Uptime

Uptime is the percentage of time a site or service was available and responsive over a period; its complement is downtime.

Uptime looks simple but two things make it subtle. First, its high numbers are deceptive: the gap between 99% and 99.9% seems small yet the first is about three and a half days of downtime a year and the second under nine hours — so uptime should be read as a “number of nines,” not a plain percentage. Second, uptime is sensitive to the definition of “available”: does a server that answers in 20 seconds count as up or down? Does a 500 error page, technically responding but broken for the user, count as up? For this reason uptime is usually measured with an external (blackbox) check and a clear response threshold. High uptime is necessary but not sufficient; a site can be 100% up yet slow and unusable.