Availability (aka. reliability)
Availability is a quality attribute of an IT system that refers to the proportion of time that a system remains operational and accessible to users, even under heavy load and even when parts of the system fail.
Availability is closely related to scalability and fault tolerance.
Availability is typically expressed as a percentage of uptime over a given period. For example, a system that has an uptime of 99.9% is considered to have "three nines of availability".
Availability % |
Downtime per year |
99% ("two nines") |
3.65 days |
99.9% ("three nines") |
8.76 hours |
99.99% ("four nines") |
52.56 minutes |
99.999% ("five nines") |
5.26 minutes |
99.9999% ("six nines") |
31.5 seconds |
What is considered to be "highly available" depends on the system’s requirements. A system with three nines of availability would actually have a downtime of 8.76 hours per year, which may be acceptable for some systems but not for others. 99.99% uptime (four nines) would mean a downtime of 52.56 minutes per year.