Availability in Numbers

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.

99.9% Availability - Three 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

99.99% Availability - Four 9s

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

Availability in parallel vs in sequence

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.

In sequence

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%.

In parallel

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%.