Web Services API Testing Interview Questions

Introduction – Why API Testing Is Important in Interviews

Modern applications rely heavily on web services to connect mobile apps, web frontends, databases, and third-party systems. Because of this, interviewers increasingly focus on web services API testing interview questions to assess whether a candidate understands backend validation, data flow, security, and automation.

Unlike UI testing, API testing validates the core business logic of an application. Interviewers want testers who can:

• Detect issues early
  
• Validate responses without UI dependency
  
• Test integrations and microservices
  
• Automate efficiently
  

This article is designed for freshers, mid-level, and experienced QA/API testers, with real-time examples, sample responses, status codes, automation snippets, and scenario-based REST API testing questions.

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What Is API Testing? (Simple & Clear)

API testing is the process of validating web services directly by sending requests and checking responses for correctness, performance, security, and reliability, without using the UI.

In short:

API testing ensures the system works correctly at the data and service layer.

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REST vs SOAP vs GraphQL

Feature	REST	SOAP	GraphQL
Type	Architectural style	Protocol	Query language
Data format	JSON (mostly)	XML only	JSON
Performance	Lightweight & fast	Heavy	Optimized
Security	OAuth, JWT	WS-Security	Token based
Usage	Most modern apps	Legacy/Enterprise	New-gen APIs

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60+ Web Services API Testing Interview Questions and Answers

Basic API Testing Questions (Freshers)

1. What are web services?
   Web services are software systems that communicate over a network using standard protocols like HTTP.
   
2. What is API?
   API (Application Programming Interface) allows two applications to communicate with each other.
   
3. What is web services API testing?
   It is the validation of APIs to ensure correct request handling and accurate responses.
   
4. Why API testing is preferred over UI testing?
   Faster execution, stable tests, and early defect detection.
   
5. What protocols are used in web services?
   HTTP/HTTPS, SOAP, REST.
   
6. What is an endpoint?
   A URL where a web service accepts requests.
   
7. What is payload in API testing?
   The request or response body sent between client and server.
   
8. What are HTTP headers?
   Metadata used for authentication, content type, caching, etc.
   
9. What is statelessness in REST?
   Each request is independent; server doesn’t store client state.
   
10. What is idempotent API?
    Repeated calls produce the same result (GET, PUT).
    

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REST API Interview Questions

11. What HTTP methods are commonly used?
    GET, POST, PUT, PATCH, DELETE.
    
12. Difference between POST and PUT?
    POST creates resources; PUT updates/replaces them.
    
13. What is JSON?
    Lightweight data format used in REST APIs.
    

{

  “userId”: 101,

  “name”: “Srushti”,

  “role”: “Tester”

}

14. What is URI vs URL?
    URI identifies resources; URL specifies location.
    
15. What is API versioning?
    Managing changes using /v1, /v2, headers, etc.
    
16. What is query parameter?
    Data passed in URL after ?.
    
17. What is path parameter?
    Variable part of endpoint URL.
    
18. What is pagination?
    Dividing response data into pages.
    
19. How do you test sorting and filtering?
    Validate response order and filtered results.
    
20. What is caching in REST?
    Storing responses to reduce server load.
    

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SOAP Web Services Questions

21. What is SOAP?
    Simple Object Access Protocol for XML-based messaging.
    
22. What is WSDL?
    XML file describing SOAP services.
    
23. What is SOAP envelope?
    Root XML element containing header and body.
    
24. SOAP vs REST – which is better?
    REST is lightweight; SOAP is more secure and structured.
    
25. How do you test SOAP APIs?
    Validate XML response using XPath.
    

<response>

  <status>SUCCESS</status>

</response>

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Status Codes – Interview Must-Knows

Code	Meaning
200	OK
201	Created
204	No Content
400	Bad Request
401	Unauthorized
403	Forbidden
404	Not Found
409	Conflict
500	Internal Server Error

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API Validation Example (Real-Time)

Request

GET /api/users/101

Response

{

  “id”: 101,

  “name”: “Srushti”,

  “email”: “srushti@test.com”

}

Validations

• Status code = 200
  
• id exists
  
• Email format valid
  

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Automation Tools & Code Snippets

Postman (Manual Validation)

• Validate status codes
  
• Check response body
  
• Use pre-request scripts
  

SoapUI Assertions

• JSONPath Match
  
• XPath Match
  
• Schema Compliance
  

RestAssured (Java)

given()

.when()

.get(“/users/101”)

.then()

.statusCode(200);

Python Requests

import requests

res = requests.get(“https://api.test.com/users/101”)

assert res.status_code == 200

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Scenario-Based REST API Testing Questions (10+)

31. API returns 200 but wrong data – what do you do?
    Validate business rules and log a defect.
    
32. How do you test token expiration?
    Wait until expiry and validate 401 response.
    
33. How to test rate limiting?
    Send rapid requests and expect 429.
    
34. What if API is slow?
    Validate response time and SLA.
    
35. How to test file upload API?
    Validate size, format, and response message.
    
36. How do you test dependent APIs?
    Chain requests and validate output.
    
37. What if backend is unavailable?
    Use mock services.
    
38. How do you test concurrency?
    Send parallel requests and check data integrity.
    
39. How do you test error handling?
    Invalid payloads and missing headers.
    
40. How to test backward compatibility?
    Validate older API versions still work.
    

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How Interviewers Evaluate Your Answers

Interviewers focus on:

• Understanding of API basics
  
• Knowledge of HTTP methods & status codes
  
• Ability to explain real-time scenarios
  
• Hands-on automation experience
  
• Clear and logical communication
  

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Web Services API Testing Cheat Sheet

• Always validate status + response body
  
• Cover positive and negative cases
  
• Use assertions
  
• Handle authentication tokens
  
• Test edge cases and boundaries
  
• Log clear defects with request/response
  

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FAQs – Web Services API Testing

Q1. Are web services API testing interview questions hard?
No, if you understand basics and practice scenarios.

Q2. Do freshers need automation knowledge?
Basics help, but fundamentals matter more.

Q3. Which tools should I know?
Postman, SoapUI, RestAssured (basic).

Q4. Are REST API interview questions common?
Yes, REST is the most widely used web service.