API Gateway Integration and Aggregation Layer

What Is an API Gateway?

An API Gateway is the entry point for all client requests in a microservices system.

It routes, aggregates, transforms, and protects traffic between clients and services.

You can think of it as the traffic controller for your backend.

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Why Use a Gateway in Microservices?

Without a gateway:

• Clients must know internal service URLs
• Every client must handle service changes
• Cross-cutting concerns (auth, rate limit, logging) are repeated

With a gateway:
✅ Simplified client access
✅ Centralized security and monitoring
✅ Flexible routing and aggregation

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Gateway Responsibilities

An API Gateway can handle:

• Reverse proxying
• Request routing
• Rate limiting
• Authentication (e.g., JWT validation)
• Aggregation (multiple services → one response)
• Caching
• Load balancing
• Logging and metrics

Tools:

• Kong
• NGINX
• Traefik
• Express (custom gateway)
• BFF or GraphQL Gateway

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Implementing a Basic Gateway (Express + Proxy)

// gateway.ts
import express from 'express';
import proxy from 'express-http-proxy';

const app = express();

app.use('/users', proxy('http://user-service:3001'));
app.use('/products', proxy('http://product-service:3002'));
app.use('/orders', proxy('http://order-service:3003'));

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

Add auth middleware if needed:

app.use('/orders', verifyJWT, proxy('http://order-service:3003'));

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Aggregation Layer (BFF or GraphQL)

Sometimes the frontend needs data from multiple services.

Instead of making 3 HTTP calls from the client:

GET /users/me  
GET /orders?userId=123  
GET /notifications?userId=123

Build an aggregation layer:

• BFF (Backend for Frontend)
• GraphQL Gateway (Apollo, Hasura, StepZen)

Example: GraphQL query to combine user + orders

{
  me {
    name
    email
    orders {
      id
      total
    }
  }
}

The gateway resolves data from multiple services and returns it as one JSON response.

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Summary

An API Gateway simplifies communication between clients and services by routing, protecting, and aggregating requests. You can implement it as a reverse proxy or a full GraphQL-powered data orchestrator.

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Next:
Lesson 19 – Testing and Debugging Microservices