API Testing

Testing below the interface: faster, more stable, and closer to the business logic than UI testing.

Fundamentals

• HTTP fundamentals — methods, status codes, headers: the language of APIs.
• What to test in an API — checklist: contract, data, errors, security, and idempotency.

API automation

• Architecture of an API testing framework — layers, service objects, and organization (Java + REST Assured + JUnit 5).
• Anatomy of an API test — Given-When-Then, the per-endpoint case matrix, and assertions.
• JSON Schema validation — validate the contract, not just the values.
• Test data and authentication — builders with random data, ephemeral users, and least privilege.
• Async APIs with Awaitility — polling with a timeout instead of sleeps.

Tools

• VS Code REST Client — testing APIs with .http files versioned in the repo.
• SQL for QA — the 20 % of SQL that solves 80 % of the job: verifying persistence, preparing data and spotting inconsistencies.
• Postman and SoapUI — testing APIs without a framework: collections with assertions, and SOAP with its WSDL contract.
• NoSQL for QA — which guarantees disappear without a fixed schema and what to validate because of it, with MongoDB as the example.
• Karate: API testing with BDD — self-contained features with match assertions, no step definitions.