Missions
Real problems, hard numbers.
You are handed requirements, a system that does not meet them, and a canvas. Traffic shifts while you work. Your architecture is evaluated on the trade-offs it makes, not on whether it matches an answer key.
Design a URL Shortener
Ten billion redirects a month against a system currently built for a few thousand a second.
beginner~30 min
Design a Notification System
Fan out millions of notifications without melting the providers you depend on.
intermediate~35 min
Design a News Feed
Fan-out on write, fan-out on read, and why a small number of huge accounts breaks both.
advanced~45 min
Design a Web Crawler
Crawl a billion pages politely, without recrawling what you already have or hammering one host.
advanced~40 min