Spiral model of software development

The spiral model (Barry Boehm) is a risk-driven, iterative process. It combines the stepwise, controlled phases of the waterfall model with the iterative prototyping model, and adds explicit risk analysis at every loop. The project moves outward through a spiral; each loop is a phase (concept, requirements, design, implementation) and produces a more complete version.

 +---------------------+---------------------+
 | 1 Objectives,       | 2 Risk analysis,    |
 |   alternatives      |   prototype         |
 +---------------------+---------------------+
 | 4 Plan next loop,   | 3 Develop and       |
 |   customer review   |   test              |
 +---------------------+---------------------+
   Loops go outward: concept -> requirements
   -> design -> code/test -> release

Process (each loop has four quadrants)

1. Determine objectives, alternatives and constraints: the requirements for this loop are set (performance, functionality, cost), the alternative solutions are listed.
2. Evaluate alternatives and identify/resolve risks: risks (technical, schedule, cost, requirement uncertainty) are analysed; prototypes, simulation or benchmarks are built to reduce them. If a risk is too high, the project may stop.
3. Develop and verify the next level of product: design, coding, and testing for the loop (e.g. concept of operation, then requirements, then design, then the code, then acceptance), using waterfall for that cycle.
4. Plan the next iteration: the customer reviews the result and the plan for the next loop (and budget) is prepared.

The first loops produce the concept and prototype; later loops produce the detailed design, code and tests, until the final system is delivered. The radius of the spiral represents cumulative cost, and the angle the progress.

Advantages: risk handled early, flexible for changes, customer feedback each loop, suits large and high-risk projects. Disadvantages: needs risk-analysis expertise, costly, complex to manage, not suitable for small projects.