Introduction – Why API Testing Is Important in Interviews
In today’s applications, APIs connect everything—frontend to backend, mobile apps to servers, and systems to third-party services. Because APIs work behind the scenes, interviewers rely heavily on api testing basic interview questions to judge whether a candidate truly understands how applications function internally.
For freshers and early-career testers, interviewers mainly look for:
- Strong fundamentals of API testing
- Understanding of REST and basic SOAP concepts
- Ability to validate responses, not just status codes
- Awareness of real-time issues like invalid data and errors
This article is designed as a complete beginner-to-intermediate guide, covering basic concepts, practical interview questions with answers, examples, tools, and scenarios—all written in simple, interview-focused language.
What Is API Testing? (Clear & Simple)
API testing is the process of testing Application Programming Interfaces to ensure they:
- Work as expected
- Return correct responses
- Follow business rules
- Handle errors properly
Unlike UI testing, API testing does not involve screens or buttons. It focuses only on requests and responses.
Simple Example
For a Login API:
- Valid username & password → 200 OK
- Invalid password → 401 Unauthorized
- Missing username → 400 Bad Request
REST vs SOAP vs GraphQL (Basic Interview Comparison)
| Feature | REST | SOAP | GraphQL |
| Protocol | HTTP | XML-based | HTTP |
| Data Format | JSON / XML | XML only | JSON |
| Performance | Fast | Slower | Optimized |
| Learning Curve | Easy | Moderate | Moderate |
| Usage | Most modern apps | Banking/legacy | Modern apps |
👉 In api testing basic interview questions, REST is asked most often, while SOAP basics are useful for enterprise roles.
API Testing Basic Interview Questions & Answers (80+)
Section 1: API Fundamentals (Q1–Q20)
1. What is an API?
An API allows two software applications to communicate with each other.
2. What is API testing?
API testing verifies requests, responses, headers, status codes, and business logic of APIs.
3. Why is API testing important?
Because APIs are the backbone of applications and errors affect multiple systems.
4. API testing vs UI testing?
API testing checks backend logic; UI testing checks frontend behavior.
5. What types of APIs are commonly tested?
REST and SOAP APIs.
6. What is REST?
Representational State Transfer, a lightweight API architecture using HTTP.
7. What is SOAP?
Simple Object Access Protocol, an XML-based messaging protocol.
8. What is an endpoint?
A URL that represents an API resource, e.g. /users/101.
9. What is request payload?
Data sent to the API in the request body.
10. What is response payload?
Data returned by the API.
11. What is statelessness?
Each API request is independent.
12. What is idempotency?
Repeating the same request gives the same result.
13. What is API versioning?
Managing API changes using /v1, /v2.
14. What is authentication?
Verifying user identity.
15. What is authorization?
Verifying user permissions.
16. What authentication methods are common?
Basic Auth, Bearer Token, API key.
17. What is JSON?
Lightweight data format used in REST APIs.
{
“id”: 101,
“name”: “Amit”
}
18. What is XML?
Markup language used in SOAP APIs.
<user>
<id>101</id>
<name>Amit</name>
</user>
19. What is negative testing?
Testing APIs with invalid input.
20. What is positive testing?
Testing APIs with valid input.
HTTP Methods – Basic Interview Questions
| Method | Purpose |
| GET | Retrieve data |
| POST | Create data |
| PUT | Update full data |
| PATCH | Update partial data |
| DELETE | Remove data |
HTTP Status Codes – Must Know for Beginners
| Code | Meaning | Example |
| 200 | OK | Successful GET |
| 201 | Created | Successful POST |
| 204 | No Content | Successful DELETE |
| 400 | Bad Request | Invalid input |
| 401 | Unauthorized | Invalid credentials |
| 403 | Forbidden | No access |
| 404 | Not Found | Invalid endpoint |
| 500 | Server Error | Backend issue |
Section 2: API Validation & Testing Types (Q21–Q45)
21. What validations are done in API testing?
Status code, response body, headers, and response time.
22. Is status code validation enough?
No, response data and business logic must be validated.
23. What is header validation?
Checking headers like Content-Type and Authorization.
24. What is schema validation?
Validating response structure.
25. What is response time testing?
Checking how fast API responds.
26. What is regression testing?
Re-testing APIs after changes.
27. What is smoke testing?
Basic API health check.
28. What is API security testing?
Testing authentication and authorization.
29. What is API performance testing?
Testing speed and scalability.
30. What is pagination testing?
Testing page-wise data responses.
31. What is filtering testing?
Testing query parameters.
32. What is sorting testing?
Testing ordered responses.
33. What is API chaining?
Using response of one API in another API.
34. What is API mocking?
Simulating API responses.
35. What is rate limiting?
Restricting number of API calls.
36. What is throttling?
Controlling API traffic.
37. What is data consistency testing?
Ensuring same data across systems.
38. What is API rollback testing?
Ensuring no partial data saved on failure.
39. What is content-type validation?
Ensuring JSON/XML format.
40. What is boundary value testing?
Testing minimum and maximum values.
41. What is concurrency testing?
Testing multiple users simultaneously.
42. What is API caching?
Storing responses temporarily.
43. What is API contract?
Agreement between client and server.
44. What is API documentation?
Defines how API works and how to use it.
45. What is OpenAPI/Swagger?
API documentation and specification tool.
Real-Time API Validation Example
Request
POST /api/login
{
“username”: “user1”,
“password”: “pass123”
}
Response
{
“token”: “abc123”,
“expiresIn”: 3600
}
Validations
- Status code = 200
- Token is not null
- expiresIn > 0
Postman / SoapUI / Automation Basics
Postman Test Script
pm.test(“Status code is 200”, function () {
pm.response.to.have.status(200);
});
SoapUI XPath Assertion
//token != ”
Rest Assured (Java – Basic)
given().when().get(“/users/1”).then().statusCode(200);
Python Requests (Basic)
import requests
res = requests.get(url)
assert res.status_code == 200
Scenario-Based API Testing Basic Interview Questions (12)
- API returns 200 but wrong data—what do you check?
- API works in Postman but fails in application—why?
- Missing parameter returns 500—is it correct?
- API allows access without token—what issue?
- Duplicate record created—what testing missed?
- API slow for large data—what test?
- API returns null values—how validate?
- Invalid input accepted—what defect?
- API returns wrong status code—impact?
- Same request gives different response—why?
- Unauthorized user accesses data—issue?
- API fails only in production—possible reasons?
How Interviewers Evaluate Your Answer
Interviewers look for:
- Clear understanding of basics
- Logical explanation, not memorization
- Ability to explain real examples
- Awareness of negative scenarios
- Confidence and clarity
👉 Explaining “why” matters more than tools.
API Testing Basic Interview Cheatsheet
- Understand REST fundamentals
- Know HTTP methods & status codes
- Validate response data, not just status
- Practice Postman basics
- Think about negative scenarios
- Keep explanations simple
FAQs – API Testing Basic Interview Questions
Q1. Is API testing mandatory for freshers?
Yes, basic API knowledge is expected.
Q2. Is Postman enough for beginners?
Yes, Postman is perfect for learning basics.
Q3. Do I need automation knowledge?
Not mandatory, but basic awareness helps.
Q4. Biggest mistake beginners make?
Only checking status codes.
Q5. How to prepare quickly?
Practice simple REST APIs daily.