Nines (Five Nines Availability)

"Nines" is shorthand for availability levels. Three nines means 99.9%, four nines 99.99% and five nines 99.999%. Each additional nine cuts the allowed downtime by a factor of ten.

Why it matters

The shorthand makes targets easy to discuss, and the numbers show how fast difficulty and cost grow. Moving from 99.9% to 99.99% usually needs redundancy, automated failover and faster detection, because the margin for human response disappears.

Example

Allowed downtime per year (365 days) is roughly:

99%      3 days 15 hours 36 minutes
99.9%    8 hours 45 minutes 36 seconds
99.99%   52 minutes 34 seconds
99.999%  5 minutes 16 seconds

At five nines, one slow human response can use up the whole year.

Common mistakes

Promising five nines for a single server in one data center.
Forgetting that your availability is limited by dependencies. If you depend on three services at 99.9% each in series, the combined figure is about 99.7%.
Ignoring the measurement window. A monthly target is much stricter than the same percentage per year.
Not agreeing on what counts as downtime.

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