API testing has become an essential component of modern software development, ensuring the reliability and functionality of web applications and services. This article provides a most frequently asked of API testing interview questions and answers, covering a wide range of topics from fundamental concepts to advanced techniques.
Whether you’re a experienced tester or just starting your career, these questions will help you demonstrate your knowledge and skills to potential employers.
API Testing interview questions and answers
1. What is API Testing?
2. What is the difference between API and Web Services?
3. What are the common types of API architectures?
4. Explain REST API and its key principles.
5. What are HTTP methods, and where are they used?
6. What is the purpose of status codes in API testing?
7. What is the difference between PUT and POST in REST APIs?
8. What is SOAP API?
9. How do you validate API responses?
10. What are the tools used for API Testing?
11. What is API Throttling?
12. What is OAuth? How is it related to API security?
13. What is CORS (Cross-Origin Resource Sharing) and why is it important in API testing?
14. How do you perform API load testing?
15. What are idempotent and safe methods in API?
16. What is an API Gateway?
17. What are API Rate Limits?
18. Explain the difference between Authorization and Authentication in API security.
19. What is API Mocking?
20. How do you handle pagination in API responses?
21. What is JSON Schema, and why is it used in API testing?
22. What is the importance of Headers in API Testing?
23. How do you test for API Security?
24. What is HATEOAS in RESTful services?
25. Explain API Versioning.
26. What is API Chaining?
27. How do you handle errors in API responses?
28. What is gRPC, and how is it different from REST?
29. What are Webhooks, and how are they different from APIs?
30. What is an API Contract?
31. How do you handle different environments (QA, Staging, Production) in API testing?
32. What is the significance of the Content-Type and Accept headers in API testing?
1. What is API Testing?
Answer:
API Testing is a type of software testing that involves testing APIs directly to ensure they meet functionality, reliability, performance, and security standards. It is usually performed at the message layer without a GUI.
2. What is the difference between API and Web Services?
Answer:
All web services are APIs, but not all APIs are web services. Web services require a network for communication (typically over HTTP), while APIs can be exposed using various protocols like HTTP, SMTP, etc.
3. What are the common types of API architectures?
Answer:
- REST (Representational State Transfer)
- SOAP (Simple Object Access Protocol)
- GraphQL (Query language for APIs)
- JSON-RPC / XML-RPC
4. Explain REST API and its key principles.
Answer: REST (Representational State Transfer) is an architectural style that uses HTTP methods like GET, POST, PUT, DELETE to perform CRUD operations on resources. Its key principles include:
- Stateless communication
- Client-server architecture
- Cacheable responses
- Uniform interface
5. What are HTTP methods, and where are they used?
Answer: HTTP methods define the type of operation to be performed:
- GET: Retrieve data
- POST: Submit data to be processed
- PUT: Update existing data
- DELETE: Remove data
- PATCH: Partially update data
6. What is the purpose of status codes in API testing?
Answer: HTTP status codes are used to indicate the outcome of the API request. Common codes include:
- 200 OK (success)
- 201 Created
- 400 Bad Request
- 401 Unauthorized
- 404 Not Found
- 500 Internal Server Error
7. What is the difference between PUT and POST in REST APIs?
Answer:
- PUT is idempotent; it updates or creates a resource with a known identifier.
- POST creates a resource with a server-assigned identifier. Multiple POSTs may create duplicate resources.
8. What is SOAP API?
Answer:
SOAP (Simple Object Access Protocol) is a protocol for exchanging structured information using XML. It is standardized, uses WSDL for operations, and is often considered more secure due to built-in ACID compliance.
9. How do you validate API responses?
Answer: API responses can be validated by:
- Status code verification
- Response time check
- Schema validation (using tools like JSON Schema Validator)
- Data accuracy and integrity
- Header validation
10. What are the tools used for API Testing?
Answer:
Popular tools include:
- Postman
- SoapUI
- JMeter
- Rest-Assured
- Katalon Studio
- Swagger UI
11. What is API Throttling?
Answer:
API Throttling limits the number of API requests a client can make in a specific time period to prevent server overload and ensure fair usage.
12. What is OAuth? How is it related to API security?
Answer: OAuth is an open-standard protocol for authorization that allows third-party services to exchange tokens (not credentials) to securely access resources on behalf of a user. OAuth enhances API security by managing and validating token-based access.
13. What is CORS (Cross-Origin Resource Sharing) and why is it important in API testing?
Answer:
CORS is a security mechanism that allows or restricts resources to be requested from another domain than the one serving the API. It is important to test CORS policies to avoid unauthorized cross-origin access.
14. How do you perform API load testing?
Answer: API load testing is done by simulating multiple users or requests hitting the API simultaneously using tools like JMeter or Gatling. The key metrics observed are response times, throughput, error rate, and server resource utilization.
15. What are idempotent and safe methods in API?
Answer:
- Idempotent methods (GET, PUT, DELETE) produce the same result regardless of how many times the request is repeated.
- Safe methods (GET, HEAD) do not modify the resource or the server’s state.
16. What is an API Gateway?
Answer:
An API Gateway acts as a reverse proxy to accept all application programming interface (API) calls, aggregate the services required to fulfill them, and return the appropriate response.
17. What are API Rate Limits?
Answer: API rate limits restrict the number of API calls a user can make within a given time period to avoid abuse and overload on the server.
18. Explain the difference between Authorization and Authentication in API security.
Answer:
- Authentication verifies who the user is (credentials or tokens).
- Authorization determines what resources the user is allowed to access after being authenticated.
19. What is API Mocking?
Answer:
API Mocking is the practice of creating a simulated version of the API to test client-side code when the real API is unavailable or still under development.
20. How do you handle pagination in API responses?
Answer:
Pagination divides a large dataset into smaller pages. API responses typically include pagination details like page number, page size, total records, and links to next and previous pages.
21. What is JSON Schema, and why is it used in API testing?
Answer:
JSON Schema defines the structure and validation constraints of JSON data. It ensures the API returns a well-structured and valid response.
22. What is the importance of Headers in API Testing?
Answer:
Headers provide metadata about the API request/response, such as content type (Content-Type), authorization tokens (Authorization), and cache-control policies (Cache-Control).
23. How do you test for API Security?
Answer:
- Test for authentication and authorization (OAuth, JWT)
- Check for data encryption (SSL/TLS)
- Test for SQL Injection, Cross-Site Scripting (XSS), and Cross-Site Request Forgery (CSRF)
- Validate error codes and responses to ensure no sensitive information is leaked.
24. What is HATEOAS in RESTful services?
Answer:
HATEOAS (Hypermedia As The Engine Of Application State) is a constraint of REST architecture. It enables API responses to include links to other actions that can be taken on a resource, promoting a self-descriptive API.
25. Explain API Versioning.
Answer:
API versioning ensures backward compatibility when changes are made. Common strategies include:
- Version in the URL (
/v1/resource) - Version in headers (
Accept: application/vnd.api.v1+json) - Version in query parameters (
?version=1)
26. What is API Chaining?
Answer:
API Chaining refers to making multiple API calls in sequence, where the output of one API serves as the input to the next API.
27. How do you handle errors in API responses?
Answer:
A good API provides detailed and meaningful error messages. HTTP error codes should be mapped to relevant messages like 400 Bad Request, 401 Unauthorized, and 500 Internal Server Error. Proper logging and exception handling mechanisms should be in place.
28. What is gRPC, and how is it different from REST?
Answer:
gRPC is a high-performance, open-source RPC framework that uses HTTP/2 for transport and Protocol Buffers for serializing data. Unlike REST, which uses text-based formats like JSON or XML, gRPC is binary and more efficient.
29. What are Webhooks, and how are they different from APIs?
Answer:
Webhooks are user-defined callbacks that are triggered by specific events and push data to a URL, whereas APIs are request-driven (pull-based). Webhooks are often used for real-time communication, while APIs are generally used for polling.
30. What is an API Contract?
Answer:
An API Contract defines the expected request and response structure, parameters, headers, and authentication mechanisms. It acts as a blueprint for both API consumers and developers, typically documented in tools like Swagger/OpenAPI.
31. How do you handle different environments (QA, Staging, Production) in API testing?
Answer:
- Use environment-specific configuration files (URLs, tokens, etc.).
- Ensure proper isolation of test data.
- Implement Continuous Integration/Continuous Deployment (CI/CD) pipelines for automated testing in different environments.
32. What is the significance of the Content-Type and Accept headers in API testing?
Answer:
Content-Typespecifies the media type of the resource being sent to the server (e.g.,application/json).Acceptheader indicates the type of response the client expects from the server (e.g.,application/json,application/xml).
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