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 26s99.99% 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 | 8.6sAvailability 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%.
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