Introduction to API Testing
What Is API Testing?
API testing verifies the logic, data, and behavior of an application's backend directly - by sending requests to its endpoints and checking responses - without going through the user interface at all.
Why Pair API Testing with UI Automation
UI tests like Selenium scripts are valuable but slower and more brittle than API tests, since they depend on rendering and page structure. API tests run faster and catch backend issues earlier, before they ever surface in the UI.
UI Testing vs API Testing
| Aspect | UI Testing (Selenium) | API Testing |
|---|---|---|
| Speed | Slower - depends on rendering | Much faster - no browser needed |
| Stability | More prone to breaking on UI changes | More stable, tied to backend contracts |
| Coverage | Validates the full user experience | Validates data and business logic directly |
Core Concepts in API Testing
Effective API testing relies on understanding a handful of core concepts: HTTP methods, status codes, request/response bodies, and headers - the same building blocks that make up every REST API interaction.
- HTTP methods - GET, POST, PUT, DELETE define the type of operation
- Status codes - indicate success (200s), client errors (400s), or server errors (500s)
- Request/response body - the data sent to and received from the API, often in JSON
With the fundamentals clear, let's look at how to actually simulate API calls in practice using REST Assured.
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