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Spiral

The Spiral Model is a software development model designed to control risk. Barry W. Boehm created the model, described in his 1986 paper “A Spiral Model of Software Development and Enhancement” (see http://portal.acm.org/citation.cfm?id=12948). Boehm states, “The major distinguishing feature of the spiral model is that it creates a risk-driven approach to the software process rather than a primarily document-driven or code-driven process. It incorporates many of the strengths of other models and resolves many of their difficulties” [8].

The spiral model repeats steps of a project, starting with modest goals, and expanding outwards in ever-wider spirals (called rounds). Each round of the spiral constitutes a project, and each round may follow traditional software development methodology such as Modified Waterfall. A risk analysis is performed in each round. Fundamental flaws in the project or process are more likely to be discovered in the earlier phases, resulting in simpler fixes. This lowers the overall risk of the project: large risks should be identified and mitigated.

Boehm used the Spiral Model to develop the TRW Software Productivity System (TRW-SPS), a complex software project that resulted in 1,300,000 computer instructions. “Round zero” was a feasibility study, a small project designed to determine if the TRW-SPS project represented significant value to the organization, and was thus worth the risk of undertaking. The feasibility study indicated that the project was worthwhile (low risk), and the project spiraled outward. The deliverables of further rounds included:

  1. Concept of Operations (COOP)
  2. Software Requirements
  3. Software Product Design
  4. Detailed Design [8]

Each round included multiple repeated steps, including prototype development and, most importantly, a risk analysis. Boehm’s spiral is shown in Fig. 9.5.

Fig. 9.5 The Spiral Model. Source: https://upload.wikimedia.org/wikipedia/commons/e/ec/Spiral_model_%28Boehm%2C_1988%29.svg. Image under permission of Creative Commons.

The spiral ended with successful implementation of the project. Any potential high risk, such as lack of value to the organization or implementation failure, was identified and mitigated earlier in the spiral, when it was cheaper and easier to mitigate.