What’s the difference between unit, functional, acceptance, and integration tests?

What is Unit Testing?

Unit testing is a software testing technique where individual code segments, typically methods or functions, are isolated and tested independently. Unit testing aims to validate that each code segment performs as expected and can be seamlessly incorporated into the larger system without creating bugs. By isolating each segment of code, unit testing facilitates easier and more focused testing and debugging.

The main rule of unit testing is to examine each code segment separately. That means any external dependencies, like databases or network connections, should be substituted with mock objects or stubs, allowing the code segment under test to rely solely on its inputs and itself.

Unit testing has various key advantages, including early detection of bugs, better collaboration, improved team confidence in changes, and improved code quality. Furthermore, testing each code segment separately allows test engineers and developers to swiftly identify and address the origin of bugs, preventing them from affecting other system segments and reducing the cost of maintaining code.

What is Functional Testing?

Functional testing is a software testing method used to validate software functionality against predetermined system requirements and specifications. It is mainly a quality assurance (QA) process that tests the software’s functionality in a black-box testing approach. The testing here is not concerned with the source code, but rather appropriate test inputs are passed through the system, and the outputs are compared to the actual expected output. This testing focuses on key areas of the application like API results, user interfaces, database results, software security, client and server application functionalities, as well as the behavior of the system under heavy load.

Functional testing involves identifying all the software functions and specifications, generating input data for those functions, determining the expected outputs for each input data and function, running the tests, and finally comparing the results. The main purpose of functional testing is to ensure that the system behaves according to the specified requirements, focusing on what the system does rather than how it does it.

What is Integration Testing?

This is a software testing type where individual units that had their unit tests done are combined and tested as a logical group. The core purpose of this testing is to expose defects in the interaction between different software modules, which, more often than not, are written in different programming languages. Ideally, integration testing comes after unit testing but before functional and acceptance testing.

Integration testing follows different methods broadly categorized as big bang or incremental integration testing. In big bang testing, all components are integrated simultaneously, and the entire system is tested as a whole.  On the other hand, incremental testing involves combining two or more logically related modules, which are then tested to ensure the correct functioning of the application. Subsequently, additional related modules are integrated incrementally, and this procedure is repeated until all the logically connected modules have been successfully integrated and tested successfully. The incremental integration testing can take different routes, including top-down, bottom-up, and sandwich methods.

What is Acceptance Testing?

Like functional testing, acceptance testing is a quality assurance black-box testing process. It determines to what extent software meets end users’ requirements and approvals. This test comes after system tests like unit, functional, and integration testing have been done and approved, but just before deployment to end users.

Acceptance testing takes various forms, and each needs feedback to ensure the software is at par with multiple stakeholders’ needs. These forms include:

• Business Acceptance Testing (BAT) focuses on business profit margins expected to be derived from the software.
• User Acceptance Testing (UAT) determines whether the software works according to the expected customer requirements.
  
• Regulation Acceptance Testing (RAT) determines if the software complies with rules and regulations defined by governments where the software is to be used.
• Contract Acceptance Testing (CAT) determines service level agreements (SLAs) that the software meets and what happens when the SLA is violated.
• Operational Acceptance Testing (OAT) determines the software’s operational readiness, including recovery testing as well as compatibility, maintainability, and reliability tests.

To make acceptance testing comprehensive, alpha and beta releases of products are rolled out to early adopters, and issues arising from those releases are ironed out before formal deployment into production.

Conclusion

From the discussion of each type of testing, it is apparent they are different but dependent on each other for comprehensive software health. Unit, functional, acceptance, and integration testing can be differentiated depending on their level, responsibility, and execution frequency. The table below shows the differences at a glance: