Spiral model

Spiral model (Boehm, 1986).

The spiral model is a software development process combining elements of both design and prototyping-in-stages, in an effort to combine advantages of top-down and bottom-up concepts. Also known as the spiral lifecycle model (or spiral development), it is a systems development method (SDM) used in information technology (IT). This model of development combines the features of the prototyping model and the waterfall model. The spiral model is intended for large, expensive and complicated projects.

This should not be confused with the Helical model of modern systems architecture that uses a dynamic programming (mathematical not software type programming!) approach in order to optimise the system's architecture before design decisions are made by coders that would cause problems.

History

The spiral model was defined by Barry Boehm in his 1986 article "A Spiral Model of Software Development and Enhancement". This model was not the first model to discuss iterative development.

As originally envisioned, the iterations were typically 6 months to 2 years long. Each phase starts with a design goal and ends with the client (who may be internal) reviewing the progress thus far. Analysis and engineering efforts are applied at each phase of the project, with an eye toward the end goal of the project.

Steps

The steps in the spiral model iteration can be generalized as follows:

1. The system requirements are defined in as much detail as possible. This usually involves interviewing a number of users representing all the external or internal users and other aspects of the existing system.
2. A preliminary design is created for the new system. 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 to use them are decided. 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.
3. A first prototype of the new system is constructed from the preliminary design. This is usually a scaled-down system, and represents an approximation of the characteristics of the final product.
4. A second prototype is evolved by a fourfold procedure:
   1. evaluating the first prototype in terms of its strengths, weaknesses, and risks;
   2. defining the requirements of the second prototype;
   3. planning and designing the second prototype;
   4. constructing and testing the second prototype.
5. At the customer's option, the entire project can be aborted if the risk is deemed too great. Risk factors might involve development cost overruns, operating-cost miscalculation, or any other factor that could, in the customer's judgment, result in a less-than-satisfactory final product.
6. The existing prototype is evaluated in the same manner as was the previous prototype, and, if necessary, another prototype is developed from it according to the fourfold procedure outlined above.
7. The preceding steps are iterated until the customer is satisfied that the refined prototype represents the final product desired.
8. The final system is constructed, based on the refined prototype.
9. The final system is thoroughly evaluated and tested. Routine maintenance is carried out on a continuing basis to prevent large-scale failures and to minimize downtime.

Applications

The spiral model is mostly used in large projects. For smaller projects, the concept of agile software development is becoming a viable alternative. The US military had adopted the spiral model for its Future Combat Systems program. The FCS project was canceled after six years (2003–2009), it had a two year iteration (spiral). The FCS should have resulted in three consecutive prototypes (one prototype per spiral—every two years). It was canceled in May 2009. The spiral model thus may suit small (up to $3 million) software applications and not a complicated ($3 billion) distributed, interoperable, system of systems.

Also it is reasonable to use the spiral model in projects where business goals are unstable but the architecture must be realized well enough to provide high loading and stress ability. For example, the Spiral Architecture Driven Development is the spiral based Software Development Life Cycle (SDLC) which shows one possible way how to reduce the risk of non-effective architecture with the help of a spiral model in conjunction with the best practices from other models.