Introduction
APIs allow systems to interact with each other efficiently. They are important for applications. Companies test APIs a lot to make sure they work properly. When getting ready for a tech interview, you should know about APIs and how to test them. This guide has API Interview Questions and Answers to help learners. It will help you understand the basics, learn more, and answer interview questions easily. The goal is to make you confident in a practical way. APIs are key to making applications work well. So understanding APIs and testing methods is crucial. This guide is here to help you prepare for your interview. Start your learning with our API Course Syllabus designed for beginners and professionals.
API Interview Questions for Freshers
1. What is an API?
An API is an interface that allows software applications to connect and exchange data. It works like a bridge. A client can ask a server for data. The server sends a response based on some rules.
2. What is the difference between an API and a Web Service?
- Web services are a type of API, but not all APIs are web-based services.
- Web services require a network (HTTP/HTTPS) to work.
- They usually follow standards like SOAP or REST.
- APIs can also work within a system without using a network.
3. What is a REST API?`
A REST API is a way to build web services. It is simple. Treats each piece of data as a resource that can be accessed using a unique web address. REST APIs do not remember what happened before. They use standard methods to get things done. It makes them quick and simple to use.
4. Explain the main HTTP methods used in REST.
- GET – Used to retrieve data.
- POST – Used to create new data.
- PUT – Used to modify or overwrite data.
- PATCH – Used to partially update data.
- DELETE – Used to remove data.
5. Can you explain the key parts of an HTTP request?
- Method (GET, POST, etc.).
- URI (resource address).
- HTTP version.
- Headers (extra information like content type).
- Body (data sent with the request, optional).
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6. What does “statelessness” mean in REST?
Statelessness means the server does not keep any information. It does not remember requests. Each request must have all the details. This makes APIs faster and more reliable.
7. How are REST and SOAP different web services?
- REST
- Lightweight and simple
- Uses JSON or XML
- Faster performance
- Uses HTTP methods
- Stateless
- SOAP
- Protocol with strict rules
- Uses only XML
- More secure
- Slower performance
- Can be stateful or stateless
8. What are common HTTP Status Codes?
- 200 OK – Request was successful.
- 201 Created – Resource created successfully.
- 400 Bad Request – Error in client request.
- 401 Unauthorized – Authentication required.
- 404 Not Found – Resource not found.
- 500 Internal Server Error – Server error.
9. What is Idempotency in APIs?
Idempotency in APIs means that doing the thing times has the same effect as doing it once. For example, asking for some data or deleting something is the same whether you do it once or many times. Creating data is different each time you do it, so it is not idempotent. APIs like GET and DELETE are idempotent. POST is not because it creates things each time.
10. What is JSON, and why is it preferred over XML?
JSON is a way to format data that is used in APIs. It is easy to understand and work with. It is fast. JSON works with modern applications, so it is used more often than XML.
11. How do you handle authentication in APIs?
- API Keys for simple access control.
- Basic Authentication using username and password.
- OAuth 2.0 for secure authorization.
- JWT (JSON Web Tokens) for verifying users.
12. What are Query Parameters vs. Path Parameters?
- Path parameters are used to identify a specific resource (e.g., /users/101).
- Query parameters are used to filter or sort data (e.g., /users?role=admin).
13. What is API Rate Limiting?
- It limits the number of requests a user can send.
- Prevents server overload.
- Ensures fair usage among users.
- Helps protect against cyber attacks.
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14. What is API Mocking?
API mocking means making an API. This helps developers and testers work on their projects even if the real API is not ready yet. It makes development and testing faster. API mocking is like having a stand-in for the API so people can keep working while they wait for the real thing.
15. What tools are used for API Testing?
- Postman for testing APIs manually and automatically.
- Swagger/OpenAPI for API documentation.
- JMeter for performance testing.
- REST Assured for Java-based API testing.
API Interview Questions for Experienced Candidates
1. What is the difference between REST and GraphQL, and when would you choose one over the other?
- REST is based on fixed resources and endpoints. This can sometimes lead to over-fetching (getting extra data) or under-fetching (not enough data).
- GraphQL lets clients specify exactly what data they want in one request.
When to use:
- Use REST for simple APIs, stable systems, and strong caching requirements.
- Use GraphQL for complex applications where different clients need flexible data.
2. How do you handle API versioning in a production environment?
API versioning helps maintain stability when APIs evolve.
Common approaches include:
- URI versioning (e.g., /v1/users)
- Header-based versioning using custom headers
- Query parameter versioning (e.g., ?version=1)
In real projects, developers keep older versions active while slowly moving users to newer versions to avoid breaking existing systems.
3. What are the primary security concerns for enterprise-grade APIs?
API development places a strong focus on security. Key concerns include:
- Strong authentication and authorization (OAuth2, JWT).
- Data encryption using TLS/SSL.
- Input validation to prevent attacks like SQL injection and XSS.
- Rate limiting to protect against DDoS attacks.
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4. How do you implement API Rate Limiting, and what algorithms are commonly used?
API rate limiting controls how many requests a user can make in a given time.
Common algorithms:
- Token Bucket – enables controlled traffic surges.
- Fixed Window – simple but may cause request spikes.
- Sliding Window – smoother and more accurate control.
5. What is HATEOAS, and how does it enhance a REST API?
HATEOAS (Hypermedia As The Engine Of Application State) means the API response includes links to guide the client on what to do next.
This helps:
- Reduce dependency on hardcoded endpoints.
- Make APIs more flexible and discoverable.
- Improve long-term maintainability.
6. Describe the role and benefits of an API Gateway.
An API Gateway acts as a single entry point for all API requests.
It helps by:
- Routing requests to the correct service.
- Handling authentication and authorization.
- Load balancing traffic.
- Protecting internal microservices.
It simplifies client communication and improves security.
7. What makes PUT and PATCH different from each other?
- PUT replaces the entire resource (full update required).
- PATCH updates only specific fields (partial update).
8. How do you handle long-running operations in a RESTful API?
Instead of keeping the request open, the API returns a 202 Accepted response.
It may also provide:
- A status-check endpoint.
- A webhook for completion notification.
This keeps the system responsive and scalable.
9. What is API Contract Testing?
- API contract testing ensures that an API follows a predefined agreement between provider and consumer (like OpenAPI specs).
- It is especially important in microservices to prevent breaking changes when services are updated.
10. How do you design an API to be resilient to failures?
To build reliable APIs, developers use:
- Circuit Breakers to stop repeated failures.
- Retries with exponential backoff.
- Timeouts to avoid system overload.
These patterns improve stability and performance under failure conditions.
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11. What are ETags, and how do they help with API performance?
ETags are unique identifiers for a version of a resource.
They help by:
- Allowing clients to check if data has changed.
- Using conditional requests (If-None-Match).
- Returning 304 Not Modified when data is unchanged.
This reduces unnecessary data transfer and improves performance.
12. How do you approach testing an API that lacks documentation?
In such cases, testers use:
- Exploratory testing.
- Tools like Postman to send requests.
- Network tools like Wireshark to inspect traffic.
This helps identify endpoints, request formats, and responses.
13. What is CORS, and why is it important for Web APIs?
- CORS (Cross-Origin Resource Sharing) is a browser security feature.
- It controls whether a web page from one domain can access resources from another domain.
- This prevents unauthorized access and improves web security.
14. How do you debug a production API failure?
To debug API issues in production:
- Check logs and monitoring tools first.
- Identify HTTP status codes (4xx or 5xx).
- Trace requests using correlation IDs across services.
- Analyze where the failure occurred in the system.
15. What are the advantages of gRPC over REST?
gRPC is faster and more efficient than REST because:
- It uses HTTP/2 instead of HTTP/1.1.
- It uses Protocol Buffers instead of JSON.
- It provides better performance for microservices.
It is commonly used for internal service-to-service communication.
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Conclusion
API knowledge is super important for both experienced people in the software industry today. API Interview Questions and Answers help you get to know the concepts. They also help you get better at solving problems and doing well in interviews. You should practice with real-world APIs. Stay updated with what’s new in API testing and development. Even if you just learn the basics, it can make an impression on the people interviewing you. The important thing is to talk clearly and present yourself confidently. These things help you make a first impression. So focus on API knowledge, practice, and clear communication. It will help you do well in API interviews. Receive expert career support from our Training and Placement Institute in Chennai.