Spiral Model

The spiral model of software development and evolution represents a risk driven approach to software process analysis and structuring (Boehm 1987, Boehm et al, 1998). This approach, developed by Barry Boehm, incorporates elements of specification-driven, prototype-driven process methods, together with the classic software life cycle. It does so by representing iterative development cycles as an expanding spiral, with inner cycles denoting early system analysis and prototyping, and outer cycles denoting the classic software life cycle.

The phases in “Spiral Model” are:

• Plan: In this phase, the objectives, alternatives and constraints of the project are determined and are documented. The objectives and other specifications are fixed in order to decide which strategies/approaches to follow during the project life cycle.

• Risk Analysis: This phase is the most important part of “Spiral Model”. In this phase all possible (and available) alternatives, which can help in developing a cost effective project are analyzed and strategies are decided to use them. This phase has been added specially in order to identify and resolve all the possible risks in the project development. If risks indicate any kind of uncertainty in requirements, prototyping may be used to proceed with the available data and find out possible solution in order to deal with the potential changes in the requirements.
• Engineering: In this phase, the actual development of the project is carried out. The output of this phase is passed through all the phases iteratively in order to obtain improvements in the same.
• Customer Evaluation: In this phase, developed product is passed on to the customer in order to receive customer’s comments and suggestions which can help in identifying and resolving potential problems/errors in the software developed. This phase is very much similar to ‘Testing’ phase.

The process progresses in spiral sense to indicate iterative path followed, progressively more complete software is built as we go on iterating through all four phases. The first iteration in this model is considered to be most important, as in the first iteration almost all possible risk factors, constraints, requirements are identified and in the next iterations all known strategies are used to bring up a complete software system. The radical dimensions indicate evolution of the product towards a complete system.

Advantages of Spiral Model:
1. Estimates (i.e. budget, schedule, etc.) become more realistic as work progresses, because important issues are discovered earlier.
2. It is more able to cope with the (nearly inevitable) changes that software development generally entails.
3. Software engineers (who can get restless with protracted design processes) can get their hands in and start working on a project earlier.

Disadvantages of Spiral Model:
1. Highly customized limiting re-usability
2. Applied differently for each application
3. Risk of not meeting budget or schedule
4. Risk of not meeting budget or schedule