What is Waterfall?
The Waterfall methodology was named after its sequential phases arranged in a downward fashion (similar to actual waterfalls), representing the various steps of software development from one end to the other. It was first mentioned in 1970 by the computer scientist Winston Walker Royce, and it originally contemplated five distinct phases: requirements, design, implementation, verification, and maintenance.
More specifically, the sequence of events in Waterfall looks something like this:
Gather and document requirements
Design
Code and unit test
Perform system testing
Perform user acceptance testing (UAT)
Fix any issues
Deliver the finished product
Waterfall uses distinct phases rather than simultaneous work. Some variations of the original model appeared in the following decades, but the logic behind it remained the same. When a phase is completed, its output becomes the input for the next one, which starts immediately after the previous one. In this way, it ensures that each phase is completed before moving on to the next, in order to avoid starting development before the design work is completed, which could give way to possible inconsistencies in both ends.
Additionally, it is possible to estimate the whole project's cost and effort needed right from the start in the requirements phase through this model. That is because all phases are passed through and completed only once, so all requirements are gathered as much as possible at the start to provide the information in creating the plans, schedules, budget, and resources. It is plan-driven, so any changes after the project has started would offset the original plan and require a restart.
