The spiral model in software development is generally used in projects where there is high possibility of risk. The waterfall model is also used within the spiral model, which I will discuss later on. All activities are arranged in the form of a spiral and hence the name.
Each ring of the spiral represents a development phase and there can be any number of rings in the spiral. Each ring is divided into 4 quarters. The first quadrant aims to determine objectives, alternatives and constraints. The second involves risk analysis and alternative approaches are evaluated in order to overcome the constraints. In the third stage the planned product is developed. This is where we see the use of the waterfall model (which I discussed in previous posts) to develop the product. In the final section of the loop the next phase is planned.
There can be any number of loops in the model, the project can be voided at any stage throughout the cycle if the risk involved becomes too immense.