The spiral software development life cycle (SDLC) model features incremental and Evolutionary development processes, which means it is developed over several iterations or loops similar to increments.
In the spiral model, each loop may not result in deliverable software. In the incremental model, every increment actually leads to a deployable increment at the customer site, one of the major differences between the spiral and incremental models.
Each loop of the spiral represents a phase of the software process. For example, the innermost loop might be concerned with system feasibility, the next loop with system requirements definition, and the next one with system design, and so on. There are no fixed phases in this model, and the phases shown in the figure are just examples.
Just for clarification: In the prototyping model, the risks which can be identified upfront can be handled, but in the spiral model, the risks which appear after the development start can be handled better.
There are four quadrants in the spiral model, and each loop of the spiral model is called a phase, and the team members decide these phases.
First Quadrant: Objective Setting
Identify the objectives of the phase and examine the risks associated with these objectives. The risk is basically any adverse circumstance that might hamper the successful completion of the software project.
Second Quadrant: Risk Assessment and Reduction
A detailed analysis of the identified features is carried out, and the risk is identified through building a prototype. For example, whether the throughput is sufficient to meet the customer requirements? In this case, the prototype is built to resolve the issue, alternate solutions may be tried out, and the best one may be used.
In contrast to the prototyping model, only one prototype is made before the project starts. It means the risks that could be identified at the beginning of the project can be resolved. However, as the project continues, more and more risks are identified and resolved in the spiral model.
Third Quadrant: Development and Validation
After the risk is resolved, the development occurs.
Fourth Quadrant: Review and Planning
The customer feedback is obtained based on the way the risk was handled and development was done. Plan the next iteration around the spiral. With each iteration around the spiral, a more complex version of the software gets built. However, every phase may not lead to deployable software at the client site.
Understanding of Spiral Model as Meta Model
A spiral model is a Meta model because it features the waterfall model, incremental model, evolutionary model, and prototyping model. A single loop of the spiral model represents the waterfall model. It uses an evolutionary approach, enabling understanding and reacting to risks during each iteration along the spiral. Also, it uses prototyping as a risk reduction mechanism.
Summary
The overall concept of the spiral model is that at every loop: the risk is identified and resolved through developing the prototypes. Finally, the customer evaluates the prototype and goes to the start of the loop once again. This loop continues till the time entire software development over.
Reference
- Fundamentals of Software Engineering Book & NPTEL Video Lectures by Rajib Mall.
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