This is one of the most popular and simplest methodologies used for project management. In this approach, a project is divided into sequential tasks that are then carried out one by one until all tasks are completed. This is similar to a waterfall, where water flows from top to bottom, hence the name. In this methodology, you can't move back to the previous phase. Instead, the only possible option is to go back to the initial phase and start again. The output generated from one stage becomes the input of the next stage.

In the Waterfall methodology, comprehensive documentation is done in every phase, which is very helpful when carrying out maintenance or when a new member joins during a project. They can easily refer to the documentation to learn about project details. The Waterfall model divides a complete project into six different stages. We can understand how these stages are implemented one by one by using the following diagram:

Let's discuss these stages in detail:

1. Requirement Analysis: This is the first phase of the Waterfall model. In this phase, we find out what is required. This phase needs a lot of interaction with the client to understand their requirements in detail. Various techniques are used to identify requirements, and we will be discussing these techniques in later chapters. All of the requirements are recorded properly in the necessary documents. Once project requirements are known, the feasibility of the solutions is discussed. All of the requirements are analyzed properly and different possibilities are considered for developing a potential system. In this phase, any existing infrastructure that clients have, such as existing servers, are also analyzed for potential use in the new system.

2. Designing: In this phase, the blueprint of the final output is prepared based on the project requirements generated in the first steps and then an appropriate technology is selected. All of the functional and technical design documents are prepared in this phase along with the system architecture. Once all of the design documents are ready, the project moves to the next phase.
3. Coding: In this phase, code is developed by team members based on design documents. Team members work on the individual modules, which are integrated with other modules once completed. Team members also perform unit testing of their code to avoid any design time or runtime errors.
4. Testing: Once all of the modules are developed, they are tested against the requirement document generated in the first phase by the quality team members. The quality team first prepares test cases using requirements, and then manual or automated testing is performed later on. Manual testing is done by a QA team member manually without using any testing tools or script, whereas automated testing is done using tools and scripts. Automated testing is useful for retesting test cases after code changes or any upgrades.
5. Deployment: In this phase, the final output is verified by clients and this involves user acceptance testing involving end users. An end user performs function testing to make sure that the project output is based on their expectations. End user training sessions are also conducted in this phase.
6. Maintenance: In this phase, post-deployed changes are implemented. This includes fixing any client-side issues, adding more functionality to a project, or upgrading software patches, if required.

Waterfall is a common method of the software and construction industries. This methodology emphasizes further collection of all of the requirements in the initial phase and documenting them properly to use them in later phases. This model is easy to understand as projects progress through easily understandable phases, one by one. However, this model can't be used for projects where it is difficult to find out all requirements at the initial stage. This is because it is very difficult to add new requirements once a project is initiated.