API Gateway

An API Gateway is a critical component in system design, particularly in microservices architecture. It acts as a single entry point for all client requests and routes them to the appropriate backend services.

Features of API Gateway

Feature	Purpose
Routing	Request goes to the right microservice
Auth	Verifies user credentials (e.g., JWT)
Rate Limiting	Protects backend from overload
Caching	Reduces load, speeds up repeated queries
Load Balancing	Distributes traffic across instances
Monitoring	Logs metrics for observability

Why Use an API Gateway?

In microservices, services are often broken into small units (e.g., auth, order, payment, etc.). If clients (like mobile apps or browsers) directly interact with all these, it:

• Increases complexity
• Exposes internal service structure
• Requires handling cross-cutting concerns multiple times

An API Gateway abstracts these concerns and centralizes them.

API Gateway Worflow

Let’s say you’re designing an e-commerce system:

Services:

• User Service (/users)
• Product Service (/products)
• Order Service (/orders)

Client Flow:

• The client sends a request to /api/products.
• The API Gateway receives the request.
• It authenticates the user (via token).
• It routes the request to the Product Service.
• The response comes back to the gateway.
• The gateway may transform the response.
• The client gets the response.

Example of API Gateway

// api-gateway.js
const express = require("express");
const { createProxyMiddleware } = require("http-proxy-middleware");

const app = express();

// Middleware: Log all requests
app.use((req, res, next) => {
  console.log(`[Gateway] ${req.method} ${req.originalUrl}`);
  next();
});

// Forward /users requests
app.use(
  "/users",
  createProxyMiddleware({
    target: "http://localhost:5001", // User Service
    changeOrigin: true,
  })
);

// Forward /products requests
app.use(
  "/products",
  createProxyMiddleware({
    target: "http://localhost:5002", // Product Service
    changeOrigin: true,
  })
);

// Forward /orders requests
app.use(
  "/orders",
  createProxyMiddleware({
    target: "http://localhost:5003", // Order Service
    changeOrigin: true,
  })
);

app.listen(3000, () => {
  console.log("API Gateway running on port 3000");
});

Benefits and Trade-offs of API Gateway

Benefits	Trade-offs
Simplifies client interaction	Single point of failure (if not replicated)
Centralizes security and logging	Adds latency (extra network hop)
Enables decoupled service development	Can become complex (requires scaling & config management)
Supports versioning, throttling, caching