The spiral model is a combination of some ideas from the iterative model but also the rigid characteristics of the waterfall model. The spiral model has four phases: identification, design, construction and evaluation. A spiral project goes through these phases in iterations, so that the cycle repeats until the deliverable is produced. The application is released gradually. The spiral model is applied in the following cases: Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get an original essayWhen the budget is low and risks need to be assessed. The project is medium to high risk. The client is unsure of his requirements. The requirements are complex and need an assessment to gain clarity. Significant changes are expected in the product during the development cycle. Advantages of the spiral model: Requirements can be changed. Promotes the use of prototypes. Requirements can be defined more precisely. Users can see the system at the early stages of development. Development can be divided into modules and difficult modules can be developed earlier. Disadvantages of the model: The end of the project may not be known early. Management is more complex .The spiral can continue indefinitely. Not suitable for low or low risk. projectsThe waterfall model is a methodology based on a sequential design process. The waterfall model flows from one step to the next and steps cannot be revisited once completed. One phase must be completed to begin the next phase. The waterfall model is composed of 6 stages, namely: requirements analysis, design, implementation, testing, launch and maintenance. The waterfall model is used for small projects that have clear requirements from the start. Applications of Waterfall Model: Can be easily used for small projects. Requirements are well defined, clearly documented and fixed. Product implementation is stable. Technology does not change and is well managed by the project team .Staff do not require any special training for this model.Advantages of this model:Simple and easy to understand and useEasy to manage due to the great rigidity of the model.The phases follow one another .Works well for small projects where requirements are very well understood. Steps clearly defined. Process and results are well documented. Disadvantages of the model: High risk and uncertainty. Not suitable for complex and long-duration projects. Not suitable to projects where requirements have a moderate to high risk of change. Once an application is tested, it is difficult to change functionality. No working software is produced until the end of the life cycle. Agile methodologies are featured in the 2001 Agile Manifesto, including: Scrum, Lean, XP, Crystal, FDD and DSDM. Agile projects are flexible and contain an iterative design and construction process. Agile models are similar to the iterative model because both models work in iterations. The model uses an adaptive approach, which allows changes to be made, unlike more rigid models such as the Waterfall model or the 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: People and interactions, Delivery of working software, customer collaboration and response to change. The advantages of this model are: Little or no planning.