Build it: Design a News Feed
An interactive sandbox where you drag real components together, connect them, and run a live traffic simulation to see where the design breaks.
Challenge: A post must be written into every follower's home timeline, but doing that while the user waits would freeze the post for popular accounts. Fan-out work is arriving at 40 jobs per second. Each worker drains 20 per second. Drop the fan-out work into a queue and add enough workers so posting stays instant and nothing piles up.