Design a Chat App

Design a Chat App

Step 1 of 12

sockets1 M

A million phones, each holding one open socket to one of many gateways.

The Idea

Deliver a message in under a second and let any device read it back later. Assume one million connected users and 50 000 messages a second at peak, so every phone holds an open socket to one of many gateways.

Real-World Example

A hotel switchboard. Rooms never dial each other directly: the board knows which line each guest is on, connects the two, and writes the message down when nobody picks up.

The Tradeoff

Persisting the message before acknowledging puts a write on the hot path, and it is the only reason delivery survives a crashed gateway. Acknowledging first is faster and loses messages. The registry mapping user to gateway is the piece that goes stale the moment a phone drops off a train.

Your turn

Put the steps in the right order.

1. 1Ask the session registry which gateway holds the recipient↑↓
2. 2Persist the message and give it a sequence number↑↓
3. 3Push it down that socket, or queue a notification if there is none↑↓
4. 4The sender's open socket carries the message to its gateway↑↓

Check order

Mini quiz

1 / 3

Storing the message before acknowledging the sender costs a write on the hot path, and buys:

• AA smaller payload
• BDelivery that survives a gateway crash
• CLower bandwidth use
• DEnd-to-end encryption