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Five Nines
Also known as: 99.999%High Availability
99.999% availability - approximately 5 minutes of downtime per year.
Definition
Five nines (99.999%) availability is an extremely high uptime standard that allows only 5.26 minutes of downtime per year. Achieving five nines requires redundant systems, automated failover, rigorous testing, and mature operational practices. While often cited as the gold standard, five nines is extremely difficult and expensive to achieve, and may be overkill for most applications. The cost and complexity increase exponentially with each additional nine.
Examples
The Nines Explained
Comparison of availability levels and their allowed downtime.
| Level | Availability | Downtime/Year | Downtime/Month |
|-------------|--------------|-----------------|-----------------|
| Two nines | 99% | 3.65 days | 7.31 hours |
| Three nines | 99.9% | 8.76 hours | 43.83 minutes |
| Four nines | 99.99% | 52.56 minutes | 4.38 minutes |
| Five nines | 99.999% | 5.26 minutes | 26.3 seconds |
| Six nines | 99.9999% | 31.56 seconds | 2.63 seconds |Use Cases
Critical infrastructure (hospitals, emergency services)
Financial trading systems
Telecommunications networks
Air traffic control systems
Best Practices
- Design for failure with redundancy at every layer
- Implement automated health checks and failover
- Use multi-region or multi-cloud deployments
- Practice chaos engineering and failure drills
- Invest in comprehensive monitoring and alerting
Common Mistakes
- Pursuing five nines when three nines is sufficient
- Underestimating the cost and complexity
- Not accounting for all failure modes
- Relying on single points of failure
FAQ
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