What is Waterfall model ?

The Waterfall Model was first Process Model to be introduced. It is also known as a linear-sequential life cycle model. It is very simple to understand and use. In a waterfall model, each phase must be completed before the next phase can begin and there is no overlapping in the phases.

The Waterfall model is the earliest SDLC (Software Development Life Cycle) approach that was used for software development. It is one of the first models that appeared in literature in late 1950s, but gained popularity during 1970s. The model is named as Waterfall model since, its graphical representation resembles a cascade of waterfall. In the Waterfall approach, the whole process of software development is divided into separate phases. In Waterfall model, typically, the outcome of one phase acts as the input for the next phase sequentially. It shows that the output of one phase flows as input to the next one. Each phase in turn, is structured as a set of sub activities. The below figure is a representation of the different phases of the Waterfall Model.

The principal phases represented in the above model are :

1. Requirement Analysis : During this phase, the requirements of the customer are properly understood and documented. The objective of this activity is to document all functions, performance, constraints and the interfacing requirements fo the software. The set of documents produced at this stage is known as Software Requirements Specification (SRS) document.

2. System Design : The requirements specification enables the software engineer to proceed further to define the overall architecture of the system. The System design helps in specifying hardware and system requirements and helps in defining the overall system architecture. Detailed design activities are performed during this phase and the document produced at this stage is known as design description document.

3. Implementation : At this stage the implementation of design is carried out. A software may comprise of several program units. Each unit is tested in isolation from the rest of the product. The implementation and testing operation ensures that each program runs smoothly and conforms to the specifications.

4. Integration and Testing : At this stage, the individual programs are integrated to produce a complete system. All the units developed in the implementation phase are integrated into a system after testing of each unit. After integration, the entire system is tested for any faults and failures. The Testing is performed to insure that the interface between various unit modules is correct and the system meets the requirements given by user.

5. Deployment : When the functional and non-functional testing is done, the product is deployed in the customer environment or released into the market.

6. Maintenance : Maintenance is an important activity which involves error correction, capability enhancement and optimization. If there are some issues which come up in the client environment. Then to fix those issues, patches are released. Also to enhance the product some better versions are released. Maintenance is done to deliver these changes in the customer environment. It is an evolution process which helps preserving the value of the software product for a longer time span. 

Applications of Waterfall Model :
  • Requirements are very well documented, clear and fixed.
  • Product definition is stable.
  • Technology is understood and is not dynamic.
  • There are no ambiguous requirements.
  • Ample resources with required expertise are available to support the product.
  • The project is short.

Advantages of Waterfall model :

Some of the major advantages of the Waterfall Model are as follows :

1. The Waterfall model is simple and easy to understand and use.

2. The model underlines the need for discipline, planning and management in the process of software development. 

3. The simplicity of this model makes it easy to explain the development process to a client who may not be familiar with software development.

4. The visibility of progress in development of the product is considerably high as each phase has a start and finish criteria.

5. The process and results are well documented.

Disadvantages of Waterfall model :

Through waterfall model is widely used, it has certain limitations because the waterfall model is liner, rigid and monolithic. Some of these limitations are :

1. The model assumes that the results of each phase are frozen before proceeding to the next phase. In reality, the software requirements evolve throughout the development life cycle. Complete freezing of requirements and design specifications at early stage will not be proper as engineers and users may not be fully aware of certain functions required for the application.

2. For large and complex systems, development time may be too long. The application may be delivered when the user's needs could have changed. This will require immediate re-work once again.

3. High amounts of risk and uncertainty.

4. A working version of the product will only be available at the later stage of the development process, hence the customer has to be patient till the product is delivered.

5. Not suitable for the projects where requirements are at a moderate to high risk of changing. So, risk and uncertainty is high with this process model.

6. It is difficult to measure progress within stages.

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