Waterfall Model

The waterfall model is one of the earliest structured models for software development. It consists of the following sequential phases through which the development life cycle progresses:

• System feasibility. In this phase, you consider the various aspects of the targeted business process, find out which aspects are worth incorporating into a system, and evaluate various approaches to building the required software.
• Requirement analysis. In this phase, you capture software requirements in such a way that they can be translated into actual use cases for the system. The requirements can derive from use cases, performance goals, target deployment, and so on.
• System design. In this phase, you identify the interacting components that make up the system. You define the exposed interfaces, the communication between the interfaces, key algorithms used, and the sequence of interaction. An architecture and design review is conducted at the end of this phase to ensure that the design conforms to the previously defined requirements.
• Coding and unit testing. In this phase, you write code for the modules that make up the system. You also review the code and individually test the functionality of each module.
• Integration and system testing. In this phase, you integrate all of the modules in the system and test them as a single system for all of the use cases, making sure that the modules meet the requirements.
• Deployment and maintenance. In this phase, you deploy the software system in the production environment. You then correct any errors that are identified in this phase, and add or modify functionality based on the updated requirements.

The waterfall model has the following advantages:

• It allows you to compartmentalize the life cycle into various phases, which allows you to plan the resources and effort required through the development process.
• It enforces testing in every stage in the form of reviews and unit testing. You conduct design reviews, code reviews, unit testing, and integration testing during the stages of the life cycle.
• It allows you to set expectations for deliverables after each phase.

The waterfall model has the following disadvantages:

• You do not see a working version of the software until late in the life cycle. For this reason, you can fail to detect problems until the system testing phase. Problems may be more costly to fix in this phase than they would have been earlier in the life cycle.
• When an application is in the system testing phase, it is difficult to change something that was not carefully considered in the system design phase. The emphasis on early planning tends to delay or restrict the amount of change that the testing effort can instigate, which is not the case when a working model is tested for immediate feedback.
• For a phase to begin, the preceding phase must be complete; for example, the system design phase cannot begin until the requirement analysis phase is complete and the requirements are frozen. As a result, the waterfall model is not able to accommodate uncertainties that may persist after a phase is completed. These uncertainties may lead to delays and extended project schedules.