An API (Application Programming Interface) is a set of protocols, tools, and definitions for building application software. It specifies how software components should interact. API testing is the process of validating these interfaces directly at the business logic layer, without relying on the user interface.
API testing has become essential in modern software development because most applications communicate with other applications or services through APIs. Unlike UI testing which focuses on what the user sees, API testing focuses on what happens behind the scenes. It validates whether the API meets functional requirements, performs well, handles errors gracefully, and maintains security. API testing is faster, more reliable, and cheaper than UI testing because it directly tests the business logic without the complexity of GUI rendering.
When you book a flight on an airline website, multiple APIs work behind the scenes: authentication API (login), search API (flight details), payment API (transaction), booking API (confirmation), and notification API (email confirmation). API testing ensures each of these works correctly before the UI presents the data to users.
Scenario: You're testing an e-commerce API. The team wants to move all payment flow testing from UI to API layer. Users are booking flights and making payments. What would be your testing approach and why is API testing better here?