Web API Testing Interview Questions

Introduction – Why Web API Testing Is Important in Interviews

Modern applications—web, mobile, cloud, and microservices—depend heavily on Web APIs for data exchange. Because UI layers change frequently, interviewers focus on web api testing interview questions to verify whether candidates can validate backend logic, integrations, security, and performance without relying on the UI.

In interviews, Web API testing questions evaluate:

• Understanding of RESTful architecture
  
• Knowledge of HTTP methods, status codes, headers
  
• Ability to validate business logic and data integrity
  
• Hands-on experience with Postman/SoapUI
  
• Awareness of automation using Java/Python
  
• Real-time problem-solving using scenario-based questions
  

This guide is interview-focused, SEO-optimised, and written for freshers to experienced testers, with clear explanations, examples, and practical snippets.

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What Is API Testing? (Clear & Simple)

API testing validates Application Programming Interfaces by checking:

• Request/response correctness
  
• Data accuracy and transformations
  
• Business rules and validations
  
• Error handling and security
  

API testing works at the service layer, making it faster, more stable, and more reliable than UI testing.

Simple Example

For a Create User Web API:

• Valid input → 201 Created
  
• Missing mandatory field → 400 Bad Request
  
• Duplicate email → 409 Conflict
  

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REST vs SOAP vs GraphQL (Web API Perspective)

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based	HTTP
Data Format	JSON / XML	XML only	JSON
Performance	Fast	Slower	Optimised
Contract	Optional (OpenAPI)	Mandatory (WSDL)	Schema
Usage	Most Web APIs	Enterprise/Banking	Modern microservices

👉 In web api testing interview questions, REST is the primary focus; SOAP knowledge helps in enterprise roles.

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Web API Testing Interview Questions & Answers (100+)

Section 1: Web API & REST Fundamentals (Q1–Q20)

1. What is a Web API?
   An interface that allows web applications to communicate over HTTP.
   
2. What is API testing?
   Testing APIs for functionality, data, security, and performance.
   
3. Why is Web API testing important?
   It validates backend logic independently of the UI.
   
4. What is REST?
   Representational State Transfer—an architectural style for APIs.
   
5. What are REST principles?
   Statelessness, client-server separation, cacheability, uniform interface.
   
6. What is an endpoint?
   A URL representing a resource/action.
   
7. What is a resource?
   An object/entity exposed by the API.
   
8. What is request payload?
   Data sent to the API.
   
9. What is response payload?
   Data returned by the API.
   
10. What is statelessness?
    Each request contains all necessary information.
    
11. What is idempotency?
    Multiple identical requests produce the same result.
    
12. Difference between PUT and PATCH?
    PUT updates full resource; PATCH updates partial fields.
    
13. What is authentication?
    Verifying identity.
    
14. What is authorization?
    Verifying access permissions.
    
15. Common auth methods?
    Bearer Token, API Key, OAuth, Basic Auth.
    
16. What is JWT?
    JSON Web Token for stateless authentication.
    
17. What is JSON?
    

{ “id”: 101, “name”: “Neha” }

18. What is XML?
    

<user><id>101</id><name>Neha</name></user>

19. What is positive testing?
    Testing with valid data.
    
20. What is negative testing?
    Testing with invalid data.
    

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HTTP Methods – Core Web API Knowledge

Method	Purpose
GET	Retrieve data
POST	Create data
PUT	Update full resource
PATCH	Update partial resource
DELETE	Remove resource

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HTTP Status Codes – Interview Essentials

Code	Meaning	Example
200	OK	Successful GET
201	Created	Successful POST
204	No Content	Successful DELETE
400	Bad Request	Invalid input
401	Unauthorized	Invalid token
403	Forbidden	No permission
404	Not Found	Missing resource
409	Conflict	Duplicate record
422	Validation error	Business rule failure
500	Server Error	Backend issue

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Section 2: Validation, Data & Tools (Q21–Q50)

21. What validations are done in Web API testing?
    Status code, body, headers, schema, response time.
    
22. Is validating status code enough?
    No, validate response data and business logic.
    
23. What is header validation?
    Checking headers like Authorization and Content-Type.
    
24. What is schema validation?
    Validating response structure.
    
25. What is response time testing?
    Measuring API performance.
    
26. What is API smoke testing?
    Basic API health check.
    
27. What is API regression testing?
    Re-testing after changes.
    
28. What is API chaining?
    Using one API’s response in another.
    
29. What is API mocking?
    Simulating API responses.
    
30. What is rate limiting?
    Restricting number of API calls.
    
31. What is concurrency testing?
    Testing simultaneous requests.
    
32. What is backend validation?
    Validating DB changes.
    
33. What is API security testing?
    Testing auth, authz, input validation.
    
34. What is environment testing?
    Testing across dev/QA/prod.
    
35. What is API contract testing?
    Validating client-server agreement.
    
36. What is data-driven API testing?
    Running tests with multiple datasets.
    
37. What is logging in API tests?
    Capturing request/response details.
    
38. What is CI/CD integration?
    Running API tests in pipelines.
    
39. What is assertion?
    Validation of expected outcome.
    
40. What tools are used for Web API testing?
    Postman, SoapUI, Rest Assured, Python requests.
    
41. What is Postman?
    A tool for manual API testing.
    
42. What is SoapUI?
    A tool for SOAP/REST testing.
    
43. What is Rest Assured?
    Java library for API automation.
    
44. What is Python requests?
    Python library for HTTP calls.
    
45. What is pagination testing?
    Validating page size and offsets.
    
46. What is filtering testing?
    Testing query parameters.
    
47. What is sorting testing?
    Testing order of results.
    
48. What is caching?
    Temporary storage of responses.
    
49. What is cache invalidation?
    Refreshing stale data.
    
50. Why API testing before UI testing?
    To catch backend issues early.
    

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Real-Time Web API Validation Example

Request

POST /api/login

{

  “username”: “webuser”,

  “password”: “pass123”

}

Response

{

  “token”: “abc123”,

  “expiresIn”: 3600

}

Validations

• Status code = 200
  
• Token is not null
  
• Token expiry > 0
  
• Token usable for secured APIs
  

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Automation Snippets (Common Interview Expectations)

Postman – Basic Test

pm.test(“Status code is 200”, function () {

  pm.response.to.have.status(200);

});

Rest Assured (Java)

given()

.when()

.get(“/users/1”)

.then()

.statusCode(200);

Python Requests

import requests

res = requests.get(url)

assert res.status_code == 200

SoapUI – XPath Assertion

//id != ”

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Scenario-Based Web API Testing Interview Questions (15)

1. API returns 200 but incorrect data – how do you detect?
   
2. Duplicate records created on retry – how to prevent?
   
3. Token expired but API still works – risk?
   
4. API returns 500 for client error – correct?
   
5. Same request returns different responses – why?
   
6. API slow for large datasets – what test?
   
7. Partial data saved after failure – how to test rollback?
   
8. API works in Postman but fails in app – reason?
   
9. Unauthorized user accesses data – issue?
   
10. Schema change breaks consumers – prevention?
    
11. Rate limiting not enforced – impact?
    
12. API fails only in production – possible causes?
    
13. Cache returns stale data – how detect?
    
14. Bulk API partially succeeds – how validate?
    
15. Backend updated but UI shows old data – issue?
    

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How Interviewers Evaluate Web API Testing Answers

Interviewers assess:

• Understanding of Web API fundamentals
  
• Ability to validate business logic
  
• Knowledge of tools and automation basics
  
• Real-world problem-solving
  
• Clear, structured communication
  

👉 Explaining the “why” with examples scores higher than memorisation.

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Web API Testing Interview Cheatsheet

• Master REST basics & HTTP status codes
  
• Validate response data, not just status
  
• Practice Postman daily
  
• Understand real-time scenarios
  
• Know basic automation concepts
  

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FAQs – Web API Testing Interview Questions

Q1. Is Web API testing mandatory for QA roles?
Yes, basic knowledge is expected.

Q2. Is Postman enough for interviews?
Yes, for manual roles; automation is a plus.

Q3. Do freshers need automation skills?
Basic awareness is sufficient.

Q4. Biggest mistake candidates make?
Only checking status codes.

Q5. How to prepare quickly?
Practice CRUD APIs and explain validations clearly.