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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.
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").
Barlow and Proschan [1975] define availability of a repairable system as "the probability that the system is operating at a specified time t." Blanchard [1998] gives a qualitative definition of availability as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called ...
Systems that exhibit truly continuous availability are comparatively rare and higher priced, and most have carefully implemented specialty designs that eliminate any single point of failure and allow online hardware, network, operating system, middleware, and application upgrades, patches, and replacements. For certain systems, scheduled ...
For databases reliability, availability, scalability and recoverability (RASR), is an important concept. Atomicity, consistency, isolation (sometimes integrity), durability is a transaction metric. When dealing with safety-critical systems, the acronym reliability, availability, maintainability and safety is frequently used.
Operational availability is used to evaluate the following performance characteristic. For a system that is expected to be available constantly, the below operational availability figures translate to the system being unavailable for approximately the following lengths of time (when all outages during a year are added together):
Reliability engineering is a sub-discipline of systems engineering that emphasizes the ability of equipment to function without failure. Reliability is defined as the probability that a product, system, or service will perform its intended function adequately for a specified period of time, OR will operate in a defined environment without failure. [1]
In engineering, reliability, availability, maintainability and safety (RAMS) [1] [2] is used to characterize a product or system: Reliability: Ability to perform a specific function and may be given as design reliability or operational reliability; Availability: Ability to keep a functioning state in the given environment