Spiral Model

The spiral model, initially proposed by Boehm, that couples the iterative nature of prototyping  and systematic aspects of the linear sequential model.

The spiral model is also known as meta-model since it encompasses all other life cycle models.

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

1. Objectives determination and identify alternative solutions: Requirements are gathered from the customers and the objectives are identified, elaborated, and analyzed at the start of every phase. Then alternative solutions possible for the phase are proposed in this quadrant.
2. 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.
3. 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.
4. Review and plan for the next Phase: In the fourth quadrant, the Customers evaluate the so far developed version of the software. In the end, planning for the next phase is started.

Advantages of Spiral Model

Flexibility in Requirements: Change requests in the Requirements at later phase can be incorporated accurately by using this model.

Customer Satisfaction: Customer can see the development of the product at the early phase of the software development and thus, they habituated with the system by using it before completion of the total product.

Good for large projects: It is recommended to use the Spiral Model in large and complex projects.

Disadvantages of Spiral Model

Documentation: is more as it has many loop iteration.

Expensive: It is not suitable for small projects.

Difficulty in time management: As the number of phases is unknown at the start of the project, so time estimation is very difficult.