Spiral Model

In this article, we will discuss about what is the spiral model? When to use spiral model? and also discuss some advantages and disadvantages of the spiral model.

What is Spiral Model?

Spiral model is an evolutionary software process model that combines the best features of the waterfall model and prototype model and incremental model.

It was first introduced by Barry Boehm in his 1988 article.

It is a risk-driven software development process model means it gives more priority to high risks in software development and tries to avoid or reduce the risks as much as possible. So the primary focus of this model is it is risk-avoidance-driven rather than document-driven, code-driven, or release-driven.

The main goal of using the spiral model is to focus on high risks features, or find what risks can be involved during the software development process. And after finding the possible risks then analyze the risks and evaluate the alternative approaches for satisfying the win conditions. So the main purpose of using this model is to reduce the risks in the project by trying to find possible solutions to minimize the risks.

The high risks implications in this model can be cost-related, schedule-related, system performance-related, safety, user interface, organizational impacts, etc.

Each loop of the spiral represents the iterative waterfall model. It uses the prototyping approach as a risk reduction mechanism. So before developing the actual product, this model uses a prototype of the product, which helps to reduce the risks and early customer feedback.

This is called the spiral model because the software project moves around every phase repeatedly in iterations called spiral. Each iteration (loop) of the spiral is divided into four phases (quadrant).

Spiral Model
Marekventur, CC BY-SA 3.0, via Wikimedia Commons


Each cycle of the spiral is divided into four quadrants

1. Objectives determination and identifying alternative solutions: Every software development starts with planning. The first quadrant starts with the feasibility study and requirement gathering and analysis.

In this phase, we determine the objective of the project means we determine the goal of the project. Requirements are gathered from the customers through interviews and discussions, analyses of the requirements, and all the requirements covert into documentation in the form of SRS. We also find alternative solutions means finding the possible ways to achieve our objective.

2. Identify and analyze the risks: In the second quadrant, all the requirements gathered in the first quadrant are studied and all the alternative solutions are evaluated and the risks associated with the alternative solutions are identified, focus on high-risk features, analyze the risks and try to minimize the risks by using the best possible strategy. In the end, after finding the best strategy a prototype product is built which helps to minimize the risks before developing the actual product.

3. Develop the next version of the product: In this third quadrant, all the required features are developed and verified through testing methodologies. At the end of this phase, a new software product or the next version of the product is ready to deliver to the customer.

This phase includes actual software development, testing the software, and deploying the software product to the customer so the customer can use the product and gives us feedback, which helps in reviewing and planning the next phase of the product.

4. Reviewing and planning the next phase: In the fourth quadrant, the customer evaluates the software product and gives us feedback. After the feedback, the software product is reviewed where the software engineers make a choice according to the feedback whether to continue with a further period of the spiral. If any further changes are required, the project continue to the next phase.

The next spiral depends on customer feedback, remaining risks, and project completion.


When to use the spiral model?

  • This model is mostly used in larger projects.
  • When the high risk is involved in developing the software product.
  • When requirements are a bit complicated and users are unclear of their needs.
  • When cost and continuous risk evaluation are important.
  • When a product needs to be released frequently.
  • When significant changes are expected.
  • When the prototype is needed to avoid the risks.

Advantages of Spiral model

  • This model is useful to avoid risks.
  • Critical high-risk functions are developed first.
  • Early and frequent customer feedback.
  • Provides an early indication of risks, without much cost.
  • This model is very useful for larger and high-risk projects.
  • Additional features can be added at the next spiral (phase).
  • This model is very flexible compared to other models
  • Product is delivered on every spiral so the end-users get a chance to see the system early.
  • Use prototype on every spiral.

Disadvantages of spiral model

  • It is a very complex model.
  • It is not beneficial for smaller projects.
  • This model is not budget-friendly.
  • Required some expert people, who can analyze the risks.
  • More time is required for planning, risk evaluation, etc.
  • Prototyping may be excessive.


You should read these articles: SDLC, Waterfall Model, Prototype Model, Incremental Model



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