Systems Development Life Cycle Models

The Waterfall Model

The Systems Development Life Cycle (SDLC) or Waterfall Model is an example of the underlying process used to develop the software in the 1970s. It was first introduced by Royce in 1970, was widely used during the 1980 and it is still used today.

It remains relevant because it provides the basis of many well-known system development methodologies. It makes a manager's task easier by providing a series of definite milestones and also helps the developer (as each stage can be viewed a separate contact). It allows the early clarification of system goals, architectures and interfaces and is often claimed to be conducive to good design practice.

The model:

• Provides an abstract description of the development process
• Separates logical and physical design
• Separates different functions and activities
• Decomposes functions and activities into discrete components

Each activity 'drives' the next: requirements (drives) specification (drives) implementation and unit testing (drives) integration and system testing (drives) operations and maintenance. Since its introduction, there have been many variations of the basic model, e.g.:

• Requirements Specification, Design Stage, Coding and Construction, Testing, Installation, Maintenance
• Problem/needs identification, Feasibility Study, Systems Analysis, Systems Design, Implementation, Review and Maintenance, De-commissioning
• etc ...

The weakness of this approach is the extent to which it relies on requirements that can be defined, in detail, in advance of any development work. Many developers will argue 'reality just isn't like that'; iteration is important in the software development process, especially:

• where the customer environment is changing
• where the regulatory environment is changing
• where the underlying technology is changing
• where the requirements are not yet understood by the customer
• where the requirements are not yet understood by the developer
• in particular types of applications, e.g. interface development

The Spiral Model

Attempts have been made to combine the rigour and management control of the SDLC with the benefits of the evolutionary approach, which is sometimes criticised as being difficult to manage effectively. The most notable attempt is that of Boehm (1988) who proposed the spiral model. This model uses the idea of working through a series of incremental releases. The development spirals outward from the centre with each cycle of the spiral resulting a further refinement of the system.

Using this approach, risks, such as design flaws, failing to meet user needs, escalating costs and losing sight of the intended system benefits, should be detected early in the development process. This aspect of the spiral model, sometimes leads to it being known as the 'risk-driven model'.

The model is split into four quadrants by a vertical axis, which represents the cumulative cost of the project, and a horizontal axis, which represents the increasing level of commitment. The four main activities in the model are each represented by one of the four quadrants.

1. Planning (bottom left) which is about planning the activities for the next phase.
2. Objectives (top left) which is about setting the objectives for the next phase.
3. Risk analysis (top right) which is concerned with the identification and resolution of risks.
4. Development (bottom right) which, in the early stages, might be the development of paper models and later might be the development of a prototype system.

Thus, for example, a prototype might be used to help define and understand the requirements or to illustrate a user interface. This prototype will then evolve and, in later cycles, become more and more complete until finally it becomes the fully tested and engineered final release of the system.

References

• W.W. Royce. (1970) Managing the Development of Large Software Systems: Concepts and Techniques. In: Technical Papers of Western Electronic Show and Convention (WesCon). August 25 - 28, 1970, Los Angeles, USA.
• B.W. Boehm. (1988) A Spiral Model of Software Development and Enhancement. IEEE Computer 21(5), pp. 61-72