Okay, let's move on to 2.4.c API Gateways.

• Definition: An API Gateway is a server or service that acts as a single entry point for client requests destined for backend services, particularly in a microservices architecture. It sits between the clients (e.g., web browsers, mobile apps) and the collection of backend microservices, acting as a reverse proxy tailored for API traffic.

• The Problem Solved: Imagine a client (like a mobile app) needing data from multiple microservices (e.g., user profile service, order history service, product recommendations service) to display a single screen. Without an API Gateway:

  • The client would need to make separate requests to each microservice.
  • The client would need to know the specific network locations (addresses, ports) of all these services.
  • Common concerns like authentication, rate limiting, and logging would need to be handled by each individual microservice, leading to duplication and inconsistency.
  • This approach makes the client complex, increases the number of network round trips (latency), and tightly couples the client to the internal microservice structure.

• Functions and Responsibilities of an API Gateway: An API Gateway abstracts the backend complexity and provides a unified interface for clients. Its key responsibilities often include:

  • Request Routing: Intelligently routes incoming client API requests to the appropriate downstream microservice based on the request path, headers, method, etc.
  • API Composition / Aggregation: Can receive a single client request and invoke multiple backend microservices, aggregating their responses into a single response for the client. This reduces chattiness between the client and backend.
  • Protocol Translation: Can translate between client-friendly protocols (like REST over HTTP) and protocols used internally between services (like gRPC).
  • Authentication & Authorization: Centralizes user authentication (verifying identity) and authorization (checking permissions) before forwarding requests to internal services. Services downstream might trust requests coming via the gateway.
  • Rate Limiting & Throttling: Protects backend services by enforcing limits on the number of requests a client can make in a given period.
  • Caching: Caches responses from backend services to reduce latency and load.
  • Logging & Monitoring: Provides a central place to log requests, responses, and errors for monitoring, analytics, and debugging.
  • SSL/TLS Termination: Handles incoming HTTPS connections, decrypts requests, and potentially communicates with internal services over plain HTTP (if the internal network is secure), reducing the SSL overhead on backend services.
  • Load Balancing: Can perform load balancing across instances of backend services (though often used in conjunction with dedicated load balancers).
  • Request/Response Transformation: Can modify requests or responses on the fly (e.g., changing data formats, adding/removing headers).

• Benefits:

  • Simplified Client: Clients interact with a single, consistent API endpoint, unaware of the underlying microservice complexity.
  • Encapsulation: Hides the internal architecture and allows backend services to be refactored or changed without impacting clients directly.
  • Centralized Cross-Cutting Concerns: Avoids duplicating logic for authentication, rate limiting, logging, etc., in every microservice.
  • Improved Security: Provides a single choke point for enforcing security policies.
  • Potentially Improved Performance: Caching and request aggregation can reduce overall latency for clients.

• Drawbacks / Challenges:

  • Potential Bottleneck / Single Point of Failure: Since all traffic flows through the gateway, it must be highly available, scalable, and performant. If it fails, all APIs become unavailable. Requires robust deployment and monitoring.
  • Increased Latency: Adds an extra network hop for requests.
  • Development & Management Complexity: The API Gateway itself is another critical component that needs to be developed, configured, deployed, managed, and scaled.
  • Risk of Becoming a "God Gateway": If too much business logic creeps into the gateway, it can become a complex monolith itself, negating some benefits of microservices.

• Popular API Gateway Technologies:

  • Cloud Provider Services: AWS API Gateway, Google Cloud API Gateway / Apigee, Azure API Management.
  • Open Source / Self-Hosted: Kong Gateway, Traefik, Spring Cloud Gateway, Ocelot.

• In an Interview: Understand the role of an API Gateway as a façade or single entry point for backend services (especially microservices). Be able to list several key functions (routing, auth, rate limiting, aggregation). Discuss the benefits (client simplification, centralized logic) and drawbacks (potential bottleneck, complexity). Propose using one when designing systems with multiple backend services accessed by external clients.