Back-of-the-Envelope Estimation

Order-of-magnitude checks catch designs that cannot work. Adjust assumptions for your interview scope — exact numbers matter less than which component becomes the bottleneck.

Assume 500M DAU, 1B video views/day, 500k new uploads/day, average watch time 10 minutes at 3 Mbps, metadata API ~2 KB per video page load.

Read traffic

• Views/day → 1B / 86,400 ≈ 11,600 views/s average.
• Peak (prime time ×5) ≈ 58,000 metadata reads/s for titles, manifests, comments.
• CDN video bytes: 1B views × 10 min × 3 Mbps / 8 ≈ 2.25 EB/day theoretical max — in practice ABR averages ~1 Mbps → ~750 PB/day egress; 95%+ served from CDN cache (long-tail + viral).
• Metadata cache (Redis): hot 1% of videos = 10M keys × 2 KB ≈ 20 GB — fits a modest cluster.

Write traffic

• Uploads: 500k/day ≈ 5.8 uploads/s average, peak ~30/s.
• Avg raw upload 500 MB → 500k × 500 MB ≈ 250 TB/day ingest to object storage.
• Transcode fan-out: 4 renditions × 500k ≈ 2M FFmpeg jobs/day → ~23 jobs/s sustained, burst queue depth in Kafka matters.
• View-count events: batched — 1B events/day to stream processor ≈ 11,600 events/s (not 1B UPDATEs on Postgres).

Storage

• Catalog: 500M videos × 2 KB metadata ≈ 1 TB (+ indexes → plan 3–5 TB).
• Raw + transcoded: 500k new/day × (500 MB raw + ~1.5 GB segments) ≈ 1 PB/year growth — tier cold content to cheaper storage class.

Network & memory

• Upload path bandwidth: 250 TB/day ÷ 86,400 ≈ 2.4 GB/s average ingest — dedicated upload endpoints, not mixed with API.
• Transcoder fleet: ~23 concurrent jobs/s × ~2 min/job ≈ 3,000 workers at peak if not batched overnight.
• Origin shield: without CDN, 750 PB/day is impossible — CDN is mandatory, not optional.

// YouTube-scale metadata reads
const viewsPerDay = 1_000_000_000;
const avgQps = viewsPerDay / 86_400;
const peakQps = avgQps * 5;
const uploadQps = 500_000 / 86_400;
const uploadBytesPerDay = 500_000 * 500 * 1024 * 1024; // 500 MB avg
console.log({
  peakMetadataQps: Math.round(peakQps),
  avgUploadQps: uploadQps.toFixed(1),
  uploadTbPerDay: (uploadBytesPerDay / 1e12).toFixed(0) + " TB",
});