What is Integration Testing? How Does It Differ from Unit Testing?

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The short answer: Integration testing is a type of software testing methodology whereby individual units or components are combined and tested as a group to ensure they work together correctly.

Integration testing involves combining multiple modules of an application and testing them as a whole. It’s designed to identify any issues in the interactions between integrated units, such as data flow problems, interface mismatches, and integration errors. Usually performed after unit testing and before system testing, integration testing bridges the gap between testing individual components and the entire system. 

Use cases for integration testing include:

• Combining Modules: When individual units or modules have been tested and are ready to be combined, integration testing ensures they interact correctly, and data flows as expected between them.

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• Detecting Interface Issues: Integration testing helps detect issues at the interfaces between components, such as mismatched data formats or incorrect method calls.

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• Data Validation: Ensuring that data is correctly passed between different parts of the application is critical, and integration testing validates this data flow.

How Does Integration Testing Differ vs. Unit Testing?

In contrast to integration testing which combines multiple modules, unit testing focuses on a single part of an application in isolation — a single class, method, or function, for example.

Unit testing is usually achieved by writing test cases that cover various scenarios, including normal inputs, edge cases, and error conditions. They’re typically automated and are the foundational testing step in the testing pyramid, the next being integration testing, followed by end-to-end and manual testing. 

Integration Testing Methods

There are four main methods that dev teams use to conduct integration testing: top-down, bottom-up, hybrid, and big-bang.

Top-down testing is an incremental approach that begins with the highest level module and moves down to the lower modules gradually. Each module is tested individually and then integrated. 

Bottom-up testing is the opposite of top-down testing and begins with the lowest level module first, and then moves up to the higher modules gradually. This approach is most suitable when all units are available for testing. 

Hybrid testing combines the top-down and bottom-up approaches. It’s the preferable approach for testing large applications and programs. 

Big-bang testing integrates all modules at once and tests them homogeneously as a single unit. This approach is better for smaller systems due to its ease of identifying errors and speeding up dev workflows.