Sdlc Models Explained: Agile, Waterfall, Iterative, Spiral

The spiral model is combined of some ideas from the iterative model but also the rigid features of the waterfall model. The spiral model has four phases: Identification, Design, Build and Evaluation. A spiral project passes through these phases in iterations, so the cycle repeats over until the deliverable is produced. The application is released incrementally. The spiral model is applied when:

• When the budget is low and risks need to be evaluated
• Project is medium to high risk.
• Customer is not sure of their requirements.
• Requirements are complex and need evaluation to get clarity.
• Significant changes are expected in the product during the development cycle.

Advantages of spiral model:

• Requirements can be changed.
• Promotes the use of prototypes.
• Requirements can be defined more accurately.
• Users can see the system in early stages of development.
• Development can be divided into modules and the difficult modules can be developed earlier.

Disadvantages of the model:

• End of the project may not be known early .
• Management is more complex.
• Spiral may go on indefinitely.
• Not suitable for small or low risk projects

The waterfall model is a methodology based on a sequential design process. The waterfall model flows from one stage to another and stages cannot be revisited once completed. One phase has to be finished to start the next phase. The waterfall model is made of 6 stages, which are: requirement analysis, design, implementation, testing, launching and maintenance. The waterfall model is used for small projects which have clear requirements from the start. Applications of the waterfall model:

• Can be easily used for small projects
• Requirements are well defined, clearly documented and fixed.
• Product implementation is stable.
• Technology doesn’t change and is well operated by the project team.
• The staff doesn’t require special training for this model.

Advantages of this model:

• Simple and easy to understand and use
• Easy to manage due to high rigidity of the model.
• Phases flow from one to another.
• Works well for smaller projects where requirements are very well understood.
• Clearly defined stages.
• Process and results are well documented.

Disadvantages of the model:

• High risk and uncertainty.
• Not a suitable for complex and long lasting projects.
• Not suitable for the projects where requirements are at a moderate to high risk of changing.
• Once an application is tested, it’s difficult to change the functionality.
• No working software is produced until late during the life cycle.

The agile methodologies, are in the agile manifesto 2001, these include: Scrum, Lean, XP, Crystal, FDD, and DSDM. Agile projects are flexible and contain iterative design and build process. Agile models are similar to the iterative model, as both models work through iterations. The model uses an adaptive approach, which allows for changes to be made, unlike more rigid models such as Waterfall or V-model, where a predictive approach with clear requirements is used. Agile development is a set of principles and values for developing a project. The four principles of agile methods are: Individuals and interactions, Delivering working software, customer collaboration and responding to change. The advantages of this model are:

• The model provides a very realistic approach to develop the product.
• Promotes teamwork and cross training.
• Functionality can be developed rapidly.
• Requirements can be changed.
• Product delivered in early stages of development.
• Minimal rules, documentation easily employed.
• Little or no planning required.
• Easy to manage.
• Gives flexibility to developers.

The disadvantages are:

• Depends heavily on customer interaction, so if customer is not clear, team can be driven in the wrong direction.
• The model is dependent on individual work, since there is minimum documentation generated.
• Transfer of technology to new team members may be quite challenging due to lack of documentation.
• Complex projects can be difficult to manage without clear requirements.

When the agile model is used:

• Scrum, XP(eXtreme Programming), lean, crystal teams all use the agile model because they work in sprints, which are iterations and they need to prioritize the user requirements then the build of the product, which is finished through a meeting with the customer to hand over the product, review the product log, and reprioritize the requirements at the end. The timeframe for each sprint is 2-4 weeks, where the project manager selects the top tasks for the team to complete.
• When the developers are knowledgeable and can solve difficult problems efficiently
• When the user base is collaborated and dedicated to the company. They have to be knowledgeable to provide useful feedback.
• The requirements are clear as the user requirements change several times. Sometimes requirements are not very clear.
• When the design of the product is flexible, as user requirements can change often.

The idea of the iterative model is to develop a system through repeated cycles (iterations) and in smaller portions at a time (incrementally). It is important that the iterative model isn’t confused with the incremental model. The difference between them is that the iterative model is developed in iterations, where the whole system is developed at an early stage and then built up in the later stages of development. The incremental model however, the iterative model begins with the requirements being created and then the application is iteratively evolved until the product is finished and ready for deployment. The development starts with just one part of the application, rather than by creating the full system based on the full requirements. Each part of the product is reviewed, so that the requirements can be expanded later. After each iteration, the developers produce a new version of software, which evolves and expands the program; designs are modified and new functional capabilities are added.