Five Nines

/faɪv naɪnz/ · Noun · Development · Origin: 1990

Definitions

  1. Availability of 99.999 percent, permitting roughly five minutes and fifteen seconds of downtime per year. The term originated in telecommunications, where carrier-grade switching equipment was held to that standard, and it became shorthand in the industry for the highest tier of reliability. Each additional nine costs disproportionately more, since it requires eliminating progressively rarer failure modes: three nines allows nearly nine hours a year and is achievable with careful operations, while five nines demands redundancy at every layer, automated failover faster than human reaction, and rigorous change control. The figure is often quoted loosely, and the important questions are usually what counts as downtime, whether planned maintenance is excluded, and over what window the average is computed.

    In plain English: A promise that a service will be working 99.999% of the time. That leaves only about 5 minutes of downtime in an entire year — an extremely high bar.

    Example: "We promised five nines in the SLA, which means our entire annual error budget is about five minutes — one bad deploy could blow it."

Related Terms