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High availability is a property of network resilience, the ability to "provide and maintain an acceptable level of service in the face of faults and challenges to normal operation." [3] Threats and challenges for services can range from simple misconfiguration over large scale natural disasters to targeted attacks. [4]
Availability, achieved (Aa) [6] The probability that an item will operate satisfactorily at a given point in time when used under stated conditions in an ideal support environment (i.e., that personnel, tools, spares, etc. are instantaneously available). It excludes logistics time and waiting or administrative downtime.
A highly available system would disable the malfunctioning portion and continue operating at a reduced capacity. In contrast, a less capable system might crash and become totally nonoperational. Availability is typically given as a percentage of the time a system is expected to be available, e.g., 99.999 percent ("five nines").
The terms high availability, continuous operation, and continuous availability are generally used to express how available a system is. [3] [4] The following is a definition of each of these terms. High availability refers to the ability to avoid unplanned outages by eliminating single points of failure. This is a measure of the reliability of ...
High availability software is measured by its performance when a subsystem fails, its ability to resume service in a state close to the state of the system at the time of the original failure, and its ability to perform other service-affecting tasks (such as software upgrade or configuration changes) in a manner that eliminates or minimizes ...
Availability includes non-operational periods associated with reliability, maintenance, and logistics. This is measured in terms of nines. Five-9's (99.999%) means less than 5 minutes when the system is not operating correctly over the span of one year. Availability is only meaningful for supportable systems.
Most modern business theorists see a continuum with pure service on one terminal point and pure commodity good on the other terminal point. [2] Most products fall between these two extremes. For example, a restaurant provides a physical good (the food), but also provides services in the form of ambience, the setting and clearing of the table ...
High availability services [ edit ] For a high-availability service, such as an Internet hosting service or Internet service provider , the purpose of stating a time period in advance is to allow clients of the service to prepare for possible disruption or prepare for any major changes to the functioning of the service.