API Basics: What It Is and How to Test Manually

In modern software development, APIs (Application Programming Interface) are a key link that enables interaction between system components. Without understanding how APIs work and having skills to test them, a significant portion of defects related to incorrect data transfer, request-handling logic errors, or mismatches with specifications often goes unnoticed at early stages. This course helps you learn manual API testing without programming, which is especially valuable for QA specialists who want to expand their competencies.

The course covers core concepts: what an API is, what request types exist (GET, POST, PUT, DELETE), and how they are used in REST architecture. You will learn how to work with Postman: build requests, pass parameters, headers, and request body, and analyze responses—status codes (2xx, 4xx, 5xx), headers (Content-Type, Authorization), and JSON format. Special attention is given to writing API test cases: defining expected results, checking boundary values, handling errors, and verifying HTTP status codes. You will also learn to read Swagger (OpenAPI) documentation to generate requests automatically and prepare checks quickly.

The course methodology is based on practical examples and analysis of common mistakes made by beginner testers. You will learn how to distinguish server-side bugs from client-side issues, why it’s important to test not only successful scenarios but also negative ones (invalid data, missing required fields), and how to interpret non-obvious API responses. Common pitfalls are covered: ignoring caching headers, incorrect authentication handling (tokens, keys), and failing to account for idempotency of requests.

The course is intended for: QA testers who want to move from manual GUI testing to API testing; junior developers who want a deeper understanding of how the frontend and backend interact; analysts involved in forming API requirements; technical support specialists who need to reproduce and accurately describe user problems.

By the end of the course, you will confidently use Postman to send and analyze requests, write API test cases, verify request and response correctness, and identify common defects (incorrect status codes, invalid data format, authorization issues). You will learn basic terminology (endpoint, request/response, payload, headers) and be able to read Swagger documentation to prepare checks quickly. These skills will help you participate effectively in testing microservices and integration solutions.