System Design (50 Qs)•Part 5: Back-of-the-Envelope Estimation & Production Calculations (44 - 50)
High Availability Sizing: 99.9% vs 99.99% ("Four Nines") Downtime Budgets.
In Simple Words: Service Level Agreements (SLAs) determine permissible annual and monthly downtime:
| Availability Level | Downtime Per Year | Downtime Per Month | Typical Architecture |
|---|---|---|---|
| 99% ("Two Nines") | 3.65 days | 7.3 hours | Single instance with automated restart |
| 99.9% ("Three Nines") | 8.76 hours | 43.8 minutes | Multi-AZ with load balancer and read replicas |
| 99.99% ("Four Nines") | 52.6 minutes | 4.38 minutes | Multi-Region Active-Active with automated failover |
| 99.999% ("Five Nines") | 5.26 minutes | 26.3 seconds | Telco-grade redundant hardware, live patching |