Rapid Application Development
In rapid application development, structured techniques and prototyping are especially used to define users' requirements and to design the final system. The development process starts with the development of preliminary data models and business process models using structured techniques. In the next stage, requirements are verified using prototyping, eventually to refine the data and process models. These stages are repeated iteratively; further development results in "a combined business requirements and technical design statement to be used for constructing new systems.
After each phase is finished, it proceeds to the next one. Reviews may occur before moving to the next phase which allows for the possibility of changes (which may involve a formal change control process). Reviews may also be employed to ensure that the phase is indeed complete. ; the phase completion criteria are often referred to as a "gate" that the project must pass through to move to the next phase.
Waterfall discourages revisiting and revising any prior phase once it's complete.
This "inflexibility" in a pure Waterfall model has been a source of criticism by supporters of other more "flexible" models. The Waterfall model is also commonly taught with the mnemonic A Dance in the Dark Every Monday, representing Analysis, Design, Implementation, Testing, Documentation and Execution, and Maintenance.
The key characteristic of a Spiral model is risk management at regular stages in the development cycle. Spiral model is a combination of some key aspect of the waterfall model and rapid prototyping methodologies.
The Spiral is visualized as a process passing through some number of iterations, with the four quadrant diagram representative of the following activities:
Risk-driven spiral model, emphasizing the conditions of options and constraints in order to support software reuse, software quality can help as a special goal of integration into the product development. However, the spiral model has some restrictive conditions, as follows:
The first stage is to formulate a plan to achieve the objectives with these constraints, and then strive to find and remove all potential risks through careful analysis and, if necessary, by constructing a prototype. If some risks can not be ruled out, the customer has to decide whether to terminate the project or to ignore the risks and continue anyway. Finally, the results are evaluated and the design of the next phase begins.