Step 1 of 13 · Intro

Design a chat app

Messages delivered in order, with ticks, to phones that are often offline.

How it works

• ›Send: the message reaches a gateway holding the connection.
• ›Deliver: the recipient's gateway pushes it, or it waits in storage.

What you'll build

1. The API and the data behind it
2. A gateway and a database for 500 requests a second
3. A queue, workers and a cache for 15,000, then 50,000
4. A rate limiter that stops a spam flood
5. A design that survives a cold cache, a lost gateway, a lost worker and a lost primary