This is Fine 🔥

Four database shards, users spread evenly by key. Tuesday, 3 AM: a brand-new account posts a meme, and by breakfast it is everywhere. Millions are loading one profile, and everyone who laughs smashes follow. Shards 1, 3, and 4 are having a quiet morning. The router is moving every request for that user to shard 2, where their key lives. The Job: Survive the user's fifteen minutes of fame before it's over for the shard.

Free to play, no account needed. A deterministic engine grades your design against this scenario's SLA - error rate, latency, and cost.