Integration testing - Integration testing is the phase in software testing in which individual software modules are combined and tested as a group. It occurs after unit testing and before system testing. Integration testing takes as its input modules that have been unit tested, groups them in larger aggregates, applies tests defined in an integration test plan to those aggregates, and delivers as its output the integrated system ready for system testing.
Purpose -

The purpose of integration testing is to verify functional, performance, and reliability requirements placed on major design items. These "design items", i.e. assemblages (or groups of units), are exercised through their interfaces using Black box testing, success and error cases being simulated via appropriate parameter and data inputs. Simulated usage of shared data areas and inter-process communication is tested and individual subsystems are exercised through their input interface. Test cases are constructed to test that all components within assemblages interact correctly, for example across procedure calls or process activations, and this is done after testing individual modules, i.e. unit testing. The overall idea is a "building block" approach, in which verified assemblages are added to a verified base which is then used to support the integration testing of further assemblages.

During Unit Testing, these assumptions are not tested. Purpose of unit testing is also to make sure that these assumptions are valid. There could be many reasons for integration to go wrong, it could be because

• Interface Misuse - A calling component calls another component and makes an error in its use of interface, probably by calling/passing parameters in the wrong sequence.
• Interface Misunderstanding - A calling component makes some assumption about the other components behavior which are incorrect.

Some different types of integration testing are big bang, top-down, and bottom-up.

1) Big Bang Integration Testing
In Big Bang Integration Testing, the individual modules are not integrated until all the modules are ready. Then they will run to check whether it is performing well.

In this type of testing, some disadvantages might occur like,

Defects can be found at the later stage.

It would be difficult to find out whether the defect arouse in Interface or in module.

2) Top Down Integration Testing
In Top Down Integration Testing, the high level modules are integrated and tested first. i.e Testing from main module to sub module. In this type of testing, Stubs are used as temporary module if a module is not ready for integration testing.

3) Bottom Up Integration Testing
In Bottom Up Integration Testing, the low level modules are integrated and tested first i.e Testing from sub module to main module. Same like Stubs, here drivers are used as a temporary module for integration testing.