Build it: Sharding
An interactive sandbox where you drag real components together, connect them, and run a live traffic simulation to see where the design breaks.
Challenge: Your app does <strong>160 writes per second</strong>. A single database tops out at 40 r/s. A shard router is already placed. Add enough <strong>shard databases</strong> behind it so writes spread across them. Each primary handles about 40 r/s.