C H A P T E R  12

Clustering for High Availability

One of the most common measures applied to modern IT systems is the Service Level Agreement (SLA) for Availability — an agreement that an IT Service will be available for a certain amount of time per year. Commonly, this measure specifies how much of the time a server is available and working – its “uptime” - and is normally defined as a percentage over a year.

The three most common uptime targets are 99.9%, 99.99%, and 99.999%; or, as they are often expressed, three 9s, four 9s, and five 9s, respectively. These require a website to run uninterrupted for an entire year with downtime not exceeding

  • Three 9s: 8.76 hours
  • Four 9s: 52.56 minutes
  • Five 9s: 5.26 minutes

There are agreements in place ...

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