Problems / Design a product, hard
Design TikTok’s For You feed from an empty canvas: rank candidate videos with a model, play them from a CDN and stream watch events back into the ranker’s features, at 3,000 requests a second.
Unlock it once for 25 credits; everything inside it is free (runs, changes, Submit, results, hints). Sign in with Google to start; new accounts get free credits.
Request types to route: video (55%, read), feed (20%, read), event (25%, write). Budget: $14,000 a month.
You start from an empty canvas. When you run the tests, each one simulates your design with its own traffic and failures:
| Test | What it does | Passes when |
|---|---|---|
| Normal day | The normal day: the traffic in the brief, nothing broken. | Success ≥ 99.9%; p99 latency ≤ 400 ms (feed); p99 latency ≤ 300 ms (video); Success ≥ 99.9% (event) |
| A trend takes off | At 5 s a trend takes off: traffic triples for 8 s. | Success ≥ 99%; p99 latency ≤ 600 ms (feed) |
| A server dies | At 4 s one server of the first app service on the request path dies for 10 s. | Success ≥ 99% |
| Ranking model 10× slower | At 5 s the ranking model answers 10× slower for 8 s, as after a bad model push or a lost GPU node. | Success ≥ 99% (feed); p99 latency ≤ 800 ms (feed) |
| Budget: $14,000/month | The normal day's design must cost at most $14,000 a month. | Cost ≤ $14,000/month |