The spiral model is a systems development lifecycle (SDLC) method used for risk management that combines the iterative development process model with elements of the Waterfall model. The spiral model is used by software engineers and is favored for large, expensive and complicated projects.
When viewed as a diagram, the spiral model of software development looks like a coil with many loops. The project manager designates the number of loops, which varies based on the project. Each loop of the spiral is a phase in the software development process model.
The spiral model enables gradual releases and refinement of a software product through each phase of the spiral. This risk-driven approach also enables the ability to build prototypes at each phase. The most important feature of the model is its ability to manage potential risks after the project has commenced; creating a prototype makes this feasible.
Uses of the spiral model
The spiral model is best used in large projects. It compartmentalizes them into phases with different people involved in each phase. Larger issues are broken down into smaller problems to solve and departments or teams are assigned specific tasks that they're responsible for. Other uses include the following:
- Projects in which frequent releases are necessary.
- Projects in which changes are required at any time.
- Long-term projects that aren't feasible because of altered economic priorities.
- Medium to high-risk projects, where iterative refinements are needed to ensure potential risks are mitigated.
- Projects in which cost and risk analysis are important.
- Projects that would benefit from the creation of a prototype to demonstrate the functionalities to stakeholders.
- Projects with unclear or complex requirements.
Real-world examples of spiral model projects
Various industries rely on the spiral model to iteratively improve projects. Examples include the following:
- Software development. Developers iteratively test software projects while adhering to feedback to guide improvements. This is especially true of mobile apps, where functionality changes rapidly and requires debugging to adhere to user and stakeholder expectations.
- Gaming. Game developers use this iterative model to test gameplay and improve graphics before a final product is released. Such refinements are based on customer feedback as well.
- Retail. E-commerce website developers use spiral modeling to continuously evolve and add new features to improve the customer experience based on consumer preferences and market trends.
- Healthcare. The spiral model is used to ensure electronic healthcare records systems meet industry standards and comply with existing regulations, such as the Health Insurance Portability and Accountability Act.
- Space. Space exploration systems, such as satellites and rovers, start as prototypes and undergo simulations for testing before being used in space. The spiral model guides their development to ensure they aren't prone to issues.
Phases of the spiral model
When looking at a diagram of a spiral model, the radius of the spiral represents the cost of the project, and the angular degree represents the progress made in the current phase. Each phase begins with a goal for the design and ends when the developer or client reviews the progress.
Every phase can be broken into four quadrants: identifying and understanding requirements, performing risk analysis, building the prototype and evaluating the software's performance.
Identifying and understanding requirements
Phases begin in the quadrant dedicated to the identification and understanding of requirements. The overall goal of the phase is determined, and all objectives are elaborated and analyzed. It's important to also identify alternative solutions in case the attempted version fails to perform.
Risk analysis
Risk analysis is performed on all possible solutions to find any faults or vulnerabilities -- such as running over budget or areas within the software open to different forms of cyberattacks. Each risk is resolved using the most efficient strategy.
Building the prototype
In the next quadrant, the prototype model is built and tested. This step includes architectural design, module design, physical product design and the final design. It takes the proposal that has been created in the first two quadrants and turns it into software that can be used.
Performance evaluation
In the fourth quadrant, the test results of the newest version are evaluated. This analysis lets programmers stop and understand what worked and didn't work before progressing with a new build. At the end of this quadrant, planning for the next phase begins and the cycle repeats. At the end of the spiral, the software is deployed in its respective market.
Steps of the spiral model
While the phases are broken down into quadrants, each quadrant is further broken down into steps. The steps in the spiral model are as follows:
- The new system requirements are defined in as much detail as possible. This usually involves interviewing several users representing all the external or internal interests and other aspects of the existing system.
- A preliminary design is created for the new system.
- The first prototype of the new system is constructed from the preliminary design. This is usually a scaled-down system that represents an approximation of the characteristics of the final software product.
- A second prototype emerges from a fourfold procedure: (1) evaluating the first prototype's strengths, weaknesses, and risks; (2) defining the requirements of the second prototype; (3) planning and designing the second prototype; (4) constructing and testing the second prototype.
- The entire project is aborted if the risk is deemed too great. Risk factors might include development cost overruns, operating cost miscalculations and other factors that could result in a less-than-satisfactory final product.
- The existing prototype is evaluated in the same manner as the previous prototype and, if necessary, another prototype is developed from it according to the fourfold procedure outlined above.
- The preceding steps are iterated until the customer is satisfied that the refined prototype represents the desired final product.
- The final product is constructed, based on the refined prototype.
- The final product is thoroughly evaluated and tested. Routine maintenance is carried out on a continuous basis to prevent large-scale failures and minimize downtime.
Advantages of the spiral model
The spiral model is a great option for large, complex projects. The progressive nature of the model lets developers break a big project into smaller ones and tackle one feature at a time, ensuring nothing is missed. The prototype building is done progressively, so the cost estimation of the whole project is sometimes easier.
Other benefits of the spiral model include the following:
- Flexibility. Changes made to the requirements after development has started are easily adopted and incorporated.
- Risk management. The spiral model involves risk analysis and handling in every phase, improving security and the chances of avoiding attacks and breakages. The iterative development process also facilitates risk mitigation.
- Customer satisfaction. The spiral model facilitates customer feedback. If the software is being designed for a customer, then the customer will be able to see and evaluate their product in every phase. This lets them raise concerns and request changes before the product is fully built, saving the development team time and money.
Disadvantages of the spiral model
Limitations of the spiral model include the following:
- High cost. The spiral model is expensive and therefore not suitable for small projects.
- Dependence on risk analysis. Successful completion of a project depends on effective risk management. Given that, it's necessary for those involved with the project to have expertise in risk assessment.
- Complexity. The spiral model is more complex than other SDLC options. For it to operate efficiently, protocols must be followed closely. Furthermore, increased documentation is required to keep track of the intermediate phases.
- Time management challenges. Going into the project, the number of required phases is often unknown, making time management almost impossible. Therefore, there's always a risk of falling behind schedule or going over budget.
The spiral model exists alongside various secure software development frameworks and lifecycle models. Learn the basics of these frameworks.