Software Development Model: Spiral

What is Spiral Model? A brief explanation:

The software development activities can be organized like a spiral that has many cycles. The radial dimension represents the cumulative cost incurred in accomplishing the steps done so far, and the angular dimension represents the progress made in completing each cycle of the spiral.

Each phase begins with identification of objectives for the cycle, the different alternatives that are possible for achieving the objectives, and the constraints that exist. This is the first quadrant of the cycle. The next step in the cycle is to evaluate these different alternatives based on the objectives and constraints. The focus of evaluation in this step is based on the risk perception for the project. Risks reflect chances that some of the objectives of the project may not be met. The next step is to develop strategies that resolve the uncertainties and risks. This step may involve activities such as benchmarking, simulation and prototyping. Next, the software is developed, keeping in mind the risks. Finally the next stage is planned.

For example: In round one, a concept of operation might be developed. The objectives are stated more precisely and quantitatively and the cost and other constraints are defined precisely. The risks here are typically whether or not the goals can be met within the constraints. The plan for the next phase will be developed, which will involve defining separate activities for the project. In round two the top-level requirements are developed. In succeeding rounds the actual development may be done.