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Software Engineering | Integration Testing

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  • Difficulty Level : Basic
  • Last Updated : 30 Nov, 2022
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Prerequisite – Software Testing | Basics, Types of Software Testing Integration testing is the process of testing the interface between two software units or modules. It focuses on determining the correctness of the interface. The purpose of integration testing is to expose faults in the interaction between integrated units. Once all the modules have been unit tested, integration testing is performed.

Integration testing can be done by picking module by module. This can be done so that there should be proper sequence to be followed. And also if you don’t want to miss out on any integration scenarios then you have to follow the proper sequence. Exposing the defects is the major focus of the integration testing and the time of interaction between the integrated units.
Integration test approaches – There are four types of integration testing approaches. Those approaches are the following: 

1. Big-Bang Integration Testing – It is the simplest integration testing approach, where all the modules are combined and the functionality is verified after the completion of individual module testing. In simple words, all the modules of the system are simply put together and tested. This approach is practicable only for very small systems. If an error is found during the integration testing, it is very difficult to localize the error as the error may potentially belong to any of the modules being integrated. So, debugging errors reported during big bang integration testing is very expensive to fix. 


  • It is convenient for small systems.


  • There will be quite a lot of delay because you would have to wait for all the modules to be integrated.
  • High risk critical modules are not isolated and tested on priority since all modules are tested at once.
  • Not Good for long Projects.

2. Bottom-Up Integration Testing – In bottom-up testing, each module at lower levels is tested with higher modules until all modules are tested. The primary purpose of this integration testing is that each subsystem tests the interfaces among various modules making up the subsystem. This integration testing uses test drivers to drive and pass appropriate data to the lower level modules.


  • In bottom-up testing, no stubs are required.
  • A principle advantage of this integration testing is that several disjoint subsystems can be tested simultaneously.
  • It is easy to create the test conditions.
  • Best for the applications that uses bottom up design approach.
  • It is Easy to observe the test results.


  • Driver modules must be produced.
  • In this testing, the complexity that occurs when the system is made up of a large number of small subsystems.
  • As Far modules have been created, there is no working model can be represented.

3. Top-Down Integration Testing – Top-down integration testing technique is used in order to simulate the behaviour of the lower-level modules that are not yet integrated. In this integration testing, testing takes place from top to bottom. First, high-level modules are tested and then low-level modules and finally integrating the low-level modules to a high level to ensure the system is working as intended. 


  • Separately debugged module.
  • Few or no drivers needed.
  • It is more stable and accurate at the aggregate level.
  • Easier isolation of interface errors.
  • In this, design defects can be found in the early stages.


  • Needs many Stubs.
  • Modules at lower level are tested inadequately.
  • It is difficult to observe the test output.
  • It is difficult to stub design.  

4. Mixed Integration Testing – A mixed integration testing is also called sandwiched integration testing. A mixed integration testing follows a combination of top down and bottom-up testing approaches. In top-down approach, testing can start only after the top-level module have been coded and unit tested. In bottom-up approach, testing can start only after the bottom level modules are ready. This sandwich or mixed approach overcomes this shortcoming of the top-down and bottom-up approaches. It is also called the hybrid integration testing. also, stubs and drivers are used  in mixed integration testing.


  • Mixed approach is useful for very large projects having several sub projects.
  • This Sandwich approach overcomes this shortcoming of the top-down and bottom-up approaches.
  • Parallel test can be performed in top and bottom layer tests.


  • For mixed integration testing, it requires very high cost because one part has Top-down approach while another part has bottom-up approach.
  • This integration testing cannot be used for smaller systems with huge interdependence between different modules.
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