An API — Application Programming Interface — is a contract that lets two pieces of software talk to each other. When your phone app shows the weather, it is calling a weather API behind the scenes. APIs are the connective tissue of modern software, and testing them is one of the highest-value things a QA team can do.
What is API testing?
API testing checks these interfaces directly — sending requests and verifying the responses — without going through a user interface. It validates the data, status codes, error handling, performance, and security of the service layer, where the real business logic usually lives.
REST vs SOAP
Two major styles of web API dominate. Most modern APIs are REST; many enterprise and legacy systems use SOAP.
REST
- Architectural style, usually over HTTP.
- Typically exchanges JSON.
- Lightweight, flexible, widely used.
- Uses HTTP methods (GET, POST, PUT, DELETE).
SOAP
- A strict protocol with an XML message format.
- Built-in standards for security and transactions.
- More rigid, heavier, contract-first (WSDL).
- Common in enterprise and legacy systems.
Why test APIs?
Where API testing sits
Conclusion
APIs let software components communicate, and API testing verifies that communication directly at the service layer. Whether the API is REST or SOAP, testing it gives fast, stable, high-value coverage of the logic that matters most.