Spiral model
Definition
The spiral model is a risk-driven software development framework in which each cycle of a spiral passes through four phases — planning, risk analysis, engineering, and evaluation — with each cycle producing an increasingly complete version of the system.
Barry Boehm introduced the spiral model in a 1986 paper as an explicit response to waterfall's failure mode: the discovery of fundamental design problems late in the project when they're most expensive to fix. The spiral model's core innovation is making risk analysis an explicit, mandatory activity at the start of every cycle.
Each loop of the spiral covers four quadrants:
- Planning: Determine objectives, alternatives, and constraints for this cycle
- Risk analysis: Identify and analyze risks. Build prototypes to resolve the biggest uncertainties before committing to a path.
- Engineering: Develop and test the deliverable for this cycle (could be a prototype, a working component, or a full release)
- Evaluation: Customer review of the cycle's output. Plan the next cycle.
The spiral grows outward with each loop, representing increasing investment and completeness. Early loops might produce paper prototypes; middle loops produce working software components; later loops produce a production-ready system.
The spiral model is most appropriate for large, complex, high-risk projects where requirements are uncertain and getting something wrong early would be catastrophically expensive. It's less practical for small teams and short timelines, where agile's lighter approach provides more development throughput with less overhead.
Examples
- Large government system: Cycle 1 = concept prototype; Cycle 2 = requirements prototype; Cycle 3 = design prototype; Cycles 4-6 = incremental releases
- Aerospace software: each cycle includes formal hazard analysis to identify safety risks before committing to the next level of design detail
- Financial trading platform: early spirals resolve regulatory uncertainty before full engineering investment
- Research software: spiral allows the research findings of one cycle to reshape the requirements of the next
Related terms
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