Availability expressed in nines
A way of stating a service's availability by the number of 9s in its percentage, each extra 9 cutting the tolerated downtime by ten.
An availability commitment of 99.9% sounds close to perfect, but it tolerates almost nine hours of downtime a year; 99.99% tolerates about fifty-three minutes, and 99.999% just over five minutes. The figure is measured over a period, often the month, and against a definition of outage written by the provider, so a service can be unusable without being deemed unavailable. Nor does the contract promise availability: it promises a service credit if it is not met, in an amount unrelated to the customer's loss. For the business interruption insurer, the number of nines states the expected frequency of outages, not their cost or their simultaneity across all customers of one region.
On December 7, 2021, Amazon Web Services' us-east-1 region suffered an outage of about seven hours: on its own, it exceeded the annual downtime tolerated by a 99.9% commitment, and affected customers were entitled only to credits on their bill.
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