Uptime
The percentage of time a system is operational and accessible.
Definition
Uptime is a measure of system reliability, expressed as the percentage of time that a service, server, or network is operational and accessible to users. In the context of web services and infrastructure, uptime is typically calculated over a specific period (monthly, quarterly, or annually) and is a key metric in Service Level Agreements (SLAs). A 99.9% uptime, often called "three nines," means the system can be down for approximately 8.76 hours per year, while 99.99% ("four nines") allows only 52.56 minutes of downtime annually.
Examples
Calculating Uptime Percentage
Uptime is calculated by dividing total operational time by total time in the period.
// Uptime calculation
const totalMinutes = 30 * 24 * 60; // 30 days in minutes
const downtimeMinutes = 45; // Total downtime
const uptimePercentage = ((totalMinutes - downtimeMinutes) / totalMinutes) * 100;
// Result: 99.896% uptimeSLA Uptime Tiers
Common uptime SLA commitments and their allowed downtime.
| Uptime % | Downtime/Year | Downtime/Month |
|----------|---------------|----------------|
| 99% | 3.65 days | 7.31 hours |
| 99.9% | 8.76 hours | 43.83 minutes |
| 99.99% | 52.56 minutes | 4.38 minutes |
| 99.999% | 5.26 minutes | 26.3 seconds |Use Cases
Best Practices
- Monitor from multiple geographic locations to detect regional issues
- Use synthetic monitoring for proactive detection
- Set up automated alerts for downtime events
- Calculate uptime excluding scheduled maintenance windows
- Document all incidents for accurate reporting
Common Mistakes
- Measuring uptime only from internal networks (missing external issues)
- Not accounting for partial failures or degraded performance
- Using overly long check intervals that miss brief outages
- Ignoring dependent service availability
FAQ
Related Terms
SLA
A Service Level Agreement defining the expected level of service between a provider and customer.
SLAs define uptime commitments
MTTR
Mean Time To Repair/Recovery - the average time to restore service after a failure.
Affects overall uptime by reducing recovery time
MTTD
Mean Time To Detect - the average time to identify that an issue has occurred.
Faster detection improves uptime
External Resources
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