System Design Interview Cheat Sheet: 15 Patterns to Know

System Design Interview Cheat Sheet: Quick Reference

This cheat sheet consolidates the essential numbers, patterns, and frameworks you need for any system design interview. Bookmark this page for quick review before your interview.

Back-of-Envelope Numbers

Metric	Value
L1 cache reference	0.5 ns
L2 cache reference	7 ns
Main memory reference	100 ns
SSD random read	150 μs
HDD seek	10 ms
Network round trip (same datacenter)	0.5 ms
Network round trip (cross-continent)	150 ms

Capacity Estimation

• 1 day = 86,400 seconds ≈ 100K seconds
• 1 million requests/day ≈ 12 requests/second
• 1 billion requests/day ≈ 12,000 requests/second
• 1 character = 1 byte, 1 image = 200KB-2MB, 1 video minute = 150MB

Design Pattern Quick Reference

• Read-heavy → Cache + Read replicas
• Write-heavy → Message queue + Write-behind cache
• Global scale → CDN + Multi-region + Eventual consistency
• Real-time → WebSocket + Redis Pub/Sub

Full guide: system design fundamentals. Practice: AissenceAI.