Availability is often quantified by uptime (or downtime) as a percentage of time the service is available. Availability is generally measured in number of 9s—a service with 99.99% availability is described as having four 9s.
Duration | Acceptable downtime
------------- | -------------
Downtime per year | 8h 41min 38s
Downtime per month | 43m 28s
Downtime per week | 10m 4.8s
Downtime per day | 1m 26s
Duration | Acceptable downtime
------------- | -------------
Downtime per year | 52min 9.8s
Downtime per month | 4m 21s
Downtime per week | 1m 0.5s
Downtime per day | 8.6s
If a service consists of multiple components prone to failure, the service’s overall availability depends on whether the components are in sequence or in parallel.
Overall availability decreases when two components with availability < 100% are in sequence:
Availability (Total) = Availability (Foo) * Availability (Bar)
If both Foo
and Bar
each had 99.9% availability, their total availability in sequence would be 99.8%.
Overall availability increases when two components with availability < 100% are in parallel:
Availability (Total) = 1 - (1 - Availability (Foo)) * (1 - Availability (Bar))
If both Foo
and Bar
each had 99.9% availability, their total availability in parallel would be 99.9999%.
To learn more, visit the following links: