Types of API Testing
Explained as simply as possible… but not simpler.
APIs need testing. But not all testing is the same. Each type catches different problems at different stages.
Here are the nine types you should know.
1. Smoke Testing
The first check after deployment. Does the API even work?
You’re testing basic connectivity, key endpoints returning 200, and authentication not failing immediately. A simple GET /health request is usually enough.
2. Functional Testing
Does the API do what it’s supposed to do?
You’re validating inputs and outputs against requirements. Send a POST /users with valid data - did it create a user? Is the response structure correct? Do all CRUD operations behave as documented?
3. Integration Testing
Does the API work with everything it connects to?
APIs don’t exist in isolation. They talk to databases, other services, message queues. Integration testing verifies data flows correctly across these boundaries.
Create a user via API, then check if the downstream service received it.
4. Regression Testing
Did the new changes break old functionality?
Every time you deploy, you risk breaking something that worked before. Regression testing runs your existing test suite to catch unintended side effects.
Automate this in CI/CD.
5. Load Testing
How does the API perform under expected traffic?
You’re measuring response times, throughput, and resource usage when many users hit the API simultaneously. Simulate 500 concurrent users and watch what happens to latency.
6. Stress Testing
What happens when traffic exceeds expectations?
Load testing checks normal conditions. Stress testing pushes past the limit. You want to know the breaking point and whether the API fails gracefully or crashes entirely.
7. Security Testing
Is the API protected from attacks?
Check authentication and authorization. Try accessing admin resources with user tokens. Test for SQL injection, XSS, and CSRF. Verify rate limiting works.
8. UI Testing (API-Driven)
Does the frontend display API data correctly?
The API might return perfect data, but if the UI mishandles it, users see bugs. This tests the contract between backend responses and frontend rendering.
9. Fuzz Testing
How does the API handle garbage input?
Send malformed JSON, random strings, oversized payloads, unexpected data types. You’re looking for unhandled exceptions and system crashes that attackers could exploit.
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Great list of API Testing here.
Here's one tip to run better Stress Tests:
Run it until the system breaks. Then, turn the stress off and see how the system recovers.
The first part will help you avoid failures; Increase Mean Time Between Failures (MTBF).
The second part will help you recover faster; Increase Mean Time To Repair (MTBR).
Nice comprehensive breakdown. The integration testing definition is particulary sharp: APIs existing in isolation is the myth that causes most production incidents. One underrated aspect here is that fuzz testing often uncovers integration issues that unit tests miss entirely, since malformed data propagating across service boundries can trigger cascading failures that look nothing like the original input error.