Availability | Software Engineering Dictionary
Availability, also known as uptime, is the percent of time that a system spends in an operational state. For example, if a system is functional for 23 hours every day, then it has 23 / 24 = 95.8% availability. If a system is down for 1 hour every three months, which we’ll approximate as 2160 hours, that means it’s up for 2159 hours, and therefore the availability is 2159 hours / 2160 hours = 99.95% availability.
People often abbreviate this by stating the number of initial 9s in the number. For example, you could say “service X has 3 9’s of availability”. 90% is one 9, 99% is 2 9s, 99.9% is 3 9s, etc. Achieving higher availabilities is quite challenging. For example, 5 9s, or 99.999%, means that a system is only down 0.001% of the time, which is 5 minutes per year!
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