API Testing

API testing — also known as Application Programming Interface testing — is a form of software testing focused on verifying the functionality, reliability, performance, and security of an application’s API.

APIs serve as communication channels between different software systems, allowing them to interact and exchange data. Testing involves sending requests and examining responses to ensure the API behaves as expected.

Testing levels

01 — Unit
Unit Testing
Testing individual API methods or functions in isolation to ensure they work correctly.
02 — Integration
Integration Testing
Verifying interactions between multiple APIs, ensuring they work together as intended.
03 — Functional
Functional Testing
Validating functional behavior by testing various scenarios against expected results.
04 — Performance
Performance Testing
Assessing responsiveness and stability under different load conditions.
05 — Security
Security Testing
Evaluating authentication, access control, and data protection to identify vulnerabilities.

What gets tested

Correctness of data formats and structures in request and response payloads

Proper handling of error conditions and error messages

Authentication and authorization mechanisms

Handling of different input types and edge cases

Performance and scalability under various loads

Compatibility with different operating systems, browsers, or devices

Manual Testing
Testers create requests and validate responses by hand — useful for exploratory or one-off checks.
Automated Testing
Specialized frameworks handle test execution — faster, more reliable for complex APIs and regression cycles.

Common tools

Postman
Manual + Auto
User-friendly interface for sending requests, inspecting responses, and automating test collections.
SoapUI
REST + SOAP
Open-source tool for functional and performance testing of web services with data-driven support.
Swagger / OpenAPI
Design + Test
Framework for designing, documenting, and manually testing RESTful APIs via Swagger UI.
JMeter
Performance
Primarily a performance tool — supports HTTP, HTTPS, SOAP, REST with high-load simulation.
Newman
CLI + CI/CD
Command-line runner for Postman collections — integrates easily into CI/CD pipelines.
RestAssured
Java
Java library for fluent, readable REST API tests with built-in JSON/XML parsing and assertions.
Karate
Open-source
Combines API testing, mocking, and test doubles into one framework with a simple syntax.
The right tool depends on the API’s complexity, the level of automation required, and team preference. Most modern QA pipelines combine two or more of the above.
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