Problems / Design a product, medium
Design a digital wallet from an empty canvas: instant balances, transfers on a ledger, a fraud check before money moves, and notifications.
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Request types to route: balance (70%, read), transfer (10%, write), history (20%, read). Budget: $265,000 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%; Success ≥ 99% (transfer); p99 latency ≤ 150 ms (balance); p99 latency ≤ 1.5 s (transfer); p99 latency ≤ 400 ms (history) |
| Festival day | At 5 s everyone sends festival money at once: traffic jumps to 3× for 8 s. | Success ≥ 99%; p99 latency ≤ 300 ms (balance); p99 latency ≤ 2.5 s (transfer) |
| Database 4× slower | At 5 s the main database on the request path answers 4× slower for 8 s. | Success ≥ 99%; p99 latency ≤ 300 ms (balance) |
| A server dies | At 4 s one server of the first app service on the request path dies for 10 s. | Success ≥ 99% |
| Budget: $265,000/month | The normal day's design must cost at most $265,000 a month. | Cost ≤ $265,000/month |