Let’s clear API Testing Interview

Introduction:

API Testing Interview is one of the pillar in preparation of Testing interviews. Testing of API Testing is one of the most critical skills for QA engineers in today’s software industry. Modern applications rely heavily on microservices, where APIs act as the backbone of communication between systems. As a result, interviewers expect candidates to have a strong understanding of API concepts, HTTP protocols, authentication mechanisms, and automation tools like REST Assured.

Preparing for an API Testing Interview requires both theoretical knowledge and hands-on experience. In a real API Testing Interview, interviewers expect you to explain concepts, write REST Assured code, and handle real-world scenarios.

In this comprehensive guide on API Testing Interview Questions, we cover the most commonly asked questions along with detailed explanations, practical examples, and real-world testing scenarios. This guide is designed not just to help you answer interview questions but to think like a tester, debug like an engineer, and perform like a professional.

📚 Table of Contents

1. Basic API Testing Concepts
2. HTTP Methods & Status Codes
3. REST Assured Interview Questions (with Code)
4. API Authentication & Security
5. Advanced API Testing Concepts
6. Real-World Scenarios
7. Best Practices & Interview Tips

With hands-on experience in API automation and real-world QA projects, this guide is designed from a practical interview perspective. As a QA professional working extensively with REST APIs, automation frameworks, and tools like REST Assured, the focus here is not just on theory but on solving real interview scenarios.

In most API Testing Interview rounds, candidates struggle not because of lack of knowledge, but due to lack of clarity in explaining concepts and applying them practically. This guide bridges that gap by combining concepts, code, and real-world examples to help you confidently crack your next interview.

Related API Testing Interview Guides

To master this API Testing Interview, explore these detailed guides:

• Advanced REST Assured Questions: https://qatribe.in/advanced-rest-assured-interview-questions
• REST Assured Beginner Questions: https://qatribe.in/rest-assured-interview-questions
• Advanced API Testing Questions: https://qatribe.in/advanced-api-testing-interview-questions-answers
• API Testing Concepts Guide: https://qatribe.in/top-api-testing-interview-questions
• API Authentication Guide:https://qatribhttps://qatribe.in/api-key-explained

SECTION 1: BASIC API TESTING INTERVIEW QUESTIONS

1. What is API?

An API (Application Programming Interface) is a set of rules and protocols that allows two software systems to communicate with each other. It acts as an intermediary between a client (Frontend) and a server (Backend).

Example:

When you open an app and fetch user details:

• The frontend sends a request to an API
• The API processes the request
• The backend sends a response back to the frontend

👉 APIs eliminate the need for direct database interaction and ensure secure, structured communication.

2. What is API Testing?

API Testing is a type of software testing where APIs are tested directly to verify:

• Functionality (Does it work?)
• Reliability (Is it stable?)
• Performance (Is it fast?)
• Security (Is it safe?)

Unlike UI testing, API testing focuses on the business logic layer, making it faster and more reliable.

3. What are the types of APIs?

One of the most asked API Interview Questions. There are mainly three types of APIs:

1. REST API

Uses HTTP methods and JSON format. Most commonly used.

2. SOAP API

Uses XML format and strict protocols.

3. GraphQL API

Allows clients to request only required data.

👉 In interviews, REST APIs are most commonly asked.

4. What is REST API?

REST (Representational State Transfer) is an architectural style where APIs use HTTP methods to perform operations on resources.

Example:

• GET → Fetch data
• POST → Create data
• PUT → Update data
• DELETE → Delete data

👉 REST APIs are stateless, meaning each request is independent.

5. What is an Endpoint?

An endpoint is a specific URL where an API can be accessed.

Example:

https://api.example.com/users

👉 This endpoint is used to fetch or manipulate user data.

6. What is Request and Response?

Request:

Sent by the client to the server. It includes:

• URL
• Headers
• Body

Response:

Returned by the server. It includes:

• Status Code
• Headers
• Body

👉 A tester validates both request and response.

7. What is JSON?

JSON (JavaScript Object Notation) is a lightweight data format used in APIs.

{
  "name": "Ajit",
  "role": "QA"
}

👉 JSON is easy to read and widely used in REST APIs.

8. What is API Contract?

An API contract defines how a client and server communicate. It includes:

• Request format
• Response format
• Status codes

👉 Tools like Swagger define API contracts.

9. What tools are used for API Testing?

Common tools include:

• Postman (Manual testing)
• REST Assured (Automation)
• SoapUI (SOAP APIs)

👉 For deep automation, Rest API Testing interview questions , refer: Rest assured interview questions blog

10. Difference between API Testing and UI Testing?

👉 API testing is preferred in automation.

SECTION 2: HTTP METHODS & STATUS CODES in API Testing

Http methods and Status code are the one of the most important API testing Interview questions, lets prepare it.

11. What are HTTP Methods?

HTTP methods define actions performed on resources:

• GET → Retrieve data
• POST → Create data
• PUT → Update data
• DELETE → Remove data

👉 Each method has a specific purpose and behavior.

12. What is Idempotency?

An API is idempotent if multiple identical requests produce the same result.

Example:

PUT request updating user name → same result every time.

👉 GET and PUT are idempotent.

13. What is HTTP Status Code?

A status code indicates the result of an API request.

One of the most common interview questions which will dig-in which status code you have tested.

Status code 200 indicates success. In my testing, I verify it along with response body to ensure data integrity.

15. How to validate status code using REST Assured?

given()
.when()
.get("/users")
.then()
.statusCode(200);

👉 This ensures API returned expected response.

16. What is 201 Created?

Indicates resource was successfully created.

17. Difference between 401 and 403?

• 401 → Authentication failed
• 403 → Access denied

18. What is 404 Not Found?

API endpoint or resource not found.

19. What is 500 Internal Server Error?

Indicates server-side issue.

20. How do you handle negative scenarios?

Test invalid inputs and validate error responses.

SECTION 3: REST ASSURED API Testing Interview With Real-World Scenarios

👉 For Advanced Rest Assured API testing Interview question , check Rest Assured API Testing Interview questions

21. What is REST Assured?

REST Assured is a Java library used for automating REST APIs.

22. Basic GET Request

given()
.baseUri("https://reqres.in/api")
.when()
.get("/users/2")
.then()
.statusCode(200);

23. What is Response Body Validation?

It means verifying the data inside the API response (JSON/XML) matches expected results.

You check things like:

• Correct values
• Correct structure
• Required fields exist
• Data types are correct

🧠 Example Response

{
  "id": 101,
  "name": "Ajit",
  "status": "active"
}

✅ Ways to Validate Response Body

1. Direct Field Validation:

Check specific values.

👉 Rest Assured example:

given()
.when()
.get("/user/101")
.then()
.body("id", equalTo(101))
.body("name", equalTo("Ajit"))
.body("status", equalTo("active"));

2. JSON Path Validation (Most Important)

Use JSONPath to extract values.

String name = response.jsonPath().getString("name");
Assert.assertEquals(name, "Ajit");

3. Multiple Field Validation

.then()
.body("id", equalTo(101))
.body("status", equalTo("active"));

👉 Cleaner + widely used in interviews

4. Validate Array Data

Example:

{
  "books": [
    { "title": "Java" },
    { "title": "Python" }
  ]
}

Validation:

.then()
.body("books[0].title", equalTo("Java"))
.body("books[1].title", equalTo("Python"));

5. Validate Entire Response (Soft Check)

String responseBody = response.getBody().asString();
Assert.assertTrue(responseBody.contains("Ajit"));

6. Schema Validation (Advanced 🔥)

Ensures structure is correct.

.then()
.assertThat()
.body(matchesJsonSchemaInClasspath("schema.json"));

Real Interview Answer (Say this)

If interviewer asks:
👉 “How do you validate response body?”

You can say:

“I validate response body using Rest Assured by checking specific fields using JSON path or direct body assertions. I also validate arrays, partial content using contains, and for advanced cases I use JSON schema validation to ensure response structure is correct.”

Quick Summary:

Schema validation → structure check

JSONPath → extract data

body() → direct validation

contains() → partial check

24. POST Request Example

given()
.header("Content-Type", "application/json")
.body("{\"name\":\"Ajit\"}")
.when()
.post("/users")
.then()
.statusCode(201);

👉 Status 201 ensures resource creation.

25. PUT Request

given()
.body("{\"name\":\"Updated\"}")
.when()
.put("/users/2")
.then()
.statusCode(200);

26. DELETE Request

when()
.delete("/users/2")
.then()
.statusCode(204);

27. What is BDD in REST Assured?

• Given → Setup
• When → Action
• Then → Validation

28. How to log response?

.then().log().all();

29. What is JSON Path?

Used to extract values from response.

JSON Path is basically the XPath of JSON.

It’s a way to navigate and extract data from a JSON document without manually looping through everything.

🔥 Simple idea

Instead of doing this in code:

• open JSON
• loop objects
• check keys
• extract values

You just write a path expression like:

$.store.book[0].title

and boom 💥 you directly get the value.

Example JSON:

{
  "store": {
    "book": [
      {
        "title": "Harry Potter",
        "price": 500
      },
      {
        "title": "Clean Code",
        "price": 700
      }
    ]
  }
}

📌 JSONPath queries

• $.store → gets the whole store object
• $.store.book → gets all books
• $.store.book[0] → first book
• $.store.book[0].title → "Harry Potter"
• $..title → all titles in JSON (recursive search 🔥)

🚀 Why it’s useful (QA/Automation POV)

In API testing (Postman, RestAssured, etc.):

• Extract response data easily
• Validate fields quickly
• Avoid writing heavy parsing code

Example in testing:

$.data.id

→ directly pulls the ID from API response

⚡ Think of it like:

👉 “Google Maps for JSON data”
You don’t search manually—you just give the path and it takes you there.

30. Extract value example

String id = response.jsonPath().getString("id");

SECTION 4: API AUTHENTICATION

👉 Full guide:
https://qatribe.in/the-ultimate-api-authentication-guide-for-beginners-basic-apikey-jwt-oauth/

31. What is Authentication?

No API testing interview will be completed without this question.Authentication verifies user identity before allowing access to API resources. It ensures that only valid users or systems can interact with APIs.

In real-world applications like banking or e-commerce, authentication prevents unauthorized access to sensitive data such as account details or payment information.

32. What are the types of API Authentication?

Common types include:

• Basic Authentication: Uses username and password encoded in base64.
• API Key Authentication: Uses a unique key passed in headers or query parameters.
• Bearer Token Authentication: Uses tokens (like JWT) passed in headers.
• OAuth 2.0: Secure authorization framework used in enterprise systems.
• JWT (JSON Web Token): Compact token containing user data and signature.

👉 Each method is used based on security requirements.

33. Bearer Token Example in REST Assured

given()
.header("Authorization", "Bearer token_value")
.when()
.get("/user/profile")
.then()
.statusCode(200);

👉 Here:

• Authorization header carries token
• API validates token before responding

34. What is JWT?

JWT (JSON Web Token) is a secure way to transmit data between client and server.

Structure:

• Header
• Payload
• Signature

👉 Widely used in modern authentication systems.

35. What is OAuth 2.0?

OAuth 2.0 is an authorization framework that allows third-party apps to access resources without sharing credentials.

👉 Example:
Login with Google.

SECTION 5: ADVANCED API TESTING

👉 Look at our Deep dive advanced API testing Interview questions

36. What is Schema Validation?

Schema validation ensures API response structure matches expected format.

Example:

.then()
.body(matchesJsonSchemaInClasspath("schema.json"));

👉 This validates:

• Fields
• Data types
• Structure

37. What is Chaining in API Testing?

Chaining means using response data from one API request in another request.

Example:

• Login API returns token
• Use token in next API

👉 Very common in real-world testing.

38. What is Mocking?

Mocking simulates API responses without actual backend.

👉 Used when:

• Backend not ready
• Testing edge cases

39. What is Rate Limiting?

Limits number of API requests per user.

👉 Example:
100 requests per minute.

40. What is Contract Testing?

Ensures API request/response follows defined contract.

👉 Prevents integration issues.

SECTION 6: REAL-WORLD SCENARIOS

41. How to test Login API?

Validate:

• Status code (200/401)
• Token generation
• Response body

👉 Also test:

• Invalid credentials
• Missing fields

42. How to handle dynamic data?

Use variables or extract values from responses.

43. How to test error responses?

Validate:

• Error message
• Status code
• Response format

44. How to validate large JSON?

Use:

• JSON Path
• Schema validation

45. How to perform API performance testing?

Use tools like JMeter to simulate load.

46. How to test API security?

Check:

• Authentication
• Authorization
• Data leaks

47. What is Pagination?

Pagination means breaking large data into smaller chunks (pages) instead of returning everything at once.

Example API:

GET /users?page=1&limit=10

Response:

• Page 1 → users 1–10
• Page 2 → users 11–20
• Page 3 → users 21–30

🧪 What do we test in Pagination?

1. Page number validation

Check correct page is returned:

?page=1
?page=2

Validate:

• Page 1 != Page 2 data
• No duplicate data across pages

2. Page size (limit)

If limit = 10:

• Each page must return max 10 records

Rest Assured:

List users = response.jsonPath().getList("data");
Assert.assertTrue(users.size() <= 10);

3. Data consistency across pages

👉 Most important interview point

Check:

• No duplicate records
• No missing records

Example approach:

Set<Integer> idsPage1 = new HashSet<>();
Set<Integer> idsPage2 = new HashSet<>();// compare sets
Assert.assertTrue(Collections.disjoint(idsPage1, idsPage2));

4. Total count validation

Many APIs return:

{
  "total": 100,
  "page": 1,
  "limit": 10
}

Validate:

Assert.assertEquals(response.jsonPath().getInt("limit"), 10);
Assert.assertEquals(response.jsonPath().getInt("page"), 1);

5. Edge cases (very important 🔥)

Page = 0 or negative

?page=0
?page=-1

Expected:

• 400 Bad Request OR default page behavior

Page exceeds total pages

?page=9999

Expected:

• Empty list OR proper error message

6. Last page validation

Check:

• Remaining records < limit
• No crash

7. Performance check (bonus point)

Pagination should reduce load time vs full dataset.

Real Interview Answer :

To test pagination, I validate page number and limit parameters, ensure correct number of records per page, and verify there is no duplication or missing data across pages. I also test edge cases like invalid page numbers, last page behavior, and page numbers exceeding total count. Additionally, I validate total record count and response consistency.

48. How to handle API versioning?

Test endpoints like:

• /v1/users
• /v2/users

49. How to debug API failures?

Check:

• Logs
• Request/response
• Headers

50. What are API testing best practices?

• Validate all responses
• Automate critical APIs
• Test edge cases
• Use proper assertions

🎯 Conclusion

Mastering these API Testing Interview Questions will help you build a strong foundation in API testing, automation, and real-world problem solving. Focus on understanding concepts, practicing with tools like REST Assured, and handling real scenarios.

Mastering API Testing Interview questions requires strong fundamentals, practical knowledge, and real-world experience. If you prepare these API Testing Interview topics properly, you can confidently crack any QA interview.

💣 Final Interview Advice

Don’t just memorize answers.

👉 Explain clearly
👉 Write code confidently
👉 Think like a tester

👉 That’s how you get selected.

🔗 External Resources & References

For deeper understanding of API testing concepts, HTTP standards, and automation tools, refer to these trusted resources:

• Swagger (API Documentation Tool)
• Mozilla Developer Network – HTTP Status Codes & Web Docs
• REST Assured Official Documentation
• OAuth 2.0 Authorization Framework
• Apache JMeter (Performance Testing Tool)
  

❓ FAQ – API Testing Interview

Q1. What is API Testing Interview?
API Testing Interview focuses on backend testing concepts, HTTP methods, REST Assured, and real-world scenarios.

Q2. Is REST Assured mandatory for API interviews?
Yes, most companies expect hands-on knowledge of REST Assured for automation.

Q3. What are important topics in API Testing Interview?
HTTP methods, status codes, authentication, JSON handling, and automation.