Spiral model

The spiral model is a risk-driven process model generator for software projects. Based on the unique risk patterns of a given project, the spiral model guides a team to adopt elements of one or more process models, such as incremental, waterfall, or evolutionary prototyping.

This model was first described by Barry Boehm in his 1986 paper "A Spiral Model of Software Development and Enhancement". In 1988 Boehm published a similar paper to a wider audience. These papers introduce a diagram that has been reproduced in many subsequent publications discussing the spiral model.

These early papers use the term "process model" to refer to the spiral model as well as to incremental, waterfall, prototyping, and other approaches. However, the spiral model's characteristic risk-driven blending of other process models' features is already present:

[R]isk-driven subsetting of the spiral model steps allows the model to accommodate any appropriate mixture of a specification-oriented, prototype-oriented, simulation-oriented, automatic transformation-oriented, or other approach to software development.

In later publications, Boehm describes the spiral model as a "process model generator", where choices based on a project's risks generate an appropriate process model for the project. Thus, the incremental, waterfall, prototyping, and other process models are special cases of the spiral model that fit the risk patterns of certain projects.

Boehm also identifies a number of misconceptions arising from oversimplifications in the original spiral model diagram. He says the most dangerous of these misconceptions are:

While these misconceptions may fit the risk patterns of a few projects, they are not true for most projects.

In a National Research Council report this model was extended to include risks related to human users.

To better distinguish them from "hazardous spiral look-alikes", Boehm lists six characteristics common to all authentic applications of the spiral model.

Authentic applications of the spiral model are driven by cycles that always display six characteristics. Boehm illustrates each with an example of a "hazardous spiral look-alike" that violates the invariant.

Sequentially defining the key artifacts for a project often lowers the possibility of developing a system that meets stakeholder "win conditions" (objectives and constraints).

This invariant excludes “hazardous spiral look-alike” processes that use a sequence of incremental waterfall passes in settings where the underlying assumptions of the waterfall model do not apply. Boehm lists these assumptions as follows:

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