What is the Spiral Model

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May 18, 2024

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These difficulties with the Waterfall and evolutionary development models have led to the development and use of several alternative process models: risk-driven (spiral), reuse-driven, legacy-driven, demonstration-driven design to cost or schedule, incremental, and hybrid combinations of these with the Waterfall or evolutionary development models.

Boehm originally proposed the Spiral model in 1988 as an evolutionary software process model that couples the iterative nature of prototyping with the controlled and systematic aspects of the linear sequential model. This was observed as an attempt to combine the strengths of the various models. It provides the potential for rapid development of incremental versions of the software. The model also takes into account the results of risk assessment activities whose outcome determines the taking up of the next phase of the development activity.

This model views software development as a spiral process with the software being developed in a series of incremental releases. Typically,

The inner cycles represent the early phases of requirement analysis along with prototyping to refine the requirement definitions, and the outer spirals are progressively representative of the classic software life cycle. The spiral model is divided into several framework activities, also called task regions. Typically, there are between three and six task regions.

Spiral model that contains six task regions as follows:

• Understanding the Context - These tasks are required to establish effective communication between the developer and the customer.
  
  
• Planning tasks - They define resources, timelines, and other project-related information.
  
  
• Risk analysis tasks - These are required to assess both technical and management risks.
  
  
• Engineering tasks - These are required to build one or more representations of the application.
  
  
• Construction and release - These tasks construct, test, install, and provide user support (for example, documentation and training).
  
  
• Customer Evaluation-tasks - These are required to obtain customer feedback based on evaluation of the software representations created during the engineering stage, and implemented during the installation stage.
  
  

Each of the defined six regions has a series of work tasks. These tasks are dependent on the type of project being undertaken. For small projects, the number of work tasks and their formality is low. For larger, more critical projects, each task region contains more work tasks that are defined to achieve a higher level of formality. In addition, all support activities like Configuration Management, and Project Management have to happen.

As this evolutionary process begins, the software engineering team moves around the spiral in a clockwise direction, beginning at the core. The first circuit around the spiral may result in the development of a product specification; subsequent passes around the spiral may be used to develop a prototype, and then progressively more sophisticated versions of the software. Each pass through the planning region results in adjustments to the project plan. Cost and schedule are adjusted based on feedback derived from customer evaluation. In addition, the project manager adjusts the planned number of iterations required to complete the software.

1. Advantages and Disadvantages of Spiral Model

The spiral model is a realistic approach to the development of large-scale systems and software. Since software evolves as the process progresses, the developer and customer better understand and react to risks at each evolutionary level. The spiral model uses prototyping as a risk reduction mechanism. It also enables the developer to apply the prototyping approach at any stage in the evolution of the product.

It maintains the systematic stepwise approach suggested by the classic life cycle but incorporates it into an iterative framework that more realistically reflects the real world. The spiral model demands a direct consideration of technical risks at all stages of the project, and if properly applied, should reduce risks before they become problematic.

However, like other paradigms, the spiral model is not a solution to all the problems faced by software developers. It may be difficult to convince customers (particularly in contract situations) that the evolutionary approach is controllable. It demands considerable risk assessment expertise and relies on this expertise for success. If major risks are not uncovered and managed, problems will undoubtedly occur.

2. Application of Spiral Model

The spiral model is very widely used in the software industry. It is chosen for projects possessing the following criteria:

• Medium to high-risk projects.
  
  
• Long-term project commitment due to changes in economic priorities as requirements change with time.
  
  
• The customer is not sure of their requirements.
  
  
• Complex requirements, which further require clarification.
  
  
• The new product line needs to be released in phases to get enough customer feedback.
  
  
• Major changes are expected in the product during the development life cycle.