SOAP and REST API Testing Interview Questions -

Introduction – Why API Testing Is Important in Interviews

In modern enterprise applications, SOAP and REST APIs are the backbone of system communication. Banking, insurance, telecom, healthcare, and e-commerce applications rely heavily on APIs to exchange data between systems.

Because of this, interviewers frequently ask soap and rest api testing interview questions to evaluate whether a candidate:

Understands both legacy SOAP and modern REST APIs
Can validate backend logic without relying on UI
Knows how to test XML and JSON responses
Can handle real-time API issues, failures, and edge cases
Has hands-on experience with tools like Postman, SoapUI, ReadyAPI

This article is a complete interview preparation guide with clear explanations, real-time examples, response samples, status codes, automation snippets, and scenario-based questions, suitable for freshers to experienced API testers.

What Is API Testing? (Clear & Simple)

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

Accept valid requests
Enforce business rules
Return correct responses and status codes
Handle errors and security properly

Simple Example

For a Money Transfer API:

Valid input → 200 OK / 201 Created
Invalid account → 400 Bad Request
Unauthorized user → 401 Unauthorized

API testing focuses on data and logic, not UI elements.

REST vs SOAP vs GraphQL (Interview Comparison)

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based	HTTP
Payload	JSON / XML	XML only	JSON
Contract	OpenAPI	WSDL	Schema
Performance	Fast	Slower	Optimized
Common Usage	Modern apps	Banking / legacy	Modern microservices

👉 Most soap and rest api testing interview questions focus on REST, but SOAP knowledge is mandatory for enterprise projects.

SOAP and REST API Testing Interview Questions & Answers (90+)

Section 1: API Basics (Q1–Q20)

1. What is an API?

An API allows two software systems to communicate with each other.

2. What is API testing?

API testing validates requests, responses, headers, status codes, and business logic.

3. Why is API testing important?

Because APIs are used by multiple applications, one defect can impact the entire system.

4. Difference between API testing and UI testing?

API testing validates backend logic, UI testing validates frontend behavior.

5. What types of APIs have you tested?

REST APIs and SOAP APIs.

6. What are HTTP methods?

GET, POST, PUT, PATCH, DELETE.

7. What is GET request?

Used to retrieve data from the server.

8. What is POST request?

Used to create new resources.

9. Difference between PUT and PATCH?

PUT → Full update
PATCH → Partial update

10. What is DELETE request?

Used to remove a resource.

11. What is an endpoint?

A URL representing an API resource, e.g. /customers/101.

12. What is request payload?

Data sent to API in request body.

13. What is response body?

Data returned by the API.

14. What is stateless API?

Each request is independent.

15. What is idempotency?

Multiple identical requests produce the same result.

16. What is authentication?

Verifying identity using token or credentials.

17. What is authorization?

Verifying user permissions.

18. What authentication methods have you used?

Basic Auth, Bearer Token, OAuth.

19. What is API versioning?

Managing API changes using /v1, /v2.

20. What is negative API testing?

Testing API behavior with invalid input.

REST vs SOAP – Interview-Focused Differences

Aspect	REST	SOAP
Data format	JSON/XML	XML only
Protocol	HTTP	HTTP/SMTP
Security	OAuth, JWT	WS-Security
Performance	Faster	Slower
Contract	Optional	Mandatory WSDL

HTTP Status Codes – Must Know

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	Access denied
404	Not Found	Invalid endpoint
409	Conflict	Duplicate record
422	Unprocessable	Business rule failure
500	Server Error	Backend crash

Section 2: REST API Testing Interview Questions (Q21–Q45)

21. What validations do you perform in REST API testing?

Status code
Response body
Headers
Schema
Response time

22. Is status code validation enough?

No. Business rules and response data must be validated.

23. What is JSON?

Lightweight data-interchange format.

{

“id”: 101,

“name”: “Ravi”,

“balance”: 5000

}

24. What is schema validation?

Ensuring response structure matches API contract.

25. What is pagination testing?

Validating page-wise responses.

26. What is filtering testing?

Validating query parameters.

27. What is sorting testing?

Validating response order.

28. What is API rate limiting?

Restricting number of requests.

29. What is API regression testing?

Re-testing APIs after changes.

30. What is API smoke testing?

Basic health check.

31. What is API security testing?

Testing authentication and authorization.

32. What is API performance testing?

Testing response time and throughput.

33. What is API mocking?

Simulating API responses.

34. What is API caching?

Storing responses temporarily.

35. What is content-type validation?

Ensuring response is JSON/XML.

36. What is header validation?

Validating Authorization, Content-Type headers.

37. What is API rollback?

Reverting data on failure.

38. What is API concurrency testing?

Testing multiple requests simultaneously.

39. What is data consistency testing?

Ensuring same data across systems.

40. What is contract testing?

Validating provider-consumer agreement.

41. What is API monitoring?

Tracking uptime and failures.

42. What is throttling?

Limiting traffic to protect backend.

43. What is API chaining?

Using one API’s response in another.

44. What is negative testing?

Testing invalid scenarios.

45. What is boundary value testing?

Testing min and max values.

Section 3: SOAP API Testing Interview Questions (Q46–Q70)

46. What is SOAP?

Simple Object Access Protocol, an XML-based messaging protocol.

47. What is WSDL?

Web Services Description Language that defines SOAP service contract.

48. What is SOAP envelope?

Root element of SOAP message.

49. What are SOAP headers?

Carry security and metadata information.

50. What is SOAP body?

Contains actual request/response data.

51. What is SOAP fault?

Error response in SOAP.

52. What is WS-Security?

Security standard for SOAP APIs.

53. What tools are used for SOAP testing?

SoapUI, ReadyAPI.

54. How do you validate SOAP responses?

Using XPath and schema assertions.

55. What is XML?

Markup language used in SOAP.

</account>

56. What is schema validation in SOAP?

Validating XML against XSD.

57. What is SOAP authentication?

Using certificates, tokens, or headers.

58. Difference between SOAP fault and HTTP error?

SOAP fault is application-level error; HTTP error is protocol-level.

59. What is SOAP version?

SOAP 1.1 and SOAP 1.2.

60. What is SOAP action?

Defines intent of SOAP request.

61. What is document vs RPC style?

Different SOAP message formats.

62. What is message-level security?

Security applied to SOAP message itself.

63. What is SOAP performance testing?

Testing response time and load.

64. What is SOAP regression testing?

Re-testing after changes.

65. What is SOAP data-driven testing?

Running tests with multiple XML inputs.

66. What is SOAP mocking?

Simulating SOAP services.

67. What is SOAP endpoint?

URL where SOAP service is hosted.

68. What is SOAP binding?

Defines protocol and format.

69. What is SOAP transport?

Usually HTTP/HTTPS.

70. What is SOAP logging?

Capturing request and response XML.

Real-Time API Validation Examples

REST API Example

POST /api/login

{

“username”: “user1”,

“password”: “pass123”

}

Validations:

Status = 200
Token exists
Token expiry > 0

SOAP API Example

<soapenv:Envelope>

<soapenv:Body>

</getBalance>

</soapenv:Body>

</soapenv:Envelope>

Validations:

No SOAP fault
Balance node exists
Balance > 0

Postman / SoapUI / Automation Snippets

Postman Test Script

pm.test(“Status code 200”, () => {

pm.response.to.have.status(200);

});

SoapUI XPath Assertion

//balance > 0

Rest Assured (Java)

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

Python Requests

import requests

res = requests.get(url)

assert res.status_code == 200

Scenario-Based SOAP and REST API Testing Questions (15)

REST API returns 200 but wrong data – what do you check?
SOAP service returns fault – how do you debug?
Token expired but API still accessible – defect?
API works in Postman but fails in app – why?
Duplicate records created – what validation missed?
SOAP response missing mandatory tag – impact?
REST API slow only in production – reason?
Same request returns different responses – why?
Unauthorized user accesses secured API – issue?
XML schema changes – what breaks?
API returns null fields – how to catch?
Payment deducted but order not created – what testing?
SOAP security header missing – impact?
API fails only in CI pipeline – reason?
Partial data saved after failure – what testing missed?

How Interviewers Evaluate Your Answers

Interviewers look for:

Clear understanding of SOAP and REST
Real-time project examples
Validation beyond status codes
Logical debugging skills
Tool knowledge (Postman + SoapUI)

👉 Explain what you test and why.

SOAP & REST API Testing Interview Cheatsheet

Validate status + data + rules
Understand JSON and XML
Know SOAP faults & REST errors
Test negative scenarios
Be ready with real examples
Practice Postman & SoapUI

FAQs – SOAP and REST API Testing Interview Questions

Q1. Is SOAP still relevant?
Yes, especially in banking and enterprise systems.

Q2. REST or SOAP – which is more important?
REST is more common, but SOAP knowledge is a big advantage.

Q3. Do interviewers expect automation?
Basic automation knowledge is a plus.

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

Q5. How to prepare quickly?
Practice both REST and SOAP APIs with tools.