System Design Interview Questions, Answered

Start from users and actions, not from servers. The whole estimate is a few multiplications with aggressively rounded numbers.

1. DAU times actions per user per day10M DAU doing 5 writes each is 50M writes/day
2. ↓
   divide by ~100,000 seconds in a day500 QPS avgrounding 86,400 up keeps the math clean
3. ↓
   apply a peak factor of 2-5x2,500 QPS targettraffic is not flat; scale by how spiky the product is
4. ↓
   storage: rows x row size x retention x replication1 KB rows at 50M/day is 50 GB/day, ~18 TB/year before 3x replication

The skill interviewers grade is rounding aggressively and stating assumptions out loud, because the answer only needs to be right within an order of magnitude.