Validation phase - Integration Testing - Top Down Integration and Bottom Up Integration

Integration testing is a systematic technique for constructing the program structure while at the same time conducting tests to uncover errors associated with interfacing. The objective is to take unit tested components and build a program structure that has been dictated by design. There are two methods of integration testing:
- Top-down integration approach
- Bottom-up integration approach

Top-down Integration Approach
It is an incremental approach to construction of program structure. Modules are integrated by moving downward through the control hierarchy beginning with the main control module. Modules subordinate to the main control module are incorporated into the structure in either a depth-first or breadth-first manner.
- The main control module is used as a test driver and stubs are substituted for all components directly subordinate to the main control module.
- Depending upon integration approach, selected subordinate stubs are replaced one at a time with actual components.
- Tests are conducted as each component is integrated.
- On completion of each set of tests, stub is replaced with real component.
- Regression testing may be conducted to ensure that new errors have not been introduced.

Bottom-Up Integration Approach
It begins construction and testing with atomic modules. Because components are integrated from bottom up, processing required for components subordinate to a given level is always available and the need for stubs is eliminated.
- Low level components are combined into clusters that perform a specific software sub function.
- A driver is written to coordinate test case input and output.
- The cluster is tested.
- Drivers are removed and clusters are combined moving upward in the program structure.