API Web Services Testing Interview Questions

Introduction – Why API Web Services Testing Is Important in Interviews

In modern software systems, web services are the backbone of application communication. Web applications, mobile apps, cloud platforms, and third-party systems all interact through API web services.

Because of this, interviewers frequently ask api web services testing interview questions to evaluate whether a candidate:

• Understands how web services work behind the UI
  
• Can test REST and SOAP APIs
  
• Knows how to validate data, business logic, and errors
  
• Can think through real-time integration issues
  
• Has hands-on experience with tools like Postman, SoapUI, Rest Assured
  

This article is a complete interview preparation guide for freshers to experienced QA/API testers, written in simple, technical, interview-focused language with examples, samples, and scenario-based questions.

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What Is API Testing? (Clear & Simple)

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

• Accept correct requests
  
• Return expected responses
  
• Follow business rules
  
• Handle errors, security, and edge cases properly
  

Unlike UI testing, API testing:

• Does not involve screens or buttons
  
• Works directly with requests and responses
  
• Finds bugs early and faster
  

Simple Example

For a User Login Web Service:

• Valid credentials → 200 OK + token
  
• Invalid password → 401 Unauthorized
  
• Missing username → 400 Bad Request
  

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REST vs SOAP vs GraphQL (Web Services Perspective)

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

👉 In api web services testing interview questions, REST is asked most often, but SOAP knowledge is very important for enterprise projects.

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API Web Services Testing Interview Questions & Answers (100+)

Section 1: Web Services & API Basics (Q1–Q20)

1. What are web services?
   Web services allow applications to communicate over a network using standard protocols.
   
2. What is API web services testing?
   Testing web service APIs to validate requests, responses, status codes, and business logic.
   
3. Difference between API and web service?
   All web services are APIs, but not all APIs are web services.
   
4. Types of web services?
   REST and SOAP.
   
5. What is REST?
   An architectural style using HTTP methods for communication.
   
6. What is SOAP?
   A protocol that uses XML for structured message exchange.
   
7. What is an endpoint?
   A URL representing a web service resource.
   
8. What is request payload?
   Data sent to the web service.
   
9. What is response payload?
   Data returned by the web service.
   
10. What is statelessness in REST?
    Each request is independent and self-contained.
    
11. What is idempotency?
    Repeating the same request gives the same result.
    
12. What is authentication?
    Verifying user or system identity.
    
13. What is authorization?
    Verifying access permissions.
    
14. Common authentication methods?
    Basic Auth, Bearer Token, API Key, OAuth.
    
15. What is API versioning?
    Managing API changes using versions like /v1, /v2.
    
16. What is JSON?
    

{ “id”: 101, “name”: “Kiran” }

17. What is XML?
    

<user><id>101</id><name>Kiran</name></user>

18. What is positive testing?
    Testing with valid input.
    
19. What is negative testing?
    Testing with invalid or unexpected input.
    
20. What is API documentation?
    Guidelines on how to use a web service.
    

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HTTP Methods – Core Web Services Knowledge

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

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HTTP Status Codes – Must-Know for Web Services Testing

Code	Meaning	Usage
200	OK	Successful request
201	Created	Resource created
204	No Content	Successful delete
400	Bad Request	Invalid input
401	Unauthorized	Invalid credentials
403	Forbidden	Access denied
404	Not Found	Invalid endpoint
409	Conflict	Duplicate data
500	Server Error	Backend failure

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Section 2: API Web Services Validation Questions (Q21–Q45)

21. What validations are done in API web services testing?
    Status code, response body, headers, schema, and response time.
    
22. Is validating status code enough?
    No, response data and business rules must also be validated.
    
23. What is header validation?
    Checking headers like Authorization and Content-Type.
    
24. What is schema validation?
    Validating response structure against schema.
    
25. What is response time testing?
    Checking how fast the web service responds.
    
26. What is smoke testing?
    Basic health check of web services.
    
27. What is regression testing?
    Re-testing APIs after changes.
    
28. What is API security testing?
    Testing authentication and authorization.
    
29. What is API performance testing?
    Testing speed, load, 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 web service in another.
    
34. What is API mocking?
    Simulating web service responses.
    
35. What is rate limiting?
    Restricting number of API calls.
    
36. What is throttling?
    Controlling traffic to protect backend.
    
37. What is boundary value testing?
    Testing minimum and maximum values.
    
38. What is data consistency testing?
    Ensuring same data across systems.
    
39. What is API rollback testing?
    Ensuring no partial data saved on failure.
    
40. What is content-type validation?
    Ensuring response format is JSON/XML.
    
41. What is Swagger/OpenAPI?
    API documentation and testing tool.
    
42. What is API contract testing?
    Validating agreement between client and server.
    
43. What is concurrency testing?
    Testing multiple users simultaneously.
    
44. What is API caching?
    Storing responses temporarily.
    
45. What is environment testing?
    Testing APIs across dev, QA, and prod.
    

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Real-Time API Web Services Validation Example

Request

POST /api/login

{

  “username”: “testuser”,

  “password”: “pass123”

}

Response

{

  “token”: “abc123”,

  “expiresIn”: 3600

}

Validations

• Status code = 200
  
• 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 API Web Services Testing Interview Questions (15)

1. API returns 200 but wrong data – what do you validate?
   
2. Web service works in Postman but fails in UI – why?
   
3. Missing parameter returns 500 – is it correct?
   
4. API allows access without authentication – what issue?
   
5. Duplicate records created – what testing missed?
   
6. API slow for large data – what test to perform?
   
7. API returns null values – how validate?
   
8. Invalid input accepted – what defect?
   
9. API returns wrong status code – impact?
   
10. Same request returns different responses – why?
    
11. Unauthorized user accesses another user’s data – issue?
    
12. API fails only in production – possible reasons?
    
13. SOAP service returns fault – how debug?
    
14. Partial data saved after failure – what testing needed?
    
15. Rate limiting not working – what is the risk?
    

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How Interviewers Evaluate Your Answer

Interviewers look for:

• Strong understanding of web services fundamentals
  
• Logical explanation of what and why
  
• Awareness of negative and edge cases
  
• Ability to relate answers to real projects
  
• Clear communication, not memorization
  

👉 Clarity and reasoning matter more than tool names.

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API Web Services Testing Interview Cheatsheet

• Understand REST & SOAP basics
  
• Know HTTP methods & status codes
  
• Validate response data, not just status
  
• Practice Postman regularly
  
• Think about security and negative cases
  
• Explain answers with simple examples
  

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FAQs – API Web Services Testing Interview Questions

Q1. Are web services testing questions asked for freshers?
Yes, basic REST API knowledge is expected.

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

Q3. Is SOAP still relevant?
Yes, especially in banking and enterprise systems.

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

Q5. How to prepare quickly?
Practice simple REST APIs daily and review status codes.