Waterfall model

The Waterfall Model was first Process Model to be introduced. It is also referred to as a linear-sequential life cycle model. It is very simple to understand and use. In a waterfall model, each phase must be completed fully before the next phase can begin. At the end of each phase, a review takes place to determine if the project is on the right path and whether or not to continue or discard the project. In waterfall model phases do not overlap.

Diagram of Waterfall-model: 

              

Advantages of waterfall model: 

• Simple and easy to understand and use. 
• Easy to manage due to the rigidity of the model – each phase has specific deliverables and a review process. 
• Phases are processed and completed one at a time. 
• Works well for smaller projects where requirements are very well understood. 

Disadvantages of waterfall model: 

• Once an application is in the testing stage, it is very difficult to go back and change something that was not well-thought out in the concept stage. 
• No working software is produced until late during the life cycle. 
• High amounts of risk and uncertainty. 
• Not a good model for complex and object-oriented projects. 
• Poor model for long and ongoing projects. 
• Not suitable for the projects where requirements are at a moderate to high risk of changing. 

When to use the waterfall model: 

• Requirements are very well known, clear and fixed. 
• Product definition is stable. 
• Technology is understood. 
• There are no ambiguous requirements 
• Ample resources with required expertise are available freely 
• The project is short.

Incremental model

In incremental model the whole requirement is divided into various builds. Multiple development cycles take place here, making the life cycle a“multi-waterfall” cycle. Cycles are divided up into smaller, more easily managed modules. Each module passes through the requirements, design, implementation and testing phases. A working version of software is produced during the first module, so you have working software early on during the software life cycle. Each subsequent release of the module adds function to the previous release. The process continues till the complete system is achieved.


For example:                                                       In the diagram above when we work incrementally we are adding piece by piece but expect that each piece is fully finished. Thus keep on adding the pieces until it’s complete.


Diagram of Incremental model:


          


Advantages of Incremental model:

• Generates working software quickly and early during the software life cycle. 
• More flexible – less costly to change scope and requirements. 
• Easier to test and debug during a smaller iteration. 
• Customer can respond to each built. 
• Lowers initial delivery cost. 
• Easier to manage risk because risky pieces are identified and handled during it’d iteration. 

Disadvantages of Incremental model:

• Needs good planning and design. 
• Needs a clear and complete definition of the whole system before it can be broken down and built incrementally. 
• Total cost is higher than waterfall. 

When to use the Incremental model:

• Requirements of the complete system are clearly defined and understood. 
• Major requirements must be defined; however, some details can evolve with time. 
• There is a need to get a product to the market early. 
• A new technology is being used 
• Resources with needed skill set are not available 
• There are some high risk features and goals.

RAD model

RAD model is Rapid Application Development model. It is a type of incremental model. In RAD model the components or functions are developed in parallel as if they were mini projects. The developments are time boxed, delivered and then assembled into a working prototype. This can quickly give the customer something to see and use and to provide feedback regarding the delivery and their requirements.

Diagram of RAD-Model:

The phases in the rapid application development (RAD) model are:

Business modeling: The information flow is identified between various business functions.

Data modeling: Information gathered from business modeling is used to define data objects that are needed for the business.

Process modeling: Data objects defined in data modeling are converted to achieve the business information flow to achieve some specific business objective. Description are identified and created for CRUD of data objects.

Application generation: Automated tools are used to convert process models into code and the actual system.

Testing and turnover: Test new components and all the interfaces.

Advantages of the RAD model: 

• Reduced development time. 
• Increases reusability of components 
• Quick initial reviews occur 
• Encourages customer feedback 
• Integration from very beginning solves a lot of integration issues. 

Disadvantages of RAD model: 

• Depends on strong team and individual performances for identifying business requirements. 
• Only system that can be modularized can be built using RAD 
• Requires highly skilled developers/designers. 
• High dependency on modeling skills 
• Inapplicable to cheaper projects as cost of modeling and automated code generation is very high. 

When to use RAD model: 

• RAD should be used when there is a need to create a system that can be modularized in 2-3 months of time. 
• It should be used if there’s high availability of designers for modeling and the budget is high enough to afford their cost along with the cost of automated code generating tools. 
• RAD SDLC model should be chosen only if resources with high business knowledge are available and there is a need to produce the system in a short span of time (2-3 months).