Five Nines (99.999%) SLA Explained
Discover what a 99.999% SLA (five nines) means for system availability. The gold standard for mission-critical infrastructure with only 5 minutes of yearly downtime.
Detailed Explanation
What Does 99.999% Uptime Mean?
A 99.999% SLA, called five nines, is considered the gold standard of availability. It allows only approximately 5 minutes and 15 seconds of total downtime per year.
Downtime Breakdown
| Period | Allowed Downtime |
|---|---|
| Per year | 5 minutes, 15 seconds |
| Per month | 26 seconds |
| Per week | 6 seconds |
| Per day | 0.86 seconds |
The Challenge of Five Nines
At 26 seconds of monthly downtime, five nines is extraordinarily difficult to achieve. Consider what this means:
- A single DNS propagation delay can exceed the monthly budget
- Any manual intervention is too slow — everything must be automated
- Even a 2-second health check interval means you might not detect a failure before the daily budget is consumed
Architecture Requirements
Five nines demands:
- Multi-region active-active — not just multi-AZ, but geographically distributed
- Zero-downtime everything — deployments, schema migrations, certificate rotations
- Chaos engineering — regularly injecting failures to validate resilience
- Sub-second failover — automated detection and switchover
- Redundant dependencies — DNS, CDN, payment processors, all with failover paths
Who Actually Achieves Five Nines?
Very few services consistently maintain five nines. Notable examples include:
- Core telecommunications infrastructure (PSTN targets 99.999%)
- Financial trading systems (exchanges require it for market integrity)
- Emergency services (911/112 systems)
- AWS S3 (designed for 99.999999999% durability, 99.99% availability)
The cost of five nines can be 10-100x that of three nines, making it appropriate only for truly critical systems where downtime has severe consequences.
Use Case
Reserve five nines for mission-critical infrastructure: payment processing core systems, telecommunications backbones, healthcare monitoring systems, and financial trading platforms where even seconds of downtime have severe regulatory or safety implications.