Web API Interview Questions and Answers

Every modern application uses APIs in one way or another. From social media feeds to online payment, APIs are a part of it all. This is why Web API questions are a common part of technical interviews, as you will be using them in your day-to-day responsibilities as a software developer. In this guide, we will walk you through all the interview questions that you will need to increase your confidence and crack the technical round on your first attempt.

Table of Contents:

• Web API Interview Questions for Freshers (Basic)
• Web API Interview Questions for Experienced (2–5 Years)
• Advanced Web API Interview Questions (5+ Years)
• Scenario-Based Web API Interview Questions
• Web API Testing Interview Questions
• ASP.NET / .NET Core Web API Interview Questions
• Web API MCQ Questions

Web API Interview Questions for Freshers (Basic)

1. What is a Web API, and why is it used?

A web API (Application Programming Interface) is what allows two applications or servers to communicate over the internet using HTTP. Simply put, it is like a messenger that takes a request from one system and delivers it to another. It facilitates data transfer and integration for various online services such as payments, e-commerce, news feeds, and pretty much anything else you can think of.

2. What is the difference between Web API, REST API, and RESTful API?

• Web API is a general interface that allows communication between applications using HTTP. It does not need to follow REST rules.
• REST API is a term for any API that follows REST principles. However, there are many API that claim to be REST APIs but do not strictly adhere to the REST principles.
• A RESTful API is a properly designed REST API that strictly respects REST constraints like statelessness, resource-based URLs, and standard HTTP methods.

3. What is the difference between MVC and Web API?

• MVC (Model-View-Controller) is mostly used to build web applications with user interfaces, as it returns HTML views.
• Web API, on the other hand, is used to build HTTP services. It returns data that can be consumed by a browser or other applications.

4. What is the difference between REST and SOAP?

• SOAP (Simple Object Access Protocol) is a protocol that relies on XML and adheres to strict rules. It is often used in large enterprise systems and is known for its security and reliability.
• REST (Representational State Transfer) is a lighter and more flexible approach that uses standard HTTP methods. It supports multiple data formats like JSON, XML, or plain text, making it faster and easier to work with for most web applications.

REST is more popular because of its simplicity and speed.

5. What is the difference between JSON and XML?

• JSON (JavaScript Object Notation) is a lightweight data format that is easy to read and write. It is ideal for modern web applications because it is fast to parse.
• XML (Extensible Markup Language) is a more verbose format that uses tags and attributes to store data. XML supports rich metadata, schemas, and validation, making it common in legacy systems and enterprise applications.

Key Difference: JSON is simpler and faster, while XML is more structured and powerful for complex data with strict rules.

6. What are the common HTTP methods used in Web APIs?

HTTP methods are used to tell the server what action a client wants to perform. Some of the most common ones used in Web APIs are:

• GET: Retrieve data
• POST: Create new data
• PUT: Update existing data (replace fully)
• PATCH: Update partially
• DELETE: Remove data

7. What are some common HTTP status codes?

HTTP status codes help us understand the status of an HTTP request. They make it easier to identify issues and debug. Some of the most common ones are:

• 200: Success
• 201: Resource created successfully
• 400: Invalid input or request format
• 401: Authentication required
• 404: Resource doesn’t exist
• 500: Something went wrong on the server

8. What is a resource in a RESTful API?

A resource in a RESTful API can be anything that can be identified and acted on. Anything from a user, product, or even an order is a resource that is usually represented by unique URLs like:

/api/users/101 

/api/products/55

9. What are the advantages of using REST in Web APIs?

• Easy to use and understand
• Works with multiple formats (JSON, XML, text)
• Stateless: each request is independent
• Scalability: easy to handle high traffic
• Widely supported by browsers, servers, and tools

10. Who can consume a Web API?

Any client that can make HTTP requests can consume Web APIs. They include, but are not limited to:

• Web browsers
• Mobile apps (Android, iOS)
• Desktop applications
• IoT devices
• Other web servers

Web API Interview Questions for Experienced (2–5 Years)

1. What are HTTP headers? 

HTTP provides additional context about HTTP requests or responses. They control things like content type, caching, authentication, and more.

Some of the most common headers in HTTP requests are:

• Content-Type: Indicates the data format (e.g., application/json).
• Authorization: Used to pass credentials or tokens.
• Accept: Specifies the response format the client can handle.
• Cache-Control: Controls caching behavior.
• User-Agent: Identifies the client software making the request.

2. What is the difference between PUT and PATCH?

PUT:

• Replaces the entire resource with the data that is sent
• If a property is missing, it can be reset or removed

PATCH:

• Uptates only specified fields
• Leaves other resources untouched.

To simplify, you can think of PUT as “replace completely” and PATCH as “update partially.”

3. What is idempotency?

A process is idempotent if performing it multiple times has the same effect as performing it once. 

For example:

• GET and PUT are usually idempotent.
• POST is not, because creating a resource multiple times generates duplicates.

Idempotency is crucial in APIs to prevent accidental duplication or inconsistent data.

4. What are some pros and cons of Statelessness in REST?

Pros:

• Each request is complete in itself, i.e, it contains all necessary info, making it easier to scale horizontally.
• Since no session state is stored, it drastically reduces server memory requirements.

Cons:

• Authentication and context must be included in every request. It can be repetitive, especially when making bulk requests.
• Could increase payload size and complexity on the client side.

5. What is content negotiation in Web API?

Content negotiation is the process by which the client and server decide on the format of the response. Let’s say the client asks for JSON or XML. The server will try to return the data in the requested format as long as it supports it. This facilitates flexibility as different clients may have different needs. 

6. Explain exception filters & error handling strategies

Exception filters let you catch unhandled errors in Web API controllers and return consistent responses instead of exposing raw exceptions.

Common error handling strategies include:

• Using try–catch blocks inside controllers for specific scenarios.
• Setting up global exception filters to handle errors in a centralized way.
• Returning clear HTTP status codes along with helpful messages.

7. What is CORS and its role in Web API?

CORS (Cross-Origin Resource Sharing) is a browser security mechanism that controls whether a web page from one domain can request resources from another domain. If CORS isn’t configured correctly on the server, the browser may block those cross-domain requests, even if the API itself is working fine.

Setting headers like Access-Control-Allow-Origin ensures that only allowed clients can consume your API.

8. What are the differences between JWT, OAuth2, and API Keys?

• JWT (JSON Web Tokens): Packs information about a user into a security token. Often used to handle authentication without storing session data on the server.
• OAuth2: Lets users grant limited access to their resources without sharing passwords.
• API Keys:  Simple tokens that identify and authorize a client or app when it makes requests.

Choosing the right method depends on the sensitivity of the data, the type of client, and the use case.

9. Why is API versioning important, and what are the common ways to implement it?

API versioning helps maintain backward compatibility as APIs evolve. Common approaches include:

• URI versioning: /api/v1/products
• Query string versioning: /api/products?version=1
• Header versioning: Custom headers to specify the API version

Proper versioning can prevent breaking existing clients when new features are added. This is crucial for scalability.

10. How does caching work in Web APIs, and what are the different types?

Caching boosts performance by temporarily storing responses and reducing server load. Types of caching include:

• Client-side caching: The browser stores API responses temporarily. If the same request is made again, it loads faster without contacting the server. Headers like Cache-Control and ETag are used to manage this.
• Server-side caching: The API server keeps frequently used data in memory or another fast store, so repeated requests are served quickly without recalculating everything.
• Proxy/CDN caching: Proxy servers or CDNs cache popular responses. This reduces load on the main server and delivers data faster to users across different regions.

Advanced Web API Interview Questions (5+ Years)

1. What is HATEOAS in REST?

HATEOAS (Hypermedia As The Engine Of Application State) is a REST principle that allows clients to discover available actions through links included in the API response. Instead of hardcoding URLs, the client follows these links to perform further operations.

For example, when you fetch a user, the response might also include links for updating or deleting that user. This makes the API more self-descriptive, flexible, and easier to maintain as it grows.

2. What are rate limiting and throttling in Web APIs?

• Rate limiting sets a cap on how many requests a client can make within a certain time window.
• Throttling slows down or temporarily restricts requests when the server is experiencing high load.

Both are used to protect APIs from abuse, keep the server stable, and ensure fair usage across all clients.

3. What is the role of an API Gateway in microservices?

An API Gateway sits between clients and microservices, acting as a single entry point. It handles:

• Request routing
• Load balancing
• Authentication and authorization
• Rate limiting and caching
• Aggregating responses from multiple microservices

This simplifies client communication and improves security and performance in microservices architectures.

4. What is the difference between synchronous and asynchronous APIs?

• Synchronous APIs: The client waits for the server to respond before continuing. 
• Asynchronous APIs: The client can continue processing while waiting for a response, often using callbacks, webhooks, or message queues.

Asynchronous APIs improve scalability and responsiveness, especially for long-running operations.

5. What are the differences between gRPC and REST?

• REST: Uses HTTP/JSON, easy to use, widely supported, human-readable.
• gRPC: Uses HTTP/2 and Protocol Buffers, faster, supports streaming, better for internal microservices communication.

Choose REST for broad client compatibility and simplicity, gRPC for performance-critical, high-throughput services.

6. How do you design scalable APIs?

Scalable APIs are designed to handle increasing traffic without slowing down or breaking. To achieve this, you can apply strategies such as:

• Stateless design (REST principles): Each request carries all the information needed, which makes it easier to distribute across servers
• Caching: Store frequently requested data so the server doesn’t have to process the same request repeatedly.
• Pagination: Break large datasets into smaller chunks instead of sending everything at once.
• Load balancing and horizontal scaling: Distribute traffic across multiple servers to prevent bottlenecks.
• Optimized database queries and indexing: Ensure the data layer is efficient enough to handle growth.
• API gateways and rate limiting: Add a layer of control, security, and traffic management.

7. What are the best practices for API logging and monitoring?

• Logging: Capture request and response info, errors, performance metrics, and user actions. Use structured logging for easier analysis.
• Monitoring: Track API health, latency, error rates, and usage patterns with tools like Prometheus, Grafana, or ELK Stack.
• Alerts: Set thresholds to notify teams when errors or slow responses occur.

Scenario-Based Web API Interview Questions

1. How would you design an API for an e-commerce app?

When designing an e-commerce API, the first step is to identify the core resources you’ll be working with, typically products, users, orders, and payments. Once that’s clear, applying RESTful principles will help keep the API clean and easy to use:

• Use meaningful endpoints: Stick to clear resource names like /products, /orders, and /users.
• Follow standard HTTP methods: Use GET to retrieve data, POST to create, PUT to update, and DELETE to remove.
• Handle large data sets smartly: Apply pagination when returning long product lists so responses stay manageable.
• Secure sensitive operations: Enforce authentication for actions such as placing orders or handling payments.
• Communicate with status codes: Return proper HTTP codes to make it obvious whether a request succeeded or failed.

The key is a clean, scalable, and easy-to-use API that developers can consume reliably.

2. How would you secure an API from misuse?

To secure an API:

• Use authentication (JWT, OAuth2) to verify users.
• Apply rate limiting to prevent abuse or excessive requests.
• Enable CORS only for trusted domains.
• Validate input data to prevent SQL injection or other attacks.
• Consider logging and monitoring suspicious activity.

3. How would you handle slow API responses?

Slow API responses can be due to a plethora of reasons. You can use the following approaches to narrow down and fix the issue. 

• Identify performance bottlenecks by using profiling tools or analyzing logs.
• Cache frequently requested data to reduce server load and speed up responses.
• Handle long-running tasks asynchronously so they don’t block other requests.
• Use pagination or lazy loading when working with large datasets to avoid overwhelming the client.
• Scale your application with load balancing or horizontal server scaling to handle increased traffic efficiently.

4. What steps would you take to debug failing API requests?

When an API request fails, the first goal is to pinpoint the location of the issue. Whether it is in the client, the server, or the network. Here is a structured debugging process that you can use to fix the problem:

• Review request and response logs to see exactly what is being sent and received.
• Check HTTP status codes and error messages to identify whether the problem is client-side, server-side, or related to permissions.
• Inspect network traffic using tools like Postman or browser Developer Tools.
• Verify request payloads, headers, and authentication tokens to ensure they are valid.
• Reproduce the issue in a test environment to isolate the cause without impacting production.

5. How would you upgrade an API version without breaking clients?

When managing changes in an API, versioning ensures backward compatibility while allowing improvements. Key practices include:

• Introduce versioned endpoints (e.g., /api/v1/products, /api/v2/products).
• Keep older versions available for a transition period so clients can adapt.
• Communicate changes clearly in documentation and release notes.
• Deprecate features gradually instead of removing fields abruptly.

Web API Testing Interview Questions

1. How do you perform manual testing using Postman?

Postman is a popular tool for testing APIs. It lets you:

• Send requests using methods like GET, POST, PUT, and DELETE.
• Inspect responses, including status codes, headers, and body.
• Organize tests into collections and use environment variables for different setups.
• Validate responses manually to confirm that the API works as expected.

2. How do you automate API testing with REST Assured?

REST Assured is a Java library for testing RESTful APIs.

• Write tests to send requests and validate responses programmatically.
• Check status codes, response body, and headers automatically.
• Integrate with JUnit or TestNG to run tests as part of CI/CD pipelines.
• Helps catch regressions early and ensures API reliability.

3. How do you unit test APIs?

Unit testing APIs involves testing individual functions or endpoints in isolation.

• Mock dependencies (databases, external services) to isolate functionality.
• Validate that each API method returns correct status codes and response data.
• Use frameworks like JUnit (Java), pytest (Python), or MSTest (C#).
• Unit tests ensure code correctness before integration.

4. What are common challenges in API testing?

• Handling dynamic data that changes with each request.
• Authentication and authorization complexities.
• Testing edge cases and error responses.
• Ensuring backward compatibility when APIs evolve.
• Performance and load testing for high-traffic APIs.

ASP.NET / .NET Core Web API Interview Questions

ASP.NET Web API is a framework provided by Microsoft to build HTTP services that can be consumed by browsers, mobile apps, IoT devices, or other applications. It runs on the .NET Framework and is mainly used for creating RESTful services.

1. What is ASP.NET Web API, and how is it different from ASP.NET Core Web API?

ASP.NET Web API was Microsoft’s earlier framework for building RESTful services on top of the .NET Framework. It worked well but was limited to Windows and is now considered part of the older .NET stack.
ASP.NET Core Web API is the newer, cross-platform version. It runs on Windows, Linux, and macOS, and is faster, lightweight, and modular. It also has built-in support for dependency injection, middleware, and cloud deployment, which makes it a better choice for modern applications and microservices.

// 1. Add CORS policy
builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowSpecific",
        policy => policy.WithOrigins("https://example.com")
                        .AllowAnyHeader()
                        .AllowAnyMethod());
});
var app = builder.Build();
// 2. Use CORS
app.UseCors("AllowSpecific");
app.MapControllers();
app.Run();