Spiral Model (Software Engineering) | SDLC- Spiral Model

The Spiral model was first introduced by Barry Boehm in 1986. It is one of the most important Software Development Life Cycle models, which provides support for Risk Handling. The spiral model is a combination of waterfall model and prototype models. This model is best used for large projects which involve continuous enhancements. 

In its diagrammatic representation, it looks like a spiral with many loops. The exact number of loops of the spiral is unknown. The number of loops varies based on each project. Each loop of the spiral is called a Phase of the software development process. The exact number of phases needed to develop the product can be varied by the project manager depending upon the project risks. As the project manager dynamically determines the number of phases, so the project manager has an important role to develop a product using the spiral model. Using the spiral model, the software is developed in a series of incremental releases.

The Radius of the spiral at any point represents the expenses (cost) of the project so far, and the angular dimension represents the progress made so far in the current phase.

The below diagram shows the different phases of the Spiral Model:

Each phase of the Spiral Model is divided into four quadrants as shown in the above figure. The functions of these four quadrants are discussed below:

Objectives determination and identify alternative solution:  At the start of every phase, requirements are collected from the customer, and objectives are identified. These objectives are elaborated on and analyzed in this quadrant. In this quadrant more discussion with clients is done to collect their requirements and if possible a prototype is created in the presence of the client to evaluate the product quality. Normally during this phase requirement gathering exercise, change requests are also seen from clients and these requests are treated as risks for the next development phases. It is important to also identify alternative solutions in case the attempted version fails to perform.

 

Identify and resolve Risks: During the second quadrant, all the possible solutions are evaluated to select the best possible solution. Then the risks associated with that solution are identified and the risks are resolved using the best possible strategy. At the end of this quadrant, the prototype is built for the best possible solution.

Develop next version of the Product: During the third quadrant, the identified features are developed and verified through testing. At the end of the third quadrant, the next version of the software is available.

Review and plan for the next Phase: In the fourth quadrant, the customers evaluate the developed version of the software. The customer tells whether any changes need to be done in the software or not. The project is reviewed, and a choice made whether to continue with a further period of the spiral. If it is determined to keep, plans are drawn up for the next step of the project.