Interview Questions on REST API Testing

Introduction – Why REST API Testing Is Important in Interviews

In modern applications, REST APIs are the backbone of communication between frontend, backend, mobile apps, and third-party systems. Because UI can change frequently, interviewers rely on interview questions on REST API testing to evaluate whether a candidate truly understands backend logic and system integration.

During interviews, REST API testing questions help assess:

• Your understanding of client–server architecture
  
• Ability to validate business logic without UI
  
• Knowledge of HTTP methods, status codes, and payloads
  
• Hands-on experience with Postman, SoapUI, Rest Assured, Python
  
• Real-time problem-solving skills using scenario-based questions
  

This article is written for freshers to experienced professionals, using simple explanations, technical clarity, and real interview-level examples.

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What Is API Testing? (Clear & Simple)

API testing is the process of validating Application Programming Interfaces to ensure they:

• Accept correct requests
  
• Return correct responses
  
• Follow business rules
  
• Handle invalid input and errors properly
  

REST API testing focuses on requests, responses, headers, status codes, and data, not UI elements.

Simple Example

For a Create User REST API:

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

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REST vs SOAP vs GraphQL (Interview Comparison)

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based	HTTP
Data Format	JSON / XML	XML only	JSON
Performance	Fast	Slower	Optimized
Contract	Optional	Mandatory (WSDL)	Schema
Usage	Most modern apps	Banking / legacy	Modern microservices

👉 In interview questions on REST API testing, REST concepts are asked most frequently.

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Interview Questions on REST API Testing (100+ with Answers)

Section 1: REST API Fundamentals (Q1–Q20)

1. What is a REST API?
   A REST API is an interface that follows REST principles and uses HTTP methods for communication.
   
2. What does REST stand for?
   Representational State Transfer.
   
3. What are the core REST principles?
   Statelessness, client-server, cacheability, uniform interface.
   
4. What is statelessness in REST?
   Each request contains all required information.
   
5. What is a REST resource?
   An object or data represented by a URL.
   
6. What is an endpoint?
   A URL that exposes a REST resource.
   
7. What is request payload?
   Data sent to the API.
   
8. What is response payload?
   Data returned by the API.
   
9. What is idempotency?
   Same request produces the same result.
   
10. What is REST API versioning?
    Managing changes using /v1, /v2.
    
11. What authentication types are used in REST APIs?
    Bearer Token, API Key, OAuth, Basic Auth.
    
12. What is JWT?
    JSON Web Token for stateless authentication.
    
13. What is JSON?
    

{ “id”: 101, “name”: “Ravi” }

14. What is XML?
    

<user><id>101</id><name>Ravi</name></user>

15. Difference between PUT and PATCH?
    PUT updates full resource; PATCH updates partial.
    
16. What is API documentation?
    Guidelines on how to use the API.
    
17. What is Swagger/OpenAPI?
    Tool for API documentation and testing.
    
18. What is positive testing?
    Testing with valid input.
    
19. What is negative testing?
    Testing with invalid input.
    
20. What is REST API testing?
    Testing REST endpoints for functionality, data, and errors.
    

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HTTP Methods – Core REST Knowledge

Method	Purpose
GET	Retrieve data
POST	Create data
PUT	Update entire resource
PATCH	Update part of resource
DELETE	Remove resource

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HTTP Status Codes – Must-Know for REST API Testing

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 access
404	Not Found	Resource missing
409	Conflict	Duplicate data
422	Validation error	Business rule failure
500	Server Error	Backend failure

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Section 2: REST API Validation Questions (Q21–Q45)

21. What validations are done in REST API testing?
    Status code, response body, headers, schema, response time.
    
22. Is validating status code enough?
    No, data and business logic must also be validated.
    
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?
    Checking how fast API responds.
    
26. What is smoke testing?
    Basic API health check.
    
27. What is regression testing?
    Re-testing APIs after changes.
    
28. What is API security testing?
    Testing authentication and authorization.
    
29. What is pagination testing?
    Testing page-wise data.
    
30. What is filtering testing?
    Testing query parameters.
    
31. What is sorting testing?
    Testing order of response data.
    
32. What is API chaining?
    Using response of one API in another.
    
33. What is API mocking?
    Simulating API responses.
    
34. What is rate limiting?
    Restricting number of API calls.
    
35. What is throttling?
    Controlling API traffic.
    
36. What is boundary value testing?
    Testing minimum and maximum values.
    
37. What is data consistency testing?
    Ensuring same data across systems.
    
38. What is rollback testing?
    Ensuring no partial data saved on failure.
    
39. What is content-type validation?
    Ensuring JSON/XML format.
    
40. What is concurrency testing?
    Testing multiple users simultaneously.
    
41. What is caching in REST APIs?
    Storing responses temporarily.
    
42. What is API contract testing?
    Validating client-server agreement.
    
43. What is environment testing?
    Testing across dev, QA, prod.
    
44. What is error handling testing?
    Validating proper error messages.
    
45. What is REST API performance testing?
    Testing speed and scalability.
    

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Real-Time REST API Validation Example

Request

POST /api/login

{

  “username”: “testuser”,

  “password”: “pass123”

}

Response

{

  “token”: “abc123”,

  “expiresIn”: 3600

}

Validations

• Status code = 200 OK
  
• Token is not null
  
• expiresIn > 0
  

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Postman / SoapUI / Automation Snippets

Postman – Basic Test Script

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

  pm.response.to.have.status(200);

});

SoapUI – XPath Assertion

//token != ”

Rest Assured (Java)

given().when().get(“/users/1”).then().statusCode(200);

Python Requests

import requests

res = requests.get(url)

assert res.status_code == 200

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Scenario-Based REST API Testing Interview Questions (15)

1. API returns 200 but wrong data – what do you check?
   
2. Missing parameter returns 500 – is it correct?
   
3. REST API allows access without authentication – issue?
   
4. Duplicate records created – what testing missed?
   
5. API slow for large data – what test needed?
   
6. Same request returns different responses – why?
   
7. Invalid input accepted – defect?
   
8. API returns wrong status code – impact?
   
9. Unauthorized user accesses data – issue?
   
10. API works in Postman but fails in UI – reason?
    
11. Partial data saved after failure – what test?
    
12. API schema changes – what breaks?
    
13. Pagination returns duplicate records – why?
    
14. Rate limiting not working – risk?
    
15. API fails only in production – possible causes?
    

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How Interviewers Evaluate Your REST API Testing Answers

Interviewers focus on:

• Understanding of REST fundamentals
  
• Ability to validate business logic
  
• Logical thinking for real-time scenarios
  
• Awareness of negative and edge cases
  
• Clear explanation, not memorization
  

👉 Explaining why you test something matters more than tool names.

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REST API Testing Interview Cheatsheet

• Understand REST principles
  
• Know HTTP methods & status codes
  
• Validate response data, not just status
  
• Practice Postman daily
  
• Think about negative and edge cases
  
• Explain answers with real examples
  

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FAQs – Interview Questions on REST API Testing

Q1. Is REST API testing mandatory for freshers?
Yes, basic REST API knowledge is expected.

Q2. Is Postman enough to prepare?
Yes, Postman is sufficient for fundamentals.

Q3. Is automation required?
Not mandatory for freshers; awareness is a plus.

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

Q5. How to prepare quickly?
Practice CRUD REST APIs daily.