Application Programming Interfaces (APIs) are essential for creating interactive, connected applications for customers. As a result, businesses are increasingly recognizing the importance of APIs in application development, testing, and documentation. When done correctly, API testing helps streamline release cycles while improving overall product quality.

What Is API Testing?

API testing validates an API by checking its functionality, reliability, performance, and security. Unlike GUI testing, API testing does not focus on the application interface or database. Instead, it tests communication between systems or system components at the business logic layer of the software architecture.

One major advantage of API testing is that it can begin before the GUI is completed, allowing teams to test early in the development process. Other key strengths of API testing include automation and continuous testing. API testing tools make it possible to create test scripts and run them on schedules or continuously.

What Are the Benefits of API Testing?

Early Testing

With API testing, teams do not need to wait for the full application to be developed. Once the business logic or service layer is designed and built, tests can be created to validate responses and data early in the development cycle.

Easy Test Maintenance

API changes are typically less frequent than GUI changes. When testing is automated, API definition files make test refactoring faster and easier. In contrast, GUI tests often require constant updates due to design changes, scope changes, browser differences, device compatibility, and other front-end variations.

API Testing Methods

Validation Tests

APIs usually consist of several methods or operations that can be tested individually. Validation tests perform syntax checks on individual methods or operations and are often used to build functional tests.

Individual API tests are similar to traditional unit tests, but they are not the same. Unit tests are typically written by developers to test small pieces of the application, while individual API tests are often used as smoke tests to confirm the API is operational before incorporating them into broader functional tests.

Functional Tests

Functional tests verify that an API or web service follows the required business logic. In general, a test sends requests and checks whether the responses meet expectations.

Functional tests combine individual test cases to verify end-to-end API behavior. These tests often consist of several requests or steps that simulate real user behavior. At each step, the test verifies that the service works as expected and returns the correct response.

Security Tests

Security tests include various checks to ensure the API is protected against common malicious attacks and does not expose sensitive information. Common security test areas include authentication, login and logout, permissions, SQL injection, malformed XML, and sensitive file exposure.

Performance Tests

Performance tests use functional test cases to perform load testing on a web service. This helps analyze how APIs behave under load and identifies their limits. Fuzz testing, in which large amounts of unexpected or random data are used to deliberately trigger errors, is one subset of performance testing.

Connectivity and Interoperability Tests

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

API Testing Best Practices

Effective API testing typically includes the following characteristics:

• Always start with validation testing.
• Test the API to determine what behaves consistently and what does not.
• Simulate the production environment during testing.
• Perform stress testing through a series of API load tests.
• Test failure conditions until the API fails in a consistent and expected way.
• Group test cases by category.
• Document selected parameters in each test case and save or archive responses.
• Prioritize API function calls to simplify testing.
• Conduct thorough security testing.
• Automate API documentation creation whenever possible, while ensuring it still meets the needs of the intended audience.
• Create test cases for all possible API input combinations to achieve complete test coverage.
• Reuse test cases whenever possible.

Summary

A successful API testing program should meet the following conditions:

• Enables fast, reliable regression testing and continuous testing through automation.
• Supports early testing in the development lifecycle.
• Verifies that APIs work as expected.
• Confirms that business rules are correctly implemented at the API level.
• Ensures the application can handle the required load.
• Verifies that APIs work seamlessly across different operating systems and devices.
• Ensures that security and compliance requirements are met.
• Provides clear API reference documentation.
• Contributes to a better user experience.