Uptime
The proportion of time a site or service is available during a given period, usually expressed as a percentage.
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Why it matters
Percentages look close but the difference is large: 99.9 percent uptime means about 43 minutes of downtime a month, while 99 percent means about 7 hours. To notice an outage before customers do, continuous external monitoring and alerts are needed. The target in a hosting contract should be judged by this calculation.
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Example
A booking site crashes at midnight and nobody notices until morning. After monitoring that checks from outside every minute is set up, the on-call person gets an instant message at the next outage and the site is back within ten minutes.
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Common mistake
Measuring whether the site works by checking only the home page. The home page may load while checkout or forms are broken; critical transactions should be monitored too.
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Related terms
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