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Top 50 API Testing Interview Questions

· 7 min read
Anush Chandra Shekar
Anush Chandra Shekar
SEO & Digital Marketing, DevAssure

API testing is a crucial aspect of software testing that ensures APIs function as expected and meet performance, security, and reliability standards. As organizations increasingly rely on APIs for their applications, demand for API testers continues to grow. If you are preparing for an interview, knowing the most common API Testing interview questions can give you a competitive edge.

In this blog, we cover the top 50 API Testing interview questions and answers, categorized into elementary and advanced sections. This will help both beginners and experienced professionals refine their knowledge and technical skills.

Fundamental Questions (1 to 25)​

1. What is API Testing?​

API Testing is a type of software testing that verifies APIs meet functionality, reliability, performance, and security expectations.

2. What are the key types of API Testing?​

  • Functional Testing - Ensures API functions correctly.
  • Performance Testing - Measures API response time.
  • Security Testing - Checks authentication & authorization.
  • Integration Testing - Tests API interactions with other components.

3. What tools are used for API Testing?​

  • DevAssure (Automation Testing)
  • REST Assured (Java-based automation)
  • SoapUI (For SOAP & REST APIs)
  • JMeter (Performance Testing)

4. What is the difference between SOAP and REST APIs?​

SOAP​

  • Uses XML-based messages
  • Strict protocol
  • WS-Security for authentication

REST​

  • Uses JSON/XML
  • Lightweight & flexible
  • Supports OAuth, JWT authentication

5. What is the difference between PUT and POST methods?​

MethodUsageIdempotent
POSTCreates new resource❌ No
PUTUpdates/replaces existing resource✅ Yes

6. What are HTTP Status Codes in API Testing?​

  • 200 OK - Success
  • 201 Created - Resource created
  • 400 Bad Request - Invalid request data
  • 401 Unauthorized - Missing authentication
  • 500 Internal Server Error - Server failure

7. What is JSON Schema Validation?​

Validates JSON structure and data types.

Example Schema:

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer" }
  }
}

8. What is API Authentication?​

9. What is an API Endpoint?​

An API endpoint is a URL that a client interacts with to access resources.

Example: https://api.example.com/users

10. What is Rate Limiting in APIs?​

Limits requests per user/IP to prevent abuse.

Example headers:

X-RateLimit-Limit: 1000  
X-RateLimit-Remaining: 500

11. What is the difference between Authentication and Authorization?​

  • Authentication - Verifies identity (e.g., Login)
  • Authorization - Grants access to resources (e.g., Admin privileges)

12. What is an API Mocking Tool?​

A tool that simulates API responses during development. Examples: WireMock, Mockoon

13. What is API Versioning?​

Methods:

  • URL versioning: /v1/users
  • Header versioning: Accept: application/vnd.api.v2+json

14. What is an API Contract?​

A contract defines API requests, responses, and data types. Used in Swagger/OpenAPI.

15. What are Idempotent Methods?​

Idempotent methods return the same result regardless of the number of calls.

✅ GET, PUT, DELETE
❌ POST (Creates a new resource every time)

16. What is API Caching?​

Stores API responses to improve performance. Example: Cache-Control: max-age=3600

17. What is a Webhook?​

A Webhook is a real-time callback URL that gets triggered when an event occurs.

18. What is GraphQL?​

A query language for APIs that fetches only required data.

Example Query:

{
  user(id: 1) {
    name
    email
  }
}

19. What is API Load Testing?​

Tests API performance under a high number of requests. Tools: JMeter, Gatling

20. What is an API Gateway?​

An intermediary that manages API authentication, rate limiting, and caching. Example: AWS API Gateway

21. What is an API Throttling Mechanism?​

Restricts API usage by limiting request frequency. Example: 500 requests per minute

22. What is an API Response Time Benchmark?​

  • Fast API (< 100ms) - Excellent
  • 100ms - 500ms - Average
  • > 1s - Needs optimization

23. How do you validate API Responses?​

  • Check status codes (200, 201, 400)
  • Validate JSON structure
  • Verify response time

24. What is SOAP Security?​

  • WS-Security (XML Encryption, Digital Signatures)
  • HTTPS (Ensures data encryption)

25. What is a Negative API Test Case?​

Test cases that check API error handling:

  • Sending invalid data
  • Exceeding rate limits
  • Testing unauthorized access

Advanced Level Questions (16 to 30)​

26. What is an API Sandbox?​

An API sandbox is a testing environment that mimics the production environment for safe API testing without affecting live data.

27. What is API Monitoring?​

Continuous tracking of API performance, availability, and correctness using tools like New Relic, Datadog, or Postman Monitors.

28. What are HTTP Headers in API Testing?​

Headers provide metadata about API requests and responses.

Examples:

  • Content-Type: application/json
  • Authorization: Bearer token123

29. What is the difference between Sync and Async APIs?​

  • Synchronous API - Request and response happen sequentially.
  • Asynchronous API - Uses callbacks or WebSockets to process requests concurrently.

30. What is API Chaining?​

Executing multiple API calls in a sequence, where the output of one API is used as input for another.

31. What is the difference between REST and GraphQL?​

GraphQL allows clients to fetch only required data, whereas REST APIs return predefined responses.

32. What is OAuth 2.0?​

OAuth 2.0 is an authorization framework allowing secure access to user data without sharing passwords.

33. What is JWT (JSON Web Token)?​

A compact, self-contained token used for authentication and authorization in APIs.

34. How do you test API Performance?​

  • Load testing (JMeter, Gatling)
  • Stress testing
  • Response time monitoring

35. What is API Pagination?​

Dividing large datasets into pages using parameters like ?page=2&limit=50.

36. What are WebSockets?​

A protocol enabling real-time bidirectional communication over a single TCP connection.

37. What is HATEOAS in REST APIs?​

Hypermedia As The Engine Of Application State (HATEOAS) helps navigate APIs dynamically by providing links within responses.

38. What is API Error Handling?​

Ensuring proper error messages and status codes for failed requests (e.g., 400 Bad Request, 500 Internal Server Error).

39. What is a Multi-Part Form Data request?​

Used to send files, images, and binary data via API requests.

Example:

Content-Type: multipart/form-data; boundary=abc123

40. What is API Contract Testing?​

Validating API request/response structure using OpenAPI, Swagger, or Pact.

41. What is a Payload in API?​

The request body containing data sent to the API, usually in JSON format.

42. What is API Mocking?​

Simulating API responses for testing purposes using tools like WireMock, Mockoon.

43. What is an API Rate Limit Exceeded Error?​

Occurs when a client exceeds the maximum allowed requests within a time frame (429 Too Many Requests).

44. What is the difference between API and Web Service?​

All web services are APIs, but not all APIs are web services. APIs can be local (e.g., OS APIs) or web-based.

45. What is the difference between RESTful APIs and Microservices?​

  • RESTful APIs are web-based interfaces.
  • Microservices are independent modules communicating via APIs.

46. What is API Security Testing?​

Ensuring APIs are protected against threats like SQL Injection, CSRF, XSS, and Broken Authentication.

47. How do you test an API without UI?​

Using tools like Postman, Curl, Swagger UI, and automated scripts (Selenium, REST Assured).

48. What is API Logging?​

Recording API requests and responses for debugging and analytics.

49. What is API Deployment Testing?​

Validating API behavior in different environments (Staging, Production) before release.

50. What are the common challenges in API Testing?​

  • Handling authentication & authorization
  • Managing dynamic response data
  • Ensuring backward compatibility
  • Testing asynchronous APIs

QA Interview Preparation Resources​