API Testing Overview and Best Practices

API Testing Overview and Best Practices

Application Programming Interfaces (APIs) are essential for creating an interactive and connected application for customers, and this is forcing businesses to recognize their importance regarding application development, testing, and documentation. When done correctly, API testing is one area where businesses can streamline release cycles while improving product quality.

What is API Testing?

API testing validates an API by checking the functionality, reliability, performance, and security of the API. API tests are different from GUI tests and do not test the application UI or database. API testing tests communication between systems or system components; this is the business logic layer of the software architecture. A benefit of this is that API testing can be done before the GUI is completed so it can be done early in the development process. Other key features of API testing are automation and continuous testing. API testing tools provide the ability to create API test scripts and to run tests on schedules or continually.

What are the benefits of API Testing?

Early Testing

With API testing you do not need to wait until other teams develop the whole application. API testing can be started early in the application development process since once the business logic (service layer) is designed and built, then tests can be built to validate the correct responses and data.

Easy Test Maintenance

API changes are less frequent and, when testing is automated, API definitions files make refactoring tests quick and easy. In normal testing, when GUIs are constantly changing because of enhancements or scope creep, or to work in different browsers and devices, etc., the tests needed to be updated constantly to keep up with the production code.

AP Testing Methods

Validation Tests

APIs usually consist of several methods or operations that can be tested individually. Validation tests perform syntax testing of individual methods or operations and are used to create functional tests. (Note: Individual API tests are like traditional unit tests, but the two are not the same. Unit tests are used by developers to test small pieces of the application from the GUI to the database, whereas individual API tests are mostly used as a “smoke” test to ensure the API is operational before incorporating that API test into a functional test.)

Functional Tests

Functional tests verify that an API or a web service follows the required business logic. In general, a test simply sends requests and checks if responses meet expectations. Functional tests use combinations of individual tests to check the end-to-end functionality of the API. Usually, the test consists of several requests or steps that simulate a real user behavior. On each step, the test checks that the service works as expected and returns the correct response.

Security Tests

Security tests include various types of checks that ensure the API is protected from most common malicious attacks and does not expose any sensitive information. Some common types of security tests are authentication, login/logout, permissions, SQL injection, malformed XML, and sensitive files exposure.

Performance Tests

Performance tests use functional test cases to perform load testing on a web service. This allows you to analyze how your APIs work under load and defines their limitations. Fuzz testing, where large amounts of data are input to deliberately cause errors, are a subset of performance testing.

Connectivity/Interoperability Tests

Automated test scripts continually verify API connectivity to systems and components. These verify that APIs are always functioning as expected and that connectivity to all required systems and components is maintained.

AP Testing Best Practices

The following characteristics are present when performing effective AP testing:

  • Always starts with validation testing.
  • Tests the API to verify what happens consistently and what does not.
  • Simulates the production environment during testing.
  • Performs stress testing on the system through a series of API load tests.
  • Tests the API for failures until you get the output as failed. Tests the API so that it fails consistently.
  • Groups the test cases by test category.
  • Documents the parameters selected in the test case itself and saves/archives responses for each test.
  • Prioritizes API function calls to simplify testing for testers.
  • Conducts thorough security testing.
  • When possible, automates the API documentation creation process, but also ensures that it meets the needs of its intended audience.
  • Creates test cases for all possible API input combinations to get complete test coverage.
  • When possible, reuses test cases.


A successful API testing program should meet the following conditions:

  • Provides the ability to do fast, reliable regression testing and continuous testing through API testing automation.
  • Provides the ability to test early.
  • Verifies that the APIs work as expected.
  • Verifies that all the business rules have been correctly implemented at the API level.
  • Verifies and ensures that the application can handle the required load.
  • Verifies that APIs work seamlessly through different operating systems and devices.
  • Verifies that security and compliance requirements are met.
  • Provides API reference documentation.
  • Provides a better user experience.