System Design•Building Blocks

Back-of-the-Envelope Estimation

Rough numbers decide the architecture. Go from daily active users to QPS, storage, and bandwidth before choosing databases or caches.

Key ideas

QPS ≈ (DAU × actions per user) / 86,400 seconds. Peak is often 2–3× average.
Storage ≈ writes per day × payload × retention. Bandwidth ≈ QPS × response size.
You only need the order of magnitude — it tells you which bottleneck to design for.

Tip

Memorize: 86,400 s/day, ~1M s in 11.5 days, 1 byte/char, and that reads usually dwarf writes.

Next chapter

Design a URL Shortener

Start with Design a URL Shortener: Requirements & Scope

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System Design•Building Blocks

Rough numbers decide the architecture. Go from daily active users to QPS, storage, and bandwidth before choosing databases or caches.

QPS ≈ (DAU × actions per user) / 86,400 seconds. Peak is often 2–3× average.
Storage ≈ writes per day × payload × retention. Bandwidth ≈ QPS × response size.
You only need the order of magnitude — it tells you which bottleneck to design for.

Tip

Memorize: 86,400 s/day, ~1M s in 11.5 days, 1 byte/char, and that reads usually dwarf writes.

Next chapter

Design a URL Shortener

Start with Design a URL Shortener: Requirements & Scope