INTEGRATION

The term integration refers to a software development activity inwhich separate software components are combined into a whole. Integration is done at several levels and stages of the implementation, for the following purposes:

To integrate the work of a team working in the same implementation subsystem before the subsystem is released to system integrators
To integrate subsystems into a complete system

The Rational Unified Process approach to integration is that the software is integrated incrementally. Incremental integration means that code is written and tested in small pieces and then is combined into a working whole by the addition of one piece at a time.

The contrasting approach is phased integration. Phased integration relies on integrating multiple (new and changed) components at the same time. The major drawback of phased integration is that it introduces multiple variables and makes it hard to locate errors. An error could be inside any of the new components, in the interaction between the new components, or in the interaction between the new components and those at the core of the system.

Incremental integration offers the following benefits:

Faults are easy to locate. When a new problem occurs during incremental integration, the new or changed component (or its interaction with the previously integrated components) is the obvious first place to look for the fault. Incremental integration also makes it likely that defects will be discovered one at a time, something that makes it easier to identify faults.
The components are tested more fully. Components are integrated and tested as they are developed. This means that the components are exercised more often than they are when integration is done in one step.
Some part of the system is running earlier. It is better for the morale of developers to see early results of their work instead of waiting until the end to test everything. It also makes it possible to receive early feedback on design, tools, rules, or style.

It is important to understand that integration occurs (at least once) during each iteration. An iteration plan defines the use cases to be designed and thus the classes to be implemented. The focus of the integration strategy is to determine the order in which classes are implemented and then combined.