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.
Advantages
o Simple and easy to use.
o Easy to manage due to the rigidity of the model – each phase has specific
Deliverables and a review process.
o Phases are processed and completed one at a time.
o Works well for smaller projects where requirements are very well understood/stable.
Disadvantages
o It’s difficult to respond to changing customer requirements.
o Adjusting scope during the life cycle can kill a project
o No working software is produced until late during the life cycle.
o High amounts of risk and uncertainty.
o Poor model for complex and object-oriented projects.
o Poor model for long run and ongoing projects.
Prototype Model
The prototyping model assumes that you do not have clear requirements at the beginning of the project. Often, customers have a vague idea of the requirements in the form of objectives that they want the system to address. With the prototyping model, you build a simplified version of the system and seek feedback from the parties who have a stake in the project. The next iteration incorporates the feedback and improves on the requirements specification.
The prototypes that are built during the iterations can be any of the following:
o A simple user interface without any actual data processing logic
o A few subsystems with functionality that is partially or completely implemented
o Existing components that demonstrate the functionality that will be incorporated into the system.
The prototyping model consists of the following steps.
o Capture requirements. This step involves collecting the requirements over a period of time as they become available.
o Design the system. After capturing the requirements, a new design is made or an existing one is modified to address the new requirements.
o Create or modify the prototype. A prototype is created or an existing prototype is modified based on the design from the previous step.
o Assess based on feedback. The prototype is sent to the stakeholders for review. Based on their feedback, an impact analysis is conducted for the requirements, the design, and the prototype. The role of testing at this step is to ensure that customer feedback is incorporated in the next version of the prototype.
o Refine the prototype. The prototype is refined based on the impact analysis conducted in the previous step.
o Implement the system. After the requirements are understood, the system is rewritten either from scratch or by reusing the prototypes.
The main advantage of the prototyping model is that it allows you to start with requirements that are not clearly defined.
The main disadvantage of the prototyping model is that it can lead to poorly designed systems. The prototypes are usually built without regard to how they might be used later, so attempts to reuse them may result in inefficient systems. This model emphasizes refining the requirements based on customer feedback, rather than ensuring a better product through quick change based on test feedback.
Incremental or Iterative Model
The incremental, or iterative, development model breaks the project into small parts. Each part is subjected to multiple iterations of the waterfall model. At the end of each iteration, a new module is completed or an existing one is improved on, the module is integrated into the structure, and the structure is then tested as a whole.
For example, using the iterative development model, a project can be divided into 12 one- to four-week iterations. The system is tested at the end of each iteration, and the test feedback is immediately incorporated at the end of each test cycle. The time required for successive iterations can be reduced based on the experience gained from past iterations. The system grows by adding new functions during the development portion of each iteration. Each cycle tackles a relatively small set of requirements; therefore, testing evolves as the system evolves. In contrast, in a classic waterfall life cycle, each phase (requirement analysis, system design, and so on) occurs once in the development cycle for the entire set of system requirements.
The main advantage of the iterative development model is that corrective actions can be taken at the end of each iteration. The corrective actions can be changes to the specification because of incorrect interpretation of the requirements, changes to the requirements themselves, and other design or code-related changes based on the system testing conducted at the end of each cycle.
The main disadvantages of the iterative development model are as follows:
o The communication overhead for the project team is significant, because each iteration involves giving feedback about deliverables, effort, timelines, and so on.
o It is difficult to freeze requirements, and they may continue to change in later iterations because of increasing customer demands. As a result, more iteration may be added to the project, leading to project delays and cost overruns.
o The project requires a very efficient change control mechanism to manage changes made to the system during each iteration.
Agile Model
Most software development life cycle methodologies are either iterative or follow a sequential model (as the waterfall model does). As software development becomes more complex, these models cannot efficiently adapt to the continuous and numerous changes that occur. Agile methodology was developed to respond to changes quickly and smoothly. Although the iterative methodologies tend to remove the disadvantage of sequential models, they still are based on the traditional waterfall approach. Agile methodology is a collection of values, principles, and practices that incorporates iterative development, test, and feedback into a new style of development.
The key differences between agile and traditional methodologies are as follows:
o Development is incremental rather than sequential. Software is developed in incremental, rapid cycles. This results in small, incremental releases, with each release building on previous functionality. Each release is thoroughly tested, which ensures that all issues are addressed in the next iteration.
o People and interactions are emphasized, rather than processes and tools. Customers, developers, and testers constantly interact with each other. This interaction ensures that the tester is aware of the requirements for the features being developed during a particular iteration and can easily identify any discrepancy between the system and the requirements.
o Working software is the priority rather than detailed documentation. Agile methodologies rely on face-to-face communication and collaboration, with people working in pairs. Because of the extensive communication with customers and among team members, the project does not need a comprehensive requirements document.
o Customer collaboration is used, rather than contract negotiation. All agile projects include customers as a part of the team. When developers have questions about a requirement, they immediately get clarification from customers.
o Responding to change is emphasized, rather than extensive planning. Extreme Programming does not preclude planning your project. However, it suggests changing the plan to accommodate any changes in assumptions for the plan, rather than stubbornly trying to follow the original plan.
