QPS & Throughput Playbook
The formula
QPS = (DAU × actions per user per day) ÷ 86,400 → peak QPS = QPS × peak factor (2–10×)
Read vs write
Split traffic into reads and writes — they hit different systems and the ratio drives your whole design. A read:write ratio of 100:1 (typical of social feeds) means caching and read replicas dominate; a write-heavy 1:1 (logging/metrics) means partitioning and write throughput dominate.
Steps
- Average QPS from DAU × actions ÷ 86,400.
- Apply a peak factor (daily peaks ~2–3×; spiky events like flash sales 10×+).
- Split by read:write; size each path separately.
Worked example
50M DAU, 20 actions/day, 90% reads:
- Total = 50M × 20 ÷ 10⁵ = ~10,000 QPS avg → peak ×3 ≈ 30K QPS.
- Reads ≈ 27K QPS, writes ≈ 3K QPS at peak.
- Gate: 27K read QPS ⇒ cache + read replicas; 3K write QPS ⇒ a single primary is still fine (see Decision Gates).
Formulas are standard/public-domain engineering math. Approach and reference-table format adapted from the System Design Primer (CC BY 4.0), Jeff Dean’s latency numbers, the DesignGurus capacity-estimation guide, and Little’s Law.
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