System Design

Anyone can build a system that works. The hard part is building one that works for 50 million users simultaneously without going down. That gap — between "it works on my machine" and "99.999% uptime" — is what system design bridges.

// what those "nines" actually mean per year
99%
two nines
87.6 hrs
downtime/year
99.9%
three nines
8.76 hrs
downtime/year
99.99%
four nines
52.6 min
downtime/year
99.999%
five nines
5.26 min
downtime/year
Users
→
CDN
→
Load
Balancer
→
App ×3
Workers
→
Redis
Queue
→
Primary DB
Replica ×2