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Five nines is the term used for describing the availability of a computer or a service at 99.999 percent of the time it is required. In other words, the system or service is only unavailable for 5.39 minutes throughout the year for planned or unplanned downtime. Five nines is recommended and required for mission-critical requirements and for certain areas such as e-commerce. However, five nines availability has always been a challenge for a service or network and is often impossible to guarantee.
Five nines ensures high availability and reliability of the computer or service. It is often the desired percentage availability of a given system or service. However, there is no ruling committee or body which formalizes the meaning or definition of five nines. In order to achieve five nines, most employ monitoring systems in order to proactively resolve all issues.
One of the approaches to achieving five nines is by duplicating the components so that backup components are always available. Along with cost involved, redundancy is also a problem with this approach. Another approach has been to build a shared component system in which another active system could be made available in case of a failure.
Ensuring five nines over a period of time is challenging. The standard is expensive due to cost of the physical infrastructure as well as software components. Additional components add to complexity and risk. High-capacity planning and multiple Tier 4 data centers are recommended for five nines. Again, for many services or networks, three nines or four nines would be more effective and justified in terms of resources and cost involved.