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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]
High availability of services, when a service is available for 99.999% of the time, or around 5 minutes of downtime per year Nine (purity) , a 99.999% pure substance German 15 cm (5.9 in) artillery shells used in World War I
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, 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.
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.
Continuous availability is an approach to computer system and application design that protects users against downtime, whatever the cause and ensures that users remain connected to their documents, data files and business applications. Continuous availability describes the information technology methods to ensure business continuity.
Availability: Ability to keep a functioning state in the given environment; Maintainability: Ability to be timely and easily maintained (including servicing, inspection and check, repair and/or modification) Safety: Ability not to harm people, the environment, or any assets during a whole life cycle.
Availability of reversion modes [clarification needed] In addition, fault-tolerant systems are characterized in terms of both planned service outages and unplanned service outages. These are usually measured at the application level and not just at a hardware level. The figure of merit is called availability and is