Load Balancer vs. Reverse Proxy vs. API Gateway

📋 Overview

While these three components often overlap in functionality (and are sometimes provided by the same software), they serve distinct architectural purposes. A Load Balancer focuses on Efficiency, a Reverse Proxy on Security/Abstraction, and an API Gateway on Management/Governance of microservices.

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🏗️ Core Principles & Characteristics

• Load Balancer: Distributes incoming traffic across a group of backend servers to prevent overload. (e.g., Round Robin, Least Connections).
• Reverse Proxy: Acts as an intermediary for servers. Hides the server's identity, handles SSL, and caches content.
• API Gateway: A specialized reverse proxy that handles "cross-cutting concerns" for microservices: Auth, Rate Limiting, Request Transformation, and Service Discovery.

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⚖️ Trade-offs: Pros & Cons

• Load Balancer:
  • Pros: Essential for horizontal scaling.
  • Cons: Simple; doesn't know about business logic or users.
• Reverse Proxy:
  • Pros: Great for security and static content caching.
  • Cons: Management overhead for multiple backend routes.
• API Gateway:
  • Pros: Simplifies client code; provides a single entry point for complex microservices.
  • Cons: Can become a Single Point of Failure and a performance bottleneck if not scaled correctly.

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🌍 Real-World Implementation

• Nginx: Can act as both a Reverse Proxy and a basic Load Balancer.
• HAProxy: The gold standard for high-performance L4/L7 Load Balancing.
• Kong / Amazon API Gateway: Used for managing API keys, rate limits, and routing in large-scale microservice environments.

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💡 Interview "Gotchas" & Tips

• The Hierarchy: A Load Balancer is a type of Reverse Proxy. An API Gateway is a type of Reverse Proxy.
• Protocol Translation: API Gateways often convert HTTP/JSON from the client into gRPC or AMQP (RabbitMQ) for internal service communication.
• Edge Functions: Modern API Gateways can run "Serverless" logic at the edge (e.g., checking a JWT before the request hits any backend).

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📐 Suggested Architecture Primitives

• ALB/NLB: Cloud-native load balancing.
• Ingress Controller: The K8s implementation of these patterns.
• Circuit Breaker: Often implemented within the API Gateway or Load Balancer.
• BFF (Backend-for-Frontend): A specific pattern where an API Gateway is built for a specific UI (Mobile vs. Web).