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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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Related Terms

Uptime

The percentage of time a system is operational and accessible.

Five nines is an uptime target

SLA

A Service Level Agreement defining the expected level of service between a provider and customer.

Often specified in SLAs for critical systems

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