API Interview Questions and Answers

Both new tech employees and senior engineering professionals will encounter API interview questions regularly. APIs are a key component in modern software as they create connections between applications, allow mobile applications to function and allow for integrated programs to operate in today's business environment.

This blog is built with practical knowledge rather than theoretical understanding from hands-on experience designing, testing and integrating APIs into real-world projects; as well as comprehensive knowledge about best practices when developing an API (like RESTful architecture, implementing security measures via authentication and optimising performance).

This resource contains 30+ questions related to APIs organised into four different categories: beginner level, intermediate level, experienced level and question/situation level. All beginner level answers provide you with an answer that is clear, concise and practical enough for you to use as you walk into your next interview full of confidence.

Now, let's move on!

API Interview Questions For Freshers

These questions test your foundational understanding. If you are new to APIs or preparing for a junior role, start here. Interviewers ask these questions to check whether you understand the basics before going deeper.

Q1. What is an API and how does it work?

An API is a contract between two software systems. It defines what requests you can make, how to make them and what response you will get back.

Here is how it works in simple terms:

• A client sends a request to an API endpoint (a specific URL).

• The API processes requests on the server side.

• The server sends back a response, usually in JSON or XML format.

For example, when you open a weather app, it sends a request to a weather API. The API fetches the current data from a server and sends it back. Your app then displays that information to you.

APIs make it possible for different systems to share data without exposing their internal logic. That is why they are so widely used across web, mobile and enterprise software.

Q2. What is REST and what makes it different from other API styles?

REST (Representational State Transfer) is an architectural style for building APIs. Roy Fielding introduced it in his 2000 doctoral dissertation. REST uses standard HTTP methods and is built around resources, which are any objects or data that you want to access.

What makes REST popular is its simplicity. It works over HTTP, returns data in JSON (which is easy to read and parse) and is stateless, meaning the server does not remember anything between requests.

Other API styles include:

• SOAP: A protocol that uses XML and has strict standards. It is more complex but better suited for enterprise systems that need formal contracts and built-in security.

• GraphQL: A query language that lets clients request exactly the data they need. It avoids over-fetching and under-fetching.

• gRPC: A high-performance framework from Google. It is great for microservices that need fast, efficient communication.

REST is the most widely used style for public APIs because it is simple, flexible and easy to build with.

Q3. What are the main HTTP methods used in REST APIs?

HTTP methods tell the server what action you want to perform on a resource. Here are the five you need to know:

Q4. What is the difference between GET and POST?

A fundamental HTTP concept, GET is used to retrieve data, while POST is used to send data to the server.

Q5. What are HTTP status codes and why do they matter?

HTTP status codes tell the client what happened with their request. They are grouped into five categories:

Status codes matter because they help clients handle responses correctly. If you get a 401, you know the user needs to log in. If you get a 500, you know the server has a problem. Well-designed APIs always return meaningful status codes so developers can debug issues quickly.

Q6. What is an API endpoint?

An API endpoint is a specific URL where an API can be accessed. It represents a resource or a collection of resources on the server.

For example:

https://api.example.com/users is the endpoint for all users.

https://api.example.com/users/42 is the endpoint for the user with ID 42.

Each endpoint is designed to accept specific HTTP methods. The /users endpoint might support GET (to list all users) and POST (to create a new user). The /users/42 endpoint might support GET, PUT, PATCH and DELETE.

Good API design uses nouns for endpoints (not verbs). So /getUsers is wrong. /users is correct.

Q7. What is JSON, and why do APIs use it?

JSON (JavaScript Object Notation) is a lightweight, text-based data format. APIs use it to send and receive data because it is easy for both humans and machines to read.

A JSON response looks like this:

{
  "id": 1,
  "name": "Priya Sharma",
  "email": "priya@example.com",
  "active": true
}

JSON is language-agnostic, meaning any programming language can parse it. It is also compact, which keeps API responses fast. Compared to XML (the older alternative), JSON is cleaner and much easier to work with.

Most modern REST APIs return JSON by default. Some also support XML when clients request it through the Accept header.

Q8. What is API authentication and why is it important?

API authentication verifies that the person or system making a request is who they claim to be. Without authentication, anyone could access your API, steal data, or abuse your system.

Q9. What is the difference between an API and a web service?

APIs are broader, while web services are a specific type of API that works over a network.

Q10. What is API documentation and what should good documentation include?

API documentation is the reference guide for developers who want to use your API. It explains what the API does, how to make requests, what parameters are required and what responses to expect.

Good API documentation includes:

• Overview: What the API does and who it is for.

• Authentication: How to get access and authenticate requests.

• Endpoints: A full list of available endpoints with their HTTP methods.

• Request parameters: Required and optional fields, data types and examples.

• Response format: What a successful response looks like, including all fields.

• Error codes: A list of error responses and what they mean.

• Code examples: Sample requests in multiple languages (curl, Python, JavaScript, etc.).

• Rate limits: How many requests are allowed per minute or hour.

Tools like Swagger (OpenAPI), Postman and Redoc are commonly used to generate and host API documentation.

Read Also: Web API Interview Questions and Answers

API Interview Questions for Intermediate

These questions go deeper into REST API design, security and testing. Expect them if you are interviewing for a mid-level developer or QA role.

Q1. What is statelessness in REST and why does it matter?

Statelessness is one of the core constraints of REST. It means the server does not store any information about previous requests. Every request from the client must contain all the information the server needs to process it, including authentication tokens, user context and session data.

Why does this matter? Stateless APIs are easier to scale. Because no session data is stored on the server, you can route requests to any server in a cluster without worrying about which server handled the previous request. This makes horizontal scaling straightforward.

The trade-off is that requests can become larger because they carry more data. But for most modern applications, this is worth the scalability benefit.

Q2. What is the difference between PUT and PATCH?

Both are used for updates, but they differ in how much data they modify.

Q3. What is REST API versioning and what are the common approaches?

API versioning lets you make breaking changes to your API without disrupting existing clients. When you release a new version, older clients can keep using the previous version while new clients adopt the latest one.

The four most common versioning strategies are:

1. URI versioning: Include the version number in the URL. GET /v1/users vs GET /v2/users This is the most common approach. It is simple and visible.

2. Query parameter versioning: Pass the version as a query parameter. GET /users?version=2 Easy to implement but less clean.

3. Header versioning: Specify the version in a custom request header. X-API-Version: 2 Cleaner URLs but harder to test directly in a browser.

4. Accept header versioning (media type): Use the Accept header. Accept: application/vnd.example.v2+json Very flexible but more complex to implement.

Most teams choose URI versioning because it is straightforward and easy to document.

Q4. What is CORS and how do APIs handle it?

CORS (Cross-Origin Resource Sharing) is a browser security mechanism that controls which domains can make requests to your API from a web page.

By default, browsers block JavaScript requests to a different domain (origin) than the one the page was loaded from. This is the same-origin policy. CORS allows servers to relax this restriction by specifying which origins are allowed.

APIs handle CORS by including response headers like:

Access-Control-Allow-Origin: https://yourapp.com

Access-Control-Allow-Methods: GET, POST, PUT, DELETE

Access-Control-Allow-Headers: Content-Type, Authorization

Setting Access-Control-Allow-Origin: * allows any domain to access the API, which is fine for public APIs but risky for APIs that handle sensitive data.

Q5. What is rate limiting in an API and why do APIs use it?

Rate limiting restricts how many requests a client can make to an API within a given time window. For example, an API might allow 1000 requests per hour per API key.

APIs use rate limiting for several reasons:

• Prevent abuse: Stop bad actors from overloading the API with too many requests.

• Ensure fair usage: Make sure one client cannot consume all the server's resources.

• Protect performance: Keep the API fast and available for all users.

• Reduce costs: Limit expensive operations.

When a client exceeds the rate limit, the API typically returns a 429 Too Many Requests status code. Good APIs also include headers like X-RateLimit-Remaining and X-RateLimit-Reset so clients know when they can make requests again.

Q6. What are the differences between REST and GraphQL?

These are two popular approaches for building APIs with different philosophies. Here is their brief differentiation:

Over-fetching means the API returns more data than you need. Under-fetching means you need to call multiple endpoints to get all the data you need. GraphQL solves both problems by letting the client define exactly what data it wants in a single query.

Use REST for straightforward APIs. Use GraphQL when you have complex data relationships or need to optimize network performance.

Q7. What is API testing and what are the main types?

API testing verifies that an API works correctly, performs well and is secure. Unlike UI testing, API testing goes straight to the backend, making it faster and more reliable.

The main types of API testing are:

• Functional testing: Checks if the API returns the correct response for a given request.

• Integration testing: Verifies that the API works correctly when combined with other services or databases.

• Load testing: Tests how the API performs under high traffic (many concurrent requests).

• Security testing: Checks for vulnerabilities like SQL injection, unauthorized access and data leaks.

• Contract testing: Verifies that the API matches its documented specification (the "contract").

• Error handling testing: Confirms the API returns meaningful error messages for invalid inputs.

Popular API testing tools include Postman, REST Assured, k6 and SoapUI.

Q8. What is the role of headers in an API request?

HTTP headers carry metadata about the request or response. They tell the server (or client) how to handle the data.

Common request headers:

Content-Type: Tells the server the format of the request body. Example: application/json.

Accept: Tells the server what response format the client expects. Example: application/json.

Authorization: Carries authentication credentials. Example: Bearer <token>.

X-Request-ID: A unique ID for tracing requests through distributed systems.

Common response headers:

Content-Type: The format of the response body.

Cache-Control: Instructions for caching the response.

X-RateLimit-Remaining: How many requests the client has left in the current window.

Headers are how clients and servers communicate context beyond just the data itself.

Q9. What is Postman and how is it used in API testing?

Postman is a popular tool for designing, testing and documenting APIs. It gives you a visual interface to send HTTP requests and inspect responses without writing any code.

With Postman, you can:

• Send GET, POST, PUT, PATCH and DELETE requests to any endpoint.

• Set headers, authentication and request bodies.

• Write automated tests using JavaScript to validate responses.

• Organize requests into collections for an entire API.

• Run collections automatically with Newman (Postman's CLI runner).

• Mock APIs to test frontend code before the backend is ready.

• Generate API documentation directly from your collections.

Postman is especially useful for manual testing during development and for sharing API examples with your team.

Q10. What is the difference between synchronous and asynchronous APIs?

This question tests how well you understand request handling and system performance—very important in real-world API design.

Read Also: What is FastAPI

API Interview Questions for Experienced Professionals

These questions test how you think and problem-solve in real-world situations. They are common in senior and technical interviews.

Q1. What are the six REST constraints and how do they guide API design?

Roy Fielding defined six architectural constraints that make an API truly RESTful:

• Client-Server: The client and server are separate. The client handles the UI; the server handles data storage and logic. This separation lets both evolve independently.

• Stateless: Every request contains all the information needed to process it. The server stores no session state. This enables horizontal scaling.

• Cacheable: Responses must indicate whether they can be cached. Caching reduces server load and improves performance.

• Layered System: A client should not be able to tell whether it is talking directly to the server or through an intermediary (like a load balancer or CDN). This allows you to add security or caching layers without changing the API.

• Uniform Interface: Resources are identified by URIs. Representations (like JSON) are separate from the resource itself. Responses include enough information for clients to understand how to modify the resource.

• Code on Demand (optional): Servers can send executable code to clients, like JavaScript. This is the only optional constraint.

These constraints together make REST APIs scalable, maintainable and interoperable.

Q2. How do you design an API for high availability and scalability?

Designing for scale involves several strategies:

• Statelessness: Store session state in a distributed cache (like Redis) instead of on the server. This allows any server instance to handle any request.

• Caching: Cache expensive or frequently read responses using HTTP Cache-Control headers, CDNs, or an in-memory cache like Redis. This reduces database load significantly.

• Rate limiting: Protect your API from traffic spikes and abuse. Implement rate limits per API key or user.

• Load balancing: Distribute traffic across multiple server instances. Use health checks to route traffic away from unhealthy instances automatically.

• Circuit breakers: Detect when a downstream service is failing and stop sending requests to it temporarily. This prevents cascading failures.

• Pagination: Never return an unbounded list of records. Always paginate large collections using limit/offset or cursor-based pagination.

• Async processing: Offload slow operations to background jobs or message queues (like RabbitMQ or Kafka).

• Monitoring and alerting: Use tools like Prometheus, Datadog, or New Relic to track latency, error rates and throughput in real time.

Q3. What is OAuth 2.0 and how does it work?

OAuth 2.0 is an authorization framework that lets users grant third-party applications access to their resources without sharing their password.

Here is how the authorization code flow works:

• The user clicks "Login with Google" on your app.

• Your app redirects the user to Google's authorization server with a request for specific permissions (scopes).

• The user logs in to Google and approves the permissions.

• Google redirects the user back to your app with an authorization code.

• Your app exchanges that code for an access token by making a server-to-server call to Google.

• Your app uses the access token to make API requests on the user's behalf.

• When the access token expires, your app uses the refresh token to get a new one.

OAuth 2.0 is the industry standard for API authorization. It is used by Google, Facebook, GitHub, Stripe and virtually every major platform.

Q4. What is JWT and how does it work in API authentication?

A JWT (JSON Web Token) is a compact, self-contained token used to transmit authentication information between a client and a server.

A JWT has three parts separated by dots:

header.payload.signature

Header: Contains the token type and the signing algorithm (e.g., HS256).

Payload: Contains claims — user ID, roles, expiration time and any other data.

Signature: A cryptographic signature that verifies the token has not been tampered with.

Here is how it works:

• The user logs in with their credentials.

• The server verifies the credentials and generates a JWT signed with a secret key.

• The server sends the JWT to the client.

• The client stores the JWT (usually in memory or an HTTP-only cookie) and sends it in the Authorization header on every subsequent request: Authorization: Bearer <token>.

• The server verifies the signature on each request. If valid, it processes the request. If not, it returns 401.

JWTs are stateless because the server does not need to look up a session in a database. The token itself carries all the information.

Q5. What is idempotency and which HTTP methods should be idempotent?

An operation is idempotent if making the same request multiple times produces the same result as making it once.

GET is idempotent because reading the same resource repeatedly does not change anything.

DELETE is idempotent because deleting a resource that is already deleted still results in the same state (the resource does not exist). You might get a 404 on subsequent calls, but the state of the system is the same.P

PUT is idempotent because replacing a resource with the same data repeatedly produces the same result.POST is not idempotent because calling it multiple times creates multiple resources.Idempotency matters a lot in distributed systems. If a network request fails, clients can safely retry idempotent requests without worrying about duplicate side effects.

Q6. How do you handle API versioning without breaking existing clients?

The key is to maintain backward compatibility as long as possible and give clients advance notice before removing old versions. Best practices:

• Add, do not remove: When evolving an API, add new fields and endpoints. Avoid removing or renaming existing ones.

• Deprecate with notice: Mark old endpoints as deprecated in the documentation and in response headers (Deprecation: true). Give clients at least 6-12 months to migrate.

• Version your API: Use URI versioning (/v1/, /v2/) so old clients can keep using the previous version while new clients adopt the latest.

• Use semantic versioning: Follow major, minor, patch versioning in your documentation. Breaking changes require a major version bump.

• Maintain old versions: Keep old versions running long enough for all clients to migrate. Use a sunset date (Sunset: Sat, 31 Dec 2025 00:00:00 GMT header) to communicate the shutdown date.

• Test backward compatibility: Add contract tests that verify your new API responses are backward compatible with what old clients expect.

Q7. What is an API gateway and what are its responsibilities?

An API gateway is a server that acts as the single entry point for all client requests to your backend services. It sits in front of your microservices and handles cross-cutting concerns.

Key responsibilities:

• Routing: Direct incoming requests to the correct backend service.

• Authentication and authorization: Validate tokens before requests reach backend services.

• Rate limiting: Enforce request limits per client.

• Load balancing: Distribute traffic across service instances.

• SSL termination: Handle HTTPS so backend services do not need to.

• Request/response transformation: Modify headers, aggregate responses from multiple services.

• Logging and monitoring: Centralize request logs and metrics.

• Caching: Cache common responses to reduce backend load.

Popular API gateways include AWS API Gateway, Kong, NGINX and Apigee. In a microservices architecture, the API gateway is essential for managing complexity.

Q8. What is the difference between SOAP and REST and when would you choose SOAP?

SOAP is more rigid and secure, while REST is lightweight and flexible.

Choose SOAP when:

• You need message-level security (not just transport-level). WS-Security encrypts the message body, not just the channel.

• You are working with enterprise systems that already use SOAP (banking, insurance, government).

• You need formal contracts (WSDL) that define the API structure strictly.

• You need ACID-compliant transactions across multiple operations.

Choose REST for everything else. REST is simpler, faster and easier to use with modern web and mobile clients.

Q9. What is API security testing and what vulnerabilities should you test for?

API security testing looks for weaknesses that attackers could exploit. The OWASP API Security Top 10 is the industry standard list of API vulnerabilities:

• Broken Object Level Authorization (BOLA): A user accesses another user's data by changing an ID in the URL. Test by swapping resource IDs across accounts.

• Broken Authentication: Weak token validation, no token expiry, tokens not invalidated on logout.

• Excessive Data Exposure: The API returns more fields than necessary. Sensitive fields (passwords, tokens, internal IDs) should never appear in responses.

• Lack of Rate Limiting: No protection against brute force or denial-of-service attacks.

• Broken Function Level Authorization: Regular users can access admin endpoints.

• Injection attacks: SQL injection, NoSQL injection, command injection through request parameters.

• Improper Assets Management: Old, undocumented API versions are still accessible.

• Security Misconfiguration: Default credentials, unnecessary HTTP methods enabled, verbose error messages exposing stack traces.

Security testing should be automated in your CI/CD pipeline and supplemented by manual penetration testing.

Q10. What is pagination in REST APIs and what are the common patterns?

Pagination splits large result sets into smaller chunks so the API does not return thousands of records in a single response.

Offset-based pagination:

GET /users?limit=20&offset=40

Returns 20 users starting from position 40. Simple to implement, but performance degrades on large datasets because the database still scans all records up to the offset.

Cursor-based (keyset) pagination:

GET /users?limit=20&after=eyJpZCI6NDB9

Uses an opaque cursor (usually a Base64-encoded value) that points to a specific record. Much more efficient for large datasets. Used by Twitter, Facebook and Stripe.

Page-based pagination:

GET /users?page=3&per_page=20

Familiar to users but has the same performance limitations as offset-based.

Link headers: Good APIs include a Link header in the response with URLs for the next, previous, first and last pages so clients do not have to construct URLs themselves.

Cursor-based pagination is the best choice for high-volume APIs where data changes frequently.

Read Also: MuleSoft API Integration

Scenario-Based API Interview Questions

These questions test how you think and problem-solve in real-world situations. They are common in senior and technical interviews.

Q1. Your API is returning 500 errors intermittently. How do you debug it?

Begin by gathering information before making any changes.

Step 1: Check your logs. Look at application logs and server logs around the time the errors occurred. Look for stack traces, database errors, or third-party service failures.

Step 2: Check your monitoring dashboards. Review metrics for CPU, memory, database connection pool usage and response times. Spikes in any of these can cause 500 errors.

Step 3: Reproduce the issue. Try to identify a specific request, payload, or condition that triggers the error. Intermittent errors are often caused by race conditions, timeouts, or specific data inputs.

Step 4: Check external dependencies. If your API calls other services or databases, check whether they are responding correctly. A timeout from a downstream service can cause a 500.

Step 5: Review recent deployments. Did a new deployment happen around the time the errors started? Roll back to confirm.

Step 6: Add tracing. If you do not already have distributed tracing (like Jaeger or Zipkin), add request IDs so you can trace a specific request through all your services.

Step 7: Fix and monitor. Once you identify the root cause, fix it and watch your error rate in real time to confirm the fix works.

Q2. A client reports that your API is very slow. How do you approach this?

Start by measuring before optimizing.

Identify where the slowness is: Use APM tools (like Datadog or New Relic) to find which endpoints are slow and what percentage of response time is spent in database queries, external API calls, or application logic.

Common causes and solutions:

• Slow database queries: Add indexes, rewrite inefficient queries, use query caching, or denormalize frequently read data.

• N+1 query problem: If your API makes one database query per item in a list, combine them into a single query using JOINs or batch lookups.

• No caching: Cache read-heavy responses in Redis. Set appropriate TTLs (time-to-live) based on how often the data changes.

• Large response payloads: Return only the fields the client needs. Support sparse fieldsets (?fields=id,name). Enable gzip compression.

• Synchronous operations that could be async: Move slow operations (sending emails, generating reports) to background jobs.

• No connection pooling: Database connections are expensive to create. Use a connection pool.

• No CDN: For static or public content, serve it from a CDN closer to the user.

After each change, measure again to confirm the improvement. Do not optimize blindly.

Q3. You need to design an API for a ride-sharing app like Uber. How do you approach it?

I will define the core resources: users, drivers, rides, payments and locations. Next, I will create a set of RESTful endpoints for booking rides, matching drivers with riders, tracking rides and processing payments. Live tracking will be accomplished through WebSockets and will support authentication, rate limiting and scalability using a microservices architecture. Our system will utilize caching and load balancing techniques and designed to support fault tolerance, high availability and secure transactions.

Q4. Your team is migrating from a monolith to microservices. How do you manage API changes during the migration?

This is a classic scenario for strangler fig pattern and careful versioning.

Use the Strangler Fig Pattern

Gradually replace pieces of the monolith with microservices, routing specific endpoints to the new services while the rest still go to the monolith. Use an API gateway to handle routing transparently.

Keep the contract the same

The clients (mobile apps, web apps, third parties) should not notice the migration. The API responses should remain identical even as the backend changes.

Run old and new in parallel

Deploy the new microservice alongside the monolith. Test the new service thoroughly before switching traffic.

Use feature flags

Route a small percentage of traffic to the new service first (canary deployment)Monitor for errors and performance issues. Gradually increase traffic.

Deprecate carefully

Once the microservice is fully validated, mark the monolith's corresponding code as deprecated and schedule its removal.

Communicate with API consumers

If you have external API consumers, inform them of the migration timeline. Give them documentation on any behavior differences.

Do not forget data migration

Moving business logic to a microservice often means splitting the database too. Plan data migration carefully to avoid data inconsistency.

Q5. How would you handle authentication for a public API that also has private premium endpoints?

The approach is tiered access control with clear separation between public and private resources.

Design the tiers:

Public (unauthenticated): Read-only, rate-limited endpoints that do not require authentication. Example: GET /products, GET /articles.

Authenticated (basic): Endpoints available to any registered user. Requires a valid API key or JWT. Example: GET /user/profile, POST /orders.

Premium: Endpoints only available to paying subscribers. Requires authentication plus a role or subscription check. Example: GET /analytics, POST /bulk-export.

Implementation:

Use OAuth 2.0 with scopes. Assign different scopes to different tiers (read:public, read:premium, write: admin).

At the API gateway level, enforce authentication for all non-public endpoints.

In the application layer, check the user's subscription status and roles before returning premium data.

Return 403 Forbidden (not 404) when an authenticated user tries to access a premium endpoint they do not have access to. Never return 404 for resources that exist but require a higher tier, as that reveals information.

Apply stricter rate limits to unauthenticated requests and more generous limits to paid subscribers.

Wrapping Up

The purpose of this blog is to offer a comprehensive guide to mastering API interview questions, beginning with basic API knowledge and including advanced real-life examples of APIs being used in business and technology.

The topics presented in this blog cover a wide range of topics related to APIs, including the following: basic concepts such as HTTP methods and JSON, all the way to more advanced topics such as API security, API scalability and system design.

Understanding the difference between the 'what' of APIs and the 'why' of APIs will provide you with confidence when answering API interview questions at any level of experience. Consistent practice and experience applying these principles will help you shine as a candidate for your desired job.

FAQs

1. How should I prepare for API interview questions effectively?

Effectively preparing yourself for API interviews should begin with knowledge of fundamental concepts, including but not limited to REST, HTTP methods and HTTP status codes. The next step in preparing for API interview questions would be to know intermediate API topics such as authentication, API versioning and API.

2. Which API topics are most important for freshers?

For entry-level API positions, a fresher's focus should be API basics, REST architecture, JSON formatting, HTTP methods, HTTP status codes and APIs' endpoints. These fundamentals are the baseline of what every fresher needs to know about API's for technical interviews.

3. Do companies expect hands-on experience with APIs?

Yes, hands-on experience is important. Using tools like Postman, building APIs and testing endpoints helps you understand real-world implementation, making you more confident and interview-ready for technical roles.