Spiral model, advantages and disadvantages

The spiral model, proposed by Barry Boehm (1988), is a risk-driven, iterative software process model. The process is shown as a spiral instead of a sequence; each loop of the spiral is one phase of development, and the radius shows the cumulative cost so far.

The four sectors of each loop

        Cumulative cost (radius grows outward)
   1. Objective setting   |  2. Risk assessment
   - objectives, limits   |     and reduction
   - alternatives         |  - analyse risks
                          |  - build prototypes
   -----------------------+-------------------------
   4. Planning            |  3. Development and
   - review with client   |     validation
   - plan next loop       |  - choose model, build,
                          |    test, validate

1. Objective setting: define the objectives of this phase, constraints on the process and product, and alternative approaches. Identify project risks.
2. Risk assessment and reduction: analyse each key risk in detail and take steps to reduce it, for example by building a prototype, doing a simulation or a benchmark.
3. Development and validation: choose a development model suited to the remaining risk (prototyping, waterfall, formal methods) and develop and validate the next version of the product.
4. Planning: review the project with the customer and decide whether to continue with another loop. If so, plan the next phase.

Typical loops move outward from concept of operation, to requirements, to design, to detailed design, code, test and deployment.

Advantages

• Explicit risk management: high-risk problems are found and handled early.
• Combines the strengths of other models; waterfall and prototyping can be used inside it.
• Iterative: requirements can be refined and changes accommodated.
• Customer reviews every loop, so there is good user feedback.
• Suitable for large, complex and high-risk projects.
• Working versions/prototypes appear early.

Disadvantages

• Success depends on risk-assessment expertise; poor risk analysis leads to failure.
• Complex to manage; more documentation and reviews.
• Costly; not suitable for small or low-risk projects.
• Hard to define fixed milestones, so cost and time are hard to estimate at the start, which makes fixed-price contracts difficult.
• The spiral may continue for a long time if there is no clear stopping point.