The Spiral model is an [iterative process model]]. The model, introduced by Barry Boehm in 1986, outlines a basic [[process]] in which development teams can design and successfully implement a software system by revisiting [phases of the process after they have been previously completed.

Phases

A simplified version of the model has four phases:

1. determine objectives
2. identify and resolve risks
3. develop and test
4. plan the next iteration

Taken in order, the four phases represent one full iteration. Each subsequent iteration has the sequence of the four phases revisited, and each iteration results in a product prototype. Early iterations lead to product ideas and concepts, while later iterations lead to working software prototypes.

Iterations and prototypes

• The first iteration may be the creation of an illustrative prototype.
• Subsequent iterations could build on this illustrative prototype with either an incremental or an evolutionary prototype approach

Pros and cons

Pros:

• development team can review their product with the client to gather feedback and improve the product
• using a repetitive process to make further refinements is a clear advantage over the earlier linear models

Cons:

• estimating work can be more difficult
• lengthy iterations can introduce more uncertainty in estimate
• requires much analytical expertise to assess risks