5. Design a Chat System

The brief

Design private one-to-one and small-group chat with multiple devices per user. Explain message acceptance, durable history, online delivery and reconnect synchronization. State what ordering and read receipts mean when two devices send concurrently or go offline.

• Support 1 million concurrently connected users and 50,000 new messages/second at peak.
• Groups contain at most 100 users, and a device may be offline for seven days.
• Clients retry after lost acknowledgements; attachments are referenced files rather than inline message bytes.

Constraints

Online delivery latency≤ 500 milliseconds

Declare p95 acceptance-to-recipient delay for connected users in a healthy region.

Acknowledgement contract

A sender must know when a message is durably accepted versus merely written to a socket.

Private history

Authorize delivery and history reads using a stated group-membership policy.

What to cover

1. 01

   Messaging protocol

   Define send IDs, acknowledgements, conversation ordering and receipt states.

2. 02

   Storage and connection routing

   Show history ownership, live connections and attachment access.

3. 03

   Reconnect walkthrough

   Trace offline catch-up, duplicates and two devices sending concurrently.

4. 04

   Capacity and failure

   Estimate connection and message load; explain gateway loss and group fan-out.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Design a Chat App like WhatsApp

Chat for a billion users: sockets on gateways, Redis pub/sub routing, an inbox per device, receipts, media in S3, encryption and regions: one-hour boards for junior, senior and staff, with the theory behind them.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.