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.
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.
The steps in the spiral model iteration can be generalized as follows:
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 SDLC which shows the possible way how to reduce a risk of non-effective architecture with the help of spiral model in conjunction with the best practices from other models.
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