Introduction to Web API Concepts
Web APIs (Application Programming Interfaces) are the backbone of modern web and mobile applications, enabling seamless communication between different software systems over the internet. They allow applications to exchange data and functionality using standard HTTP/HTTPS protocols, making them essential for building scalable, distributed systems.
Whether you’re developing a RESTful API, integrating third-party services, or securing your endpoints, understanding key Web API concepts is crucial for software engineers. This guide explores 25 fundamental Web API interview questions, covering topics such as:
- API design principles (REST, SOAP, GraphQL)
- Security (authentication, CORS, rate limiting)
- Performance optimization (caching, async operations)
- Testing and documentation (Swagger, Postman)
By the end of this guide, you’ll have a solid grasp of Web API best practices and common pitfalls, preparing you for technical interviews and real-world development scenarios.
Explore the 25+ Web API Interview Questions and Answers Below to Boost Your Preparation.
1. What is a Web API?
A Web API is an interface that allows applications to interact with each other over the web using HTTP protocols. It provides endpoints to expose functionalities and data for use in external systems like web or mobile apps.
2. Difference between SOAP and Web API?
SOAP is a protocol with strict standards and only XML data format, used mainly in enterprise-level services. Web APIs are typically RESTful, lightweight, and support multiple formats like JSON and XML, offering more flexibility.
3. How do you handle rate limiting in Web APIs?
Rate limiting controls the number of requests a client can make to an API. This can be achieved through middleware, API gateways, or by using tokens and headers like X-RateLimit-Limit and Retry-After.
4. How does CORS work in Web API?
CORS (Cross-Origin Resource Sharing) allows a server to specify who can access its resources by adding headers in the response. Browsers enforce CORS policy to prevent security issues like cross-site request forgery.
5. What is the role of API Gateway in Web API architecture?
An API Gateway acts as a reverse proxy, routing client requests to the appropriate backend services. It also handles security, logging, throttling, and transformation of requests/responses.
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6. Explain token-based authentication in Web API?
Token-based authentication involves sending a token (like JWT) with each request. The server validates the token to authenticate the client, avoiding the need to send credentials repeatedly.
7. What is Swagger/OpenAPI in Web API context?
Swagger or OpenAPI is a specification used to describe RESTful APIs. It helps in documenting APIs, generating client SDKs, and testing endpoints using interactive tools like Swagger UI.
8. How to secure a Web API?
Web APIs can be secured using HTTPS, authentication mechanisms (JWT, OAuth2), API keys, input validation, rate limiting, and proper CORS policies.
9. What is dependency injection in Web API?
Dependency injection is a design pattern used to achieve Inversion of Control (IoC). In Web APIs, it allows services to be injected into controllers for better modularity and testability.
10. What is versioning in Web API and why is it needed?
API versioning allows developers to make changes to APIs without breaking existing clients. It can be implemented via URL, query string, headers, or media type.
11. Explain middleware in ASP.NET Web API?
Middleware is software that’s assembled into an application pipeline to handle requests and responses. In ASP.NET Web API, middleware can perform tasks like logging, authentication, and exception handling.
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12. What is the difference between synchronous and asynchronous Web API calls?
Synchronous calls block execution until the response is received. Asynchronous calls allow execution to continue, improving performance in scenarios with I/O operations.
13. Explain the use of filters in Web API?
Filters in Web API (like Authorization, Action, and Exception filters) are used to execute code before or after specific stages in the request pipeline.
14. How do you implement logging in Web API?
Logging can be implemented using built-in .NET Core logging, third-party libraries like Serilog, or middleware to log request/response information and errors.
15. What is content negotiation in Web API?
Content negotiation is the process by which the client and server agree on the format of the response (JSON, XML, etc.) using headers like Accept and Content-Type.
16. What are custom headers in Web API?
Custom headers are user-defined HTTP headers used to send additional metadata with API requests/responses. Examples include X-Correlation-ID or X-Auth-Token.
17. What is the use of HttpClient in Web API testing?
HttpClient is a class used to send HTTP requests and receive responses from a resource. It’s widely used for integration testing Web APIs.
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18. What is throttling in Web APIs?
Throttling is used to control the amount of traffic to an API. It prevents overuse and abuse by limiting the number of requests a client can make in a specific period.
19. What is the difference between 401 and 403 HTTP status codes?
401 Unauthorized indicates missing or invalid authentication. 403 Forbidden means authentication succeeded but the user does not have permission.
20. What is the difference between RESTful Web API and RESTless Web API?
RESTful APIs follow the REST constraints such as statelessness, resource-based URIs, and uniform interface. RESTless APIs do not follow all constraints strictly and may mix RPC-like behavior.
21. How do you handle exceptions globally in Web API?
Using middleware or exception filters to catch unhandled exceptions and return custom error responses with logging mechanisms.
22. Explain the difference between IHttpActionResult and HttpResponseMessage.
IHttpActionResult is an abstraction introduced for unit testing and flexibility. HttpResponseMessage provides more control over the HTTP response content.
23. How does attribute routing work in Web API?
Attribute routing allows defining routes using attributes directly on controller methods, providing more control and flexibility than conventional routing.
24. What is model binding in Web API?
Model binding maps incoming request data to action method parameters. Web API uses binders to convert query strings, form data, or JSON into .NET types.
25. How to implement file upload in Web API?
Use MultipartFormDataStreamProvider or IFormFile in .NET Core to handle multipart/form-data POST requests to upload files.
Conclusion:
Web APIs play a vital role in modern software development, acting as the bridge between frontend applications and backend services. Mastering concepts like RESTful design, authentication (JWT/OAuth), rate limiting, and error handling ensures that your APIs are scalable, secure, and maintainable.
Key takeaways from this guide:
- RESTful APIs are preferred for their simplicity and flexibility over SOAP.
- Security is critical—always use HTTPS, validate inputs, and implement proper CORS policies.
- Performance can be improved with async programming, caching, and throttling.
- Documentation (Swagger/OpenAPI) and testing (Postman, unit tests) are essential for API maintainability.
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Author’s Bio:
As CEO of TestLeaf, I’m dedicated to transforming software testing by empowering individuals with real-world skills and advanced technology. With 24+ years in software engineering, I lead our mission to shape local talent into global software professionals. Join us in redefining the future of test engineering and making a lasting impact in the tech world.
Babu Manickam
CEO – Testleaf
