown in Figure 1.14, “The Spiral Model” of development is iterative, with each iteration involving planning, risk analysis, engineering (from design, to coding, testing, installation and then release) and customer evaluation (including comments, changes and further requirements). More advanced forms of this model are available for dealing with further communication with the client.

The spiral model is particulary suited to large scale software development projects and needs constant review. For smaller projects an agile development model is more suitable (discussed later).

One object-oriented methodology is based around the re-use of development modules and components. As such, a new development model is required that takes this re-use into account. The object-oriented model as shown in Figure 1.15, “The Object-Oriented Design Model” builds integration of existing software modules into the system development. A database of reusable components supplies the components for re-use. The object-oriented model starts with the formulation and analysis of the problem. The design phase is followed by a survey of the component library to see if any of the components can be re-used in the system development. If the component is not available in the library then a new component must be developed, involving formulation, analysis, coding and testing of the module. The new component is added to the library and used to construct the new application.

This model aims to reduce costs by integrating existing modules into development. These modules are usually of a higher quality as they have been tested in the field by other clients and should have been debugged. The development time using this model should be lower as there is less code to write.

Figure 1.15. The Object-Oriented Design Model

The object-oriented model should provide advantages over the other models as the libary of components grows over time.

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