Build it: Design a URL Shortener
An interactive sandbox where you drag real components together, connect them, and run a live traffic simulation to see where the design breaks.
Challenge: Design the read path for a URL shortener. The user sends 150 redirects/sec. Each app server handles 40 r/s; the database only 30 r/s. Most reads are for the same hot links. Build a system that serves all 150 r/s without melting the database.