Building a Scalable Chat App

Real-time + Horizontal Scalability

Introduction

When I first started building this chat app, there was one requirement I had in mind: it has to be real-time, scalable, and easy to scale up. Sounds easy—until you introduce multiple servers to the mix.

In this article, I'm going to walk you through how I built a horizontally scalable chat app using:

• Socket.io for real-time messaging

• Express.js as the backend framework

• Redis Pub/Sub for synchronizing messages between servers

This is a simple, yet strong—and horizontally scalable for production traffic capable—system.

Architecture Overview

Here’s a quick look at the architecture I implemented:

• Users 1–3 are connected to App Server A

• User 4 is connected to App Server B

• Both servers are connected to Redis

• Redis handles message broadcasting between servers

With this setup, the system is horizontally scalable—add app servers whenever needed.

Why Redis Pub/Sub?

As your application grows, you can no longer rely on a single server. However, if there are lots of servers, broadcasting a chat message to everyone is tricky.

Here's why:

• Suppose User 1 is on Server A, and User 4 is on Server B. A message from one server will never reach the other user—unless servers talk to one another.

That's when Redis Pub/Sub comes into play:

• Each app server subscribes to a Redis channel (e.g. chat)

• When a message is posted, it's published to the channel

• All servers receive it and emit to their connected clients

So Redis is essentially a router of messages between servers.

Test Scenario

Here’s what I tested:

• User 1 sends a message → hits Server A → published to Redis

• Server B receives it via Redis → emits to User 4

• Everyone sees the message in real-time

So even though users may be on different app servers, the chat is synchronized.

Why This Architecture Works

• Stateless app servers – easy to scale horizontally

• Redis handles cross-server communication

• No message loss or delay

• Production-ready

Try It Out

• Live Demo: errasor.io

• Source Code: GitHub Repo

Final Thoughts

Redis Pub/Sub + Socket.IO is an excellent pairing to build highly scalable real-time systems. If you're working on anything from chat through multiplayer games or real-time dashboards—this pattern is worth learning.

I hope you enjoyed this..! Feel free to share your thoughts, questions, or ideas for future topics in the comments.

Thanks for reading

Happy coding!

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