System Design Simulator

Chat App

Design 1:1 and group chat with online presence and fan-out.

Al Beltran Engineering Lab

System design

Senior

Chat App

Design a chat system for 5 million daily users. Support 1:1 and group rooms, presence, and near-real-time delivery. History must be searchable later, but the live path should stay thin.

Components

How do you deliver a group message?

Write a copy into every member inbox on sendWrite once to the room; push via connections or a queueEmail every member

Where does presence live?

A SQL row updated on every heartbeatAn in-memory store with TTLs and heartbeatsObject storage

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How the challenge works

Live delivery and durable history are different jobs. Split them.
Presence is ephemeral. Do not treat it like an orders table.

Key concepts

Fan-out on write vs read

Huge groups should not clone a message into millions of inboxes on send.

Backpressure

A queue plus workers keep typing indicators and push from blocking the write API.

Common mistakes

Using a CDN as the realtime plane.
Storing last-seen timestamps in a heavily locked SQL table.

Recommended architecture

Websocket or long-poll API, cache for presence, database for messages, queue/workers for push and search indexing.

Interview tips

Ask about group size. 8-person rooms and 200k-person rooms are different products.

Related challenges

Built by Al Beltran