Spiral Model – Software Engineering

The Spiral Model is one of the most important Software Development Life Cycle models, which provides support for Risk Handling. This article focuses on discussing the Spiral Model in detail.

What is the Spiral Model?

The Spiral Model is a Software Development Life Cycle (SDLC) model that provides a systematic and iterative approach to software development. In its diagrammatic representation, looks like a spiral with many loops. The exact number of loops of the spiral is unknown and can vary from project to project. Each loop of the spiral is called a Phase of the software development process.

1. The exact number of phases needed to develop the product can be varied by the project manager depending upon the project risks.
2. As the project manager dynamically determines the number of phases, the project manager has an important role in developing a product using the spiral model. 
3. It is based on the idea of a spiral, with each iteration of the spiral representing a complete software development cycle, from requirements gathering and analysis to design, implementation, testing, and maintenance.

What Are the Phases of Spiral Model?

The Spiral Model is a risk-driven model, meaning that the focus is on managing risk through multiple iterations of the software development process. It consists of the following phases:

1. Planning

The first phase of the Spiral Model is the planning phase, where the scope of the project is determined and a plan is created for the next iteration of the spiral.

2. Risk Analysis

In the risk analysis phase, the risks associated with the project are identified and evaluated.

3. Engineering

In the engineering phase, the software is developed based on the requirements gathered in the previous iteration.

4. Evaluation

In the evaluation phase, the software is evaluated to determine if it meets the customer’s requirements and if it is of high quality.

5. Planning

The next iteration of the spiral begins with a new planning phase, based on the results of the evaluation.

The Spiral Model is often used for complex and large software development projects, as it allows for a more flexible and adaptable approach to software development. It is also well-suited to projects with significant uncertainty or high levels of risk.

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. 

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:

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 the 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.

Risk Handling in Spiral Model

A risk is any adverse situation that might affect the successful completion of a software project. The most important feature of the spiral model is handling these unknown risks after the project has started. Such risk resolutions are easier done by developing a prototype.

1. The spiral model supports coping with risks by providing the scope to build a prototype at every phase of software development. 
2. The Prototyping Model also supports risk handling, but the risks must be identified completely before the start of the development work of the project.
3. But in real life, project risk may occur after the development work starts, in that case, we cannot use the Prototyping Model.
4. In each phase of the Spiral Model, the features of the product dated and analyzed, and the risks at that point in time are identified and are resolved through prototyping.
5. Thus, this model is much more flexible compared to other SDLC models. 

Why Spiral Model is called Meta Model?

The Spiral model is called a Meta-Model because it subsumes all the other SDLC models. For example, a single loop spiral actually represents the Iterative Waterfall Model.

1. The spiral model incorporates the stepwise approach of the Classical Waterfall Model.
2. The spiral model uses the approach of the Prototyping Model by building a prototype at the start of each phase as a risk-handling technique.
3. Also, the spiral model can be considered as supporting the Evolutionary model – the iterations along the spiral can be considered as evolutionary levels through which the complete system is built. 

Advantages of the Spiral Model

Below are some advantages of the Spiral Model. 

1. Risk Handling: The projects with many unknown risks that occur as the development proceeds, in that case, Spiral Model is the best development model to follow due to the risk analysis and risk handling at every phase.
2. Good for large projects: It is recommended to use the Spiral Model in large and complex projects.
3. Flexibility in Requirements: Change requests in the Requirements at a later phase can be incorporated accurately by using this model.
4. Customer Satisfaction: Customers 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.
5. Iterative and Incremental Approach: The Spiral Model provides an iterative and incremental approach to software development, allowing for flexibility and adaptability in response to changing requirements or unexpected events.
6. Emphasis on Risk Management: The Spiral Model places a strong emphasis on risk management, which helps to minimize the impact of uncertainty and risk on the software development process.
7. Improved Communication: The Spiral Model provides for regular evaluations and reviews, which can improve communication between the customer and the development team.
8. Improved Quality: The Spiral Model allows for multiple iterations of the software development process, which can result in improved software quality and reliability.

Disadvantages of the Spiral Model

Below are some main disadvantages of the spiral model. 

1. Complex: The Spiral Model is much more complex than other SDLC models.
2. Expensive: Spiral Model is not suitable for small projects as it is expensive.
3. Too much dependability on Risk Analysis: The successful completion of the project is very much dependent on Risk Analysis. Without very highly experienced experts, it is going to be a failure to develop a project using this model.
4. Difficulty in time management: As the number of phases is unknown at the start of the project, time estimation is very difficult.
5. Complexity: The Spiral Model can be complex, as it involves multiple iterations of the software development process.
6. Time-Consuming: The Spiral Model can be time-consuming, as it requires multiple evaluations and reviews.
7. Resource Intensive: The Spiral Model can be resource-intensive, as it requires a significant investment in planning, risk analysis, and evaluations.

The most serious issue we face in the cascade model is that taking a long length to finish the item, and the product became obsolete. To tackle this issue, we have another methodology, which is known as the Winding model or spiral model. The winding model is otherwise called the cyclic model.

When To Use the Spiral Model?

1. When a project is vast in software engineering, a spiral model is utilized.
2. A spiral approach is utilized when frequent releases are necessary.
3. When it is appropriate to create a prototype
4. When evaluating risks and costs is crucial
5. The spiral approach is beneficial for projects with moderate to high risk.
6. The SDLC’s spiral model is helpful when requirements are complicated and ambiguous.
7. If modifications are possible at any moment
8. When committing to a long-term project is impractical owing to shifting economic priorities.

Questions For Practice

1. Match each software lifecycle model in List – I to its description in List – II: [UGC NET CSE 2016]

Choose the Correct Option:

Solution: Correct Answer is (A).

2. In the Spiral model of software development, the primary determinant in selecting activities in each iteration is [ISRO 2016]

(A) Iteration Size

(B) Cost

(C) Adopted process such as Rational Unified Process or Extreme Programming

(D) Risk

Solution: Correct Answer is (D).

FAQs on Spiral Model – Software Engineering

1. How does Spiral Model differ from Waterfall Model?

Ans: Spiral Model is different from Waterfall Model as Waterfall Model follows a linear and sequential approach whereas Spiral Model has repeated cycles of development.

2. What are the places where the Spiral Model is commonly used?

Ans: Spiral Model is commonly used in industries where risk management is critical like software development medical device manufacturing, etc.