Problems / Design a product, medium
Design Slack from an empty canvas: clients connect over WebSockets, messages fan out to channel members in real time, history is scrollable, everything is searchable and presence is live.
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Request types to route: read (50%, read), send (30%, write), presence (15%, write), search (5%, read). Budget: $6,500 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 ≥ 99.9%; p99 latency ≤ 200 ms (read); p99 latency ≤ 250 ms (send) |
| Monday morning | At 5 s everyone logs in: traffic doubles for 8 s. | Success ≥ 99%; p99 latency ≤ 400 ms (read) |
| Cache flushed | At 5 s the cache on the request path is emptied, as after a restart or a bad deploy. | 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% |
| Budget: $6,500/month | The normal day's design must cost at most $6,500 a month. | Cost ≤ $6,500/month |