API testing checks whether a programming interface returns correct data, handles errors according to spec and holds up under real load — independently of whichever user interface actually consumes that API. Good API tests catch regressions faster and more cheaply than end-to-end tests through the UI, because they don’t depend on page rendering.
Quick Overview
What you’ll learn from this article:
- The four layers of API testing: functional, contract, performance and security
- How to match a tool (Postman, SoapUI, REST Assured) to a specific testing layer
- A step-by-step plan for rolling out API tests in an existing project
- The most common API security gaps from the OWASP API Security Top 10
Who this article is for: testers and QA Engineers expanding into API testing, developers responsible for the quality of their own endpoints, QA team leads planning a testing strategy.
Reading time: 7 minutes
API testing: tools and best practices
API testing splits into four layers, which one single tool rarely covers in full. Functional tests check whether an endpoint returns the expected response for a given request — the most basic layer, usually done in Postman or SoapUI. Contract tests verify that an API’s response matches its specification (e.g. OpenAPI/Swagger) and that a change in one service doesn’t break that API’s consumers’ expectations — critical in a microservices architecture. Performance tests measure response time and how the API behaves under load. Security tests check resilience against common gaps, such as broken object level authorization (OWASP API1:2023) or excessive data exposure in a response.
Which tool for which testing layer
| Testing layer | What it checks | Typical tool |
|---|---|---|
| Functional | Correctness of the response for a given request | Postman, SoapUI |
| Contract | Compliance with the API spec, no broken consumers | Pact, REST Assured with schema validation |
| Performance | Response time, behaviour under load | k6, JMeter, Postman (collections via Newman) |
| Security | Resilience against OWASP API Security Top 10 gaps | OWASP ZAP, manual authorization review |
Postman dominates the functional layer thanks to its low barrier to entry — tests can be written as simple JavaScript assertions directly in the interface, without configuring a development environment. SoapUI remains strong for testing SOAP services and older enterprise systems, where REST isn’t the only standard. REST Assured, a Java library, works best when API tests need to live in the same repository as the application code and run automatically in a CI/CD pipeline.
How to roll out API testing step by step
- Start with functional tests for the most important endpoints — the ones used by the most API consumers, or that handle critical business flows — before expanding coverage to the rest.
- Automate the test collection in a CI/CD pipeline (e.g. via Newman for Postman), so tests run on every pull request instead of only manually before a release.
- Add API contract validation once a project has more than one consumer of the same service — this catches breaking changes before they reach production and break another team’s integration.
- Introduce security tests based on the OWASP API Security Top 10, starting with checking object-level authorization — the most common and most costly gap in APIs according to OWASP.
- Monitor response time in the production environment, not only in tests — performance tests in an isolated environment don’t always reflect real load and interactions with other services.
The OWASP API Security Top 10 lists broken object level authorization as the most common category of API gaps — a situation where a user can access another user’s data simply by changing an identifier in a request. Standard functional tests rarely catch this, because they check whether the API returns the correct response for a valid request — they don’t deliberately test a malicious request with someone else’s identifier, which is why security tests need to be designed separately, with a different intent than functional tests.
Read Also
- What is Postman? A Tool for Testing and Working with APIs
- From Manual Tester to QA Engineer - Skills You Must Acquire
Develop Your Skills
Want to learn how to test APIs in practice? Check out our training led by experienced EITT instructors.
➡️ API Testing with Postman: Automation and Performance — EITT training ➡️ SoapUI in API Testing — EITT training
Frequently Asked Questions (FAQ)
How do functional tests differ from contract tests in API testing?
Functional tests check whether an endpoint returns the expected response for a specific request. Contract tests check whether the response matches the API specification and whether a change in one service breaks other teams’ expectations of that same API — critical in a microservices architecture with multiple consumers.
Is Postman enough for full API testing?
Postman excels at functional testing and can be automated in CI/CD via Newman, but it doesn’t replace dedicated performance tools (like k6) or security tools (like OWASP ZAP). A mature API testing strategy usually combines several tools, each covering a different layer.
What’s the most common API security gap?
According to the OWASP API Security Top 10, it’s broken object level authorization — a situation where a user can access someone else’s data by changing an identifier in an API request. Standard functional tests rarely catch it, since they don’t deliberately test malicious requests.
When is it worth introducing API contract testing?
When a project has more than one consumer of the same service — for example several microservices, or a mobile and web app sharing the same backend. Contract tests catch breaking API changes before they reach production and break an integration on the other side.