A software development lifecycle model describes the types of activity performed at each stage in a software development project, and how the activities relate to one another logically and chronologically.
There are numerous development life cycles that have been developed in order to achieve different required objectives. Each of these methodologies has its place in modern software development and the most appropriate development process should be applied to each project. The models specify the various stages of the process and the order in which they are carried out.
Sequential development models
A sequential development model describes the software development process as a linear, sequential flow of activities. This means that any phase in the development process should begin when the previous phase is complete. In theory, there is no overlap of phases, but in practice, it is beneficial to have early feedback from the following phase.
Sequential development models deliver software that contains the complete set of features, but typically
require months or years for delivery to stakeholders and users.
Waterfall model
In the Waterfall model, the development activities (e.g., requirements analysis, design, coding, testing) are completed one after another. In this model, test activities only occur after all other development activities have been completed.
With this model it has been difficult to get feedback passed backwards up the waterfall and there are difficulties if we need to carry out numerous iterations for a particular phase.
Defects were being found too late in the life cycle, as testing was not involved until the end of the project. Testing also added lead time due to its late involvement.
V-model
The V-model is a model that illustrates how testing activities (verification and validation) can be integrated into each phase of the life cycle. Within the V-model, validation testing takes place especially during the early stages, e.g. reviewing the user requirements, and late in the life cycle, e.g. during user acceptance testing.
A common type of V-model uses four test levels. The four test levels used, each with their own objectives, are component testing, integration testing, system testing, acceptance testing.
Iterative and incremental development models
There are iterative or incremental life cycles where, instead of one large development time line from beginning to end, we cycle through a number of smaller self-contained life cycle phases for the same project.
A common feature of iterative approaches is that the delivery is divided into increments or builds with each increment adding new functionality. The initial increment will contain the infrastructure required to support the initial build functionality.
The increment produced by an iteration may be tested at several levels as part of its development. Subsequent increments will need testing for the new functionality, regression testing of the existing functionality, and integration testing of both new and existing parts. Regression testing is increasingly important on all iterations after the first one. This means that more testing will be required at each subsequent delivery phase which must be allowed for in the project plans.
This life cycle can give early market presence with critical functionality, can be simpler to manage because the workload is divided into smaller pieces, and can reduce initial investment although it may cost more in the long run. Also early market presence will mean validation testing is carried out at each increment, thereby giving early feedback on the business value and fitness for-use of the product.
Examples of iterative or incremental development models are prototyping, Rapid Application Development (RAD), Rational Unified Process (RUP) and agile development.
