Problems / Design a product, medium
Design Reddit from an empty canvas: subreddit front pages from a cache, write-heavy vote counters that feed hot rankings, and comment threads.
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Request types to route: front (55%, read), thread (20%, read), vote (20%, write), comment (5%, write). Budget: $7,500 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 ≤ 150 ms (front); p99 latency ≤ 200 ms (vote) |
| A post hits r/all | At 5 s a post goes viral on r/all: traffic triples for 8 s. | Success ≥ 99%; p99 latency ≤ 300 ms (front) |
| 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 (front) |
| A server dies | At 4 s one server of the first app service on the request path dies for 10 s. | Success ≥ 99% |
| Budget: $7,500/month | The normal day's design must cost at most $7,500 a month. | Cost ≤ $7,500/month |