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Reliability, availability and serviceability (RAS), also known as reliability, availability, and maintainability (RAM), is a computer hardware engineering term involving reliability engineering, high availability, and serviceability design. The phrase was originally used by IBM as a term to describe the robustness of their mainframe computers.
The subject of the terms is thus important here: whether the focus of a discussion is the server hardware, server OS, functional service, software service/process, or similar, it is only if there is a single, consistent subject of the discussion that the words uptime and availability can be used synonymously.
High availability software is software used to ensure that systems are running and available most of the time. High availability is a high percentage of time that the system is functioning. It can be formally defined as (1 – (down time/ total time))*100%.
In reliability engineering, the term availability has the following meanings: . The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e. a random, time.
Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network.
Availability Every request received by a non-failing node in the system must result in a response. This is the definition of availability in CAP theorem as defined by Gilbert and Lynch. [1] Note that availability as defined in CAP theorem is different from high availability in software architecture. [5] Partition tolerance
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 clusters (also known as HA clusters, fail-over clusters) are groups of computers that support server applications that can be reliably utilized with a minimum amount of down-time. They operate by using high availability software to harness redundant computers in groups or clusters that provide continued service when system ...