One of the most difficult aspects of software development is the integration and testing of large, untested sub-systems. The integrated system frequently fails in significant and mysterious ways, and it is difficult to fix it
Integration testing exercises several units that have been combined to form a module, subsystem, or system. Integration testing focuses on the interfaces between units, to make sure the units work together. The nature of this phase is certainly ‘white box’, as we must have a certain knowledge of the units to recognize if we have been successful in fusing them together in the module.
There are three main approaches to integration testing: top-down, bottom-up and ‘big bang’. Top-down combines, tests, and debugs top-level routines that become the test ‘harness’ or ’scaffolding’ for lower-level units. Bottom-up combines and tests low-level units into progressively larger modules and subsystems. ‘Big bang’ testing is, unfortunately, the prevalent integration test ‘method’. This is waiting for all the module units to be complete before trying them out together.
Integration tests can rely heavily on stubs or drivers. Stubs stand-in for finished subroutines or sub-systems. A stub might consist of a function header with no body, or it may read and return test data from a file, return hard-coded values, or obtain data from the tester. Stub creation can be a time consuming piece of testing.
The cost of drivers and stubs in the top-down and bottom-up testing methods is what drives the use of ‘big bang’ testing. This approach waits for all the modules to be constructed and tested independently, and when they are finished, they are integrated all at once. While this approach is very quick, it frequently reveals more defects than the other methods. These errors have to be fixed and as we have seen, errors that are found ‘later’ take longer to fix. In addition, like bottom up, there is really nothing that can be demonstrated until later in the process.
