1 min lesson
The reliability math, made concrete
Make the call in this situation: "An interviewer says: 'We're at over a million daily active users. Why does that change how you'd think about a 99.9% success rate?' Sketch a strong answer." Explain what supports it.
Step 1 of 2
The reliability math, made concretewhy 0.1% is not small
Translate percentages into people. It's the fastest way to show an interviewer you reason about scale rather than reciting it.
- Metric
- 99.9% success
- Sounds like
- Three nines, fine
- At ~1M DAU actually means
- ~1,000 affected users per million requests - a steady stream of failures
- Metric
- p99 latency
- Sounds like
- 1% of requests
- At ~1M DAU actually means
- Tens of thousands of sessions per day landing in the slow tail
- Metric
- 1 hour of downtime
- Sounds like
- A blip
- At ~1M DAU actually means
- A visible, talked-about outage for a developer tool people live in all day
- Metric
- One hot endpoint
- Sounds like
- Edge case
- At ~1M DAU actually means
- Abuse or a retry storm that can saturate a service before alerts fire
| Metric | Sounds like | At ~1M DAU actually means |
|---|---|---|
| 99.9% success | Three nines, fine | ~1,000 affected users per million requests - a steady stream of failures |
| p99 latency | 1% of requests | Tens of thousands of sessions per day landing in the slow tail |
| 1 hour of downtime | A blip | A visible, talked-about outage for a developer tool people live in all day |
| One hot endpoint | Edge case | Abuse or a retry storm that can saturate a service before alerts fire |
Percentages hide the headcount. Translate them and the stakes become obvious.