Explain the concept of an API Gateway in a microservices architecture. What are its primary functions, and how does it contribute to the overall system design?

MECE Framework: Define API Gateway as a single entry point. Detail its primary functions: Request Routing (directing to microservices), API Composition (aggregating multiple services), Authentication/Authorization (security enforcement), Rate Limiting/Throttling (traffic management), and Caching (performance optimization). Explain its contribution to system design by decoupling clients from microservices, simplifying client-side development, enhancing security, improving performance, and enabling easier microservice evolution without client impact. Emphasize its role in managing cross-cutting concerns.

An API Gateway acts as a single entry point for all client requests, sitting in front of a collection of microservices. Its primary functions include Request Routing, directing incoming requests to the appropriate microservice based on predefined rules; API Composition, aggregating responses from multiple microservices into a single client-friendly response; Authentication and Authorization, enforcing security policies and validating client credentials; Rate Limiting and Throttling, controlling traffic to prevent abuse and ensure service stability; and Caching, storing frequently accessed data to improve response times and reduce microservice load. It contributes significantly to system design by decoupling clients from the underlying microservice architecture, simplifying client-side development by providing a consistent interface, enhancing security through centralized policy enforcement, improving performance via caching and load balancing, and enabling independent evolution of microservices without impacting client applications. This centralizes cross-cutting concerns, making the system more manageable and resilient.