Design Reddit - Scale and Constraints

User Scale

• Registered Users: 500M
• Daily Active Users: 50M (10%)
• Monthly Active Users: 430M (86%)
• Subreddits: 3M+ active communities
• Moderators: 100K+ volunteer moderators

Content Volume

• Posts per Day: 2M (23 TPS, peak 70 TPS)
• Comments per Day: 20M (231 TPS, peak 700 TPS)
• Votes per Day: 500M (5,787 TPS, peak 17K TPS)
• Average Post Size: 1KB (text + metadata)
• Average Comment Size: 500 bytes

Traffic Patterns

• Page Views: 20B per month
• API Requests: 100M per day (1,157 RPS)
• Read:Write Ratio: 100:1 (read-heavy)
• Peak Hours: 6 PM - 11 PM EST

Storage Estimates

Posts (5 years):
2M/day × 365 × 5 × 1KB = 3.65TB

Comments (5 years):
20M/day × 365 × 5 × 500 bytes = 18.25TB

Votes (1 year):
500M/day × 365 × 16 bytes = 2.92TB

Subreddit Data:
3M subreddits × 100KB = 300GB

Total: ~25TB

Performance Targets

• Feed Load: <1s p95
• Comment Thread: <500ms p95
• Vote Registration: <100ms p95
• Search: <300ms p95
• Uptime: 99.9%

Compute Resources

• Application Servers: 5,000 servers
• Database Servers: 500 shards + 2,500 replicas
• Cache Servers: 200 Redis nodes (5TB total)
• Search Servers: 100 Elasticsearch nodes

Cost Estimates

Monthly Infrastructure:
- Compute: $1M
- Database: $500K
- Cache: $100K
- Storage: $50K
- CDN: $200K
Total: ~$1.85M/month
Cost per DAU: $0.037/month

This scale analysis provides the foundation for Reddit's architecture.