Incremental

Iterative and Incremental development is a cyclic software development process developed in response to the weaknesses of the waterfall model. It starts with an initial planning and ends with deployment with the cyclic interaction in between.

An iterative development model

The iterative and incremental development is an essential part of the Rational Unified Process, the Dynamic Systems Development Method, Extreme Programming and generally the agile software development frameworks

Overview

Incremental development is a scheduling and staging strategy, in which the various parts of the system are developed at different times or rates, and integrated as they are completed. It does not imply, require nor preclude iterative development or waterfall development - both of those are rework strategies. The alternative to incremental development is to develop the entire system with a "big bang" integration.

Iterative development is a rework scheduling strategy in which time is set aside to revise and improve parts of the system. It does not presuppose incremental development, but works very well with it. A typical difference is that the output from an increment is not necessarily subject to further refinement, and its testing or user feedback is not used as input for revising the plans or specifications of the successive increments. On the contrary, the output from an iteration is examined for modification, and especially for revising the targets of the successive iterations.^{[clarification needed]}

The two terms were merged in practical use in the mid-1990s. The authors of the Unified Process (UP) and the Rational Unified Process (RUP) selected the term "iterative development", and "iterations" to generally mean any combination of incremental and iterative development. Most people saying "iterative" development mean that they do both incremental and iterative development. Some project teams get into trouble by doing only one and not the other without realizing it

The Basic idea

The basic idea behind iterative enhancement is to develop a software system incrementally, allowing the developer to take advantage of what was being learned during the development of earlier, incremental, deliverable versions of the system. Learning comes from both the development and use of the system, where possible. Key steps in the process were to start with a simple implementation of a subset of the software requirements and iteratively enhance the evolving sequence of versions until the full system is implemented. At each iteration, design modifications are made and new functional capabilities are added.

The Procedure itself consists of the Initialization step, the Iteration step, and the Project Control List. The initialization step creates a base version of the system. The goal for this initial implementation is to create a product to which the user can react. It should offer a sampling of the key aspects of the problem and provide a solution that is simple enough to understand and implement easily. To guide the iteration process, a project control list is created that contains a record of all tasks that need to be performed. It includes such items as new features to be implemented and areas of redesign of the existing solution. The control list is constantly being revised as a result of the analysis phase.

The iteration involves the redesign and implementation of a task from the project control list, and the analysis of the current version of the system. The goal for the design and implementation of any iteration is to be simple, straightforward, and modular, supporting redesign at that stage or as a task added to the project control list. The level of design detail is not dictated by the interactive approach. In a light-weight iterative project the code may represent the major source of documentation of the system; however, in a mission-critical iterative project a formal Software Design Document may be used. The analysis of an iteration is based upon user feedback, and the program analysis facilities available. It involves analysis of the structure, modularity, usability, reliability, efficiency, & achievement of goals. The project control list is modified in light of the analysis results

Iterative development

Iterative development slices the deliverable business value (system functionality) into iterations. In each iteration a slice of functionality is delivered through cross-discipline work, starting from the model/requirements through to the testing/deployment. The unified process groups iterations into phases: inception, elaboration, construction, and transition.

• Inception identifies project scope, risks, and requirements (functional and non-functional) at a high level but in enough detail that work can be estimated.
• Elaboration delivers a working architecture that mitigates the top risks and fulfills the non-functional requirements.
• Construction incrementally fills-in the architecture with production-ready code produced from analysis, design, implementation, and testing of the functional requirements.
• Transition delivers the system into the production operating environment.

Each of the phases may be divided into 1 or more iterations, which are usually time-boxed rather than feature-boxed. Architects and analysts work one iteration ahead of developers and testers to keep their work-product backlog full