Proposed by Barry Boehm in 1988, the Spiral Model is risk-driven. Each loop of the spiral goes through four quadrants, and the project can be terminated after any loop if the risk is too high.
The Four Quadrants (per loop)
- Determine objectives — what are we building this loop? What are the alternatives?
- Identify and resolve risks — analyze each risk, build prototypes to reduce uncertainty.
- Develop and test — produce the next-level artifact (could be a prototype, a design doc, or working code).
- Plan the next iteration — review results, plan what comes next.
The model is called “spiral” because you go through these quadrants repeatedly, each loop producing a more complete version of the product while addressing the top risks.
Risk-Driven Thinking
What’s unique: the first loop might only produce a paper prototype if the risk is “we don’t understand the user’s needs.” The second loop might produce a UI prototype. The third loop might produce working code for the riskiest module. You don’t start coding until the code-level risks are the highest remaining risks.
| Risk | Mitigation in Spiral |
|---|---|
| Unclear requirements | Build a throwaway prototype |
| Technology uncertainty | Build a proof-of-concept |
| Performance risk | Build a benchmark |
| Schedule risk | Build a minimal feature set first |
Q: When would you use the Spiral Model?
A: Large, high-risk projects — NASA systems, defense contracts, enterprise ERP implementations. Any project where a failure would be catastrophic and expensive.
Q: What’s the downside of the Spiral Model?
A: It requires expert risk assessors. If the risk analysis is poor, the model provides no benefit. It’s also expensive and heavy — overkill for small, low-risk projects.
Q: How is Spiral different from Agile?
A: Spiral is risk-driven — the next iteration’s work is determined by the highest remaining risk. Agile is feature-driven — the next iteration’s work is determined by business priority. Both are iterative, but the selector of what to do next differs.
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