Objectives

• Understand the Pub/Sub (Publish–Subscribe) model.
• Install and run RabbitMQ using Docker.
• Use amqplib to connect to RabbitMQ in Node.js.
• Publish messages from order-service.
• Consume messages in notification-service.

1. What is RabbitMQ?

RabbitMQ is a message broker that enables services to send messages to each other via queues, allowing asynchronous, decoupled communication.

Instead of one service calling another directly, it publishes a message to a queue — and other services subscribe to that queue and act upon it.

2. Set Up RabbitMQ with Docker

Add this service to your main docker-compose.yml (you’ll complete this in Epoch 9 — for now, run it separately):

• rabbitmq-docker.yml

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  version: '3'
  services:
  rabbitmq:
    image: rabbitmq:3-management
    ports:
      - "5672:5672"       # AMQP protocol
      - "15672:15672"     # Management UI
  

Start RabbitMQ:

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docker compose -f rabbitmq-docker.yml up -d

• Check the UI at: http://localhost:15672
• Login: guest / guest

  3. Publish Events from Order Service

Step 1: Install amqplib

In the order-service:

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npm install amqplib

Step 2: Create rabbitmq.js utility

In utils/rabbitmq.js:

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import amqplib from 'amqplib';

let channel;

export async function connectRabbitMQ() {
  const connection = await amqplib.connect('amqp://localhost');
  channel = await connection.createChannel();
  await channel.assertQueue('order_created');
}

export function publishOrderCreated(order) {
  channel.sendToQueue('order_created', Buffer.from(JSON.stringify(order)));
}

Step 3: Modify order creation to publish event

In controller after order.save()

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import { publishOrderCreated } from '../utils/rabbitmq.js';

await publishOrderCreated({
  orderId: order._id,
  userId: order.userId,
  totalItems: order.items.length,
  createdAt: order.createdAt,
});

Call connectRabbitMQ() when server starts.

4. Consume Events in Notification Service

Step 1: Install amqplib

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npm install amqplib

Step 2: Create consumer logic

In src/consumer.js

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import amqplib from 'amqplib';

async function consumeNotifications() {
  const conn = await amqplib.connect('amqp://localhost');
  const channel = await conn.createChannel();
  await channel.assertQueue('order_created');

  channel.consume('order_created', (msg) => {
    const content = JSON.parse(msg.content.toString());
    console.log('🔔 Received Order Event:', content);
    channel.ack(msg);
  });
}

consumeNotifications();

Run this file along with the notification server:

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node src/consumer.js

5. Diagram – Event Flow