API Design Fundamentals
Interview Questions

RESTful API design is based on six principles: stateless operations, which ensure each request is completely independent; client-server architecture, dividing responsibilities; cacheability, to improve network efficiency; a uniform interface, simplifying system interaction; layered systems, to allow intermediaries for scalabilities such as load balancers; and optional code on demand, enabling client-side script execution for flexibility.

Versioning in an API can be managed through URI versioning (e.g., /v1/resource), query parameters, or custom headers. Maintaining backward compatibility is essential to avoid breaking existing client applications. Good practices include clear documentation and a structured deprecation policy.

Ensuring security in API design involves implementing robust authentication mechanisms such as OAuth, encrypting data in transit using SSL/TLS, validating inputs to prevent injections, applying rate limiting to mitigate denial-of-service attacks, and logging requests for monitoring and auditing.

In my previous role, I was tasked with designing an API for a complex microservices architecture. The main challenge was ensuring seamless data flow among multiple services while maintaining performance. After identifying bottlenecks, I restructured the API endpoints to optimize request handling and incorporated load balancing. This resulted in a 30% performance improvement and enhanced system reliability.

Effective API documentation can be achieved using Swagger to create dynamic, self-updating documentation from the source code. Including comprehensive user guides, code examples, and clear error message descriptions enhances user understanding. Maintaining a detailed change log aids developers in keeping track of version changes.

Adapting an API for mobile involves reducing payload sizes to improve load times, optimizing latency with efficient data formats (like JSON), and applying token-based authentication to manage limited resources securely. Ensuring the API functionally suits offline capabilities can also enhance mobile usability.

Idempotency in API design ensures that repeated requests produce the same result as a single request, crucial for operations like PUT and DELETE. Implementing idempotency involves ensuring these HTTP methods handle repeat requests correctly, often using unique request identifiers to track operations and maintain consistent responses.

Graceful error handling involves defining standard error codes (e.g., 400, 404), consistent message formats, and documenting these errors thoroughly. Including details that guide users toward solving issues or understanding the nature of the error, such as rate limits exceeded or invalid request parameters, improves user experience.

To optimize an API for high traffic, caching responses using tools like Redis can significantly reduce server load. Implementing load balancing distributes requests efficiently across servers. Asynchronous processing, using queues such as RabbitMQ, offloads intense processing tasks, improving API response times. Monitoring and scaling resources based on traffic demands ensure optimal performance.

API throttling is achieved through rate limiting, which restricts the number of requests a client can make in a given timeframe. Quotas allocate resource usage efficiently, ensuring fair access to all clients. Employing API gateways like AWS API Gateway or Apigee provides built-in throttling and monitoring features, enhancing resource management.

Ensuring API compliance involves implementing data encryption both in transit and at rest, strict access controls to protect sensitive information, comprehensive logging for audit trails, and adherence to regulations such as GDPR by implementing and reflecting privacy notices. Regular audits and assessments help maintain compliance.

API testing methods include functional testing with tools like Postman ensuring endpoints work as expected, performance testing with Apache JMeter to simulate high load and assess throughput, and continuous integration tests to verify stability in production environments. Automated regression tests further enhance long-term reliability.