Waterfall model

The waterfall model is a sequential software development model (a process for the creation of software) in which development is seen as flowing steadily downwards (like a waterfall) through the phases of requirements analysis, design, implementation, testing (validation), integration, and maintenance. The origin of the term "waterfall" is often cited to be an article published in 1970 by Winston W. Royce (1929–1995), although Royce did not use the term "waterfall" in this article. Ironically, Royce was actually presenting this model as an example of a flawed, non-working model.

History of the waterfall model
In 1970 Royce proposed what is presently referred to as the waterfall model as an initial concept, a model which he argued was flawed. His paper explored how the initial model could be developed into an iterative model, with feedback from each phase influencing subsequent phases. It is only the initial model that received notice; his own criticism of this initial model has been largely ignored. The "waterfall model" quickly came to refer not to Royce's final, iterative design, but rather to his purely sequentially ordered model. This article will use this popular meaning of the phrase waterfall model. For an iterative model similar to Royce's final vision.Despite Royce's intentions for the waterfall model to be modified into an iterative model, use of the "waterfall model" as a purely sequential process is still popular, and, for some, the phrase "waterfall model" has since come to refer to any approach to software creation which is seen as inflexible and non-iterative. Those who use the phrase "waterfall model" pejoratively for non-iterative models that they dislike usually see the waterfall model itself as naive and unsuitable for an "iterative" process.

The model
The unmodified "waterfall model". Progress flows from the top to the bottom, like a waterfall.
In Royce's original waterfall model, the following phases are followed in order:
1. Requirements specification
2. Design
3. Construction (AKA implementation or coding)
4. Integration
5. Testing and debugging (AKA validation)
6. Installation
7. Maintenance

To follow the waterfall model, one proceeds from one phase to the next in a purely sequential manner. For example, one first completes "requirements specification" — they set in stone the requirements of the software. When the requirements are fully completed, one proceeds to design. The software in question is designed and a "blueprint" is drawn for implementers (coders) to follow — this design should be a plan for implementing the requirements given. When the design is fully completed, an implementation of that design is made by coders. Towards the later stages of this implementation phase, disparate software components produced by different teams are integrated. After the implementation and integration phases are complete, the software product is tested and debugged; any faults introduced in earlier phases are removed here. Then the software product is installed, and later maintained to introduce new functionality and remove bugs.
Thus the waterfall model maintains that one should move to a phase only when its preceding phase is completed and perfected. Phases of development in the waterfall model are discrete, and there is no jumping back and forth or overlap between them.
However, there are various modified waterfall models (including Royce's final model) that may include slight or major variations upon this process.