hard Drill: Twitter timeline sizing
Drill
300M DAU, 2 tweets/user/day (~300 B each), each user follows ~200 people and reads their timeline ~10×/day. Estimate write QPS, read QPS, yearly tweet storage, and name the key bottleneck.
✅ Worked solution & bounds check
Worked solution
- Writes: 300M × 2 = 6×10⁸/day ÷ 10⁵ = ~6,000 writes/s (peak ×5 ≈ 30K/s).
- Timeline reads: 300M × 10 = 3×10⁹/day ÷ 10⁵ = ~30,000 reads/s (peak ≈ 150K/s).
- Tweet storage: 6×10⁸ × 300 B × 365 ≈ ~65 TB/yr (×3 ≈ 200 TB).
Bottleneck / gate: read-heavy + fan-out. Naive read-time fan-out (query 200 followees per timeline load) is far too expensive ⇒ fan-out-on-write (precompute timelines into a cache), with a hybrid for celebrity accounts (millions of followers ⇒ fan-out-on-read for them to avoid write storms).
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