Problems / Design a product, hard
Design WhatsApp from an empty canvas: phones connect over WebSockets, messages are stored and then delivered in real time or by push, presence is tracked, and no accepted message is ever lost.
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: send (45%, write), sync (35%, read), presence (20%, write). Budget: $14,500 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 ≥ 99.8%; p99 latency ≤ 200 ms (send); p99 latency ≤ 200 ms (sync) |
| The message broker crashes | At 6 s the queue or stream that carries messages to recipients crashes and takes 10 s to restart. | No lost events; Success ≥ 99% |
| A server dies | At 4 s one server of the first app service on the request path dies for 10 s. | Success ≥ 99% |
| DDoS attack | At 5 s 20,000 requests a second of flood traffic arrive for 10 s, alongside real users. | Success ≥ 99%; Attack blocked ≥ 95% |
| Budget: $14,500/month | The normal day's design must cost at most $14,500 a month. | Cost ≤ $14,500/month |