Problems / Design a product, hard
Design ad click aggregation from an empty canvas: take 2,850 clicks a second without losing or double-counting one, aggregate them per ad per minute on a stream, and serve advertisers’ reports.
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Request types to route: click (95%, write), report (5%, read). Budget: $4,000 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 ≤ 80 ms (click); p99 latency ≤ 1.5 s (report) |
| A Super Bowl ad | At 5 s a Super Bowl ad airs: traffic triples for 8 s. | Success ≥ 99%; p99 latency ≤ 100 ms (click) |
| The event stream goes down | At 5 s the event stream on the request path crashes; it comes back 10 s later. While this test runs, brokers redeliver unacknowledged messages after 5 s and 0.2% of acknowledgements are lost, as in production: only idempotent consumers avoid duplicates. | No lost events; Success ≥ 99% (click); No duplicate side effects |
| A server dies | At 4 s one server of the first app service on the request path dies for 10 s. | Success ≥ 99% |
| Budget: $4,000/month | The normal day's design must cost at most $4,000 a month. | Cost ≤ $4,000/month |