Lesson 3 / 32

Back-of-the-Envelope Estimation

~100 QPS. 8.6M / 86,400s ≈ 100 QPS average; peak would be higher.

Quick math for QPS, storage, and bandwidth.

From DAU to QPS

A day has ~~86,400 seconds (~~100k). So 1M requests/day ≈ ~12 QPS average. Multiply by a peak factor of 2–10x for the real target.

Estimate per-record size, multiply by write rate and retention.

500 bytes/tweet x 100M tweets/day
= 50 GB/day
x 365 x 5 years
~= 90 TB (before replication)

Output:

Plan for 3x with replicas ~= 270 TB

Use powers of 10 and 2. The goal is the right order of magnitude — is it GB or PB? — not a precise figure.

Quick check: A service gets 8.6M requests/day evenly. Roughly what is the average QPS?

Answer

~100 QPS — 8.6M / 86,400s ≈ 100 QPS average; peak would be higher.