Week 8 Day 5: Mini Project - Scalable Chat App

This is it. The Final Project. We will build a Chat App that scales across multiple servers using Redis Adapter.

The Problem with simple WebSockets

Using socket.io on one server works. But if User A is on Server 1, and User B is on Server 2, they cannot talk. Server 1 doesn’t know about User B’s socket.

Solution: Redis Pub/Sub Adapter

When A sends message, Server 1 publishes it to Redis. Redis broadcasts to Server 2. Server 2 sees the message and pushes it to User B.

Implementation

1. Setup

npm install express socket.io socket.io-redis

2. Server Code (server.js)

const express = require('express');
const http = require('http');
const { Server } = require("socket.io");
const { createClient } = require("redis");
const { createAdapter } = require("@socket.io/redis-adapter");

const app = express();
const server = http.createServer(app);

// Redis Client
const pubClient = createClient({ url: "redis://localhost:6379" });
const subClient = pubClient.duplicate();

Promise.all([pubClient.connect(), subClient.connect()]).then(() => {
    const io = new Server(server);
    
    // Bind Redis Adapter
    io.adapter(createAdapter(pubClient, subClient));

    io.on('connection', (socket) => {
        console.log(`User connected on ${process.env.PORT}`);

        socket.on('chat message', (msg) => {
            // Broadcast to everyone (across all servers)
            io.emit('chat message', msg);
        });
    });

    const PORT = process.env.PORT || 3000;
    server.listen(PORT, () => {
        console.log(`Server running on port ${PORT}`);
    });
});

3. Running multiple instances

1. PORT=3001 node server.js
2. PORT=3002 node server.js
3. Connect Client A to 3001.
4. Connect Client B to 3002.
5. Send message. It works! Redis bridged the servers.

Graduation 🎓

You have completed the 8-Week System Design Roadmap. You built:

• URL Shortener
• Load Balanced Site
• Cached API
• Replicated Store
• Async Job Queue
• Microservices
• Secured API
• Global Chat

You are now ready to tackle scalable systems. Go build something huge. 🚀