Problems / Design a product, easy
Design Calendly from an empty canvas: anyone with a link sees a host’s free slots and books one, never twice, with calendars kept in sync.
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Request types to route: availability (87%, read), sync (10%, write), book (3%, write). Budget: $3,600 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 ≤ 150 ms (availability); p99 latency ≤ 500 ms (book); No lost updates |
| A popular host shares a link | At 5 s a popular host’s link goes round: traffic triples for 8 s, many people racing for the same slots. | Success ≥ 99%; p99 latency ≤ 300 ms (availability); No lost updates |
| Cache flushed | At 5 s the cache on the request path is emptied, as after a restart or a bad deploy. | Success ≥ 99%; p99 latency ≤ 500 ms (availability) |
| A server dies | At 4 s one server of the first app service on the request path dies for 10 s. | Success ≥ 99% |
| Budget: $3,600/month | The normal day's design must cost at most $3,600 a month. | Cost ≤ $3,600/month |