High Availability(HA) is a class of availability. According to the Havard Research Group (HRG), high-availability systems and networks are 99.999% available and have an annual downtime of less than 8 minutes. Since high availability consists of five 9s, the rule is also called "Fife Nine."
The Harvard Research Group has developed a classification for high availability: the Availability Environment Classification( AEC) from which the constraints and fault tolerance are derived. There are a total of six AEC classifications, labeled AEC-0 through AEC-5.
High availability refers to a high level of provisioning of system hardware and software that ensures virtually uninterrupted operation of computer systems. Corresponding systems are redundant throughout. This applies equally to servers and storage systems as well as to all network and security components, which continue to operate without restriction if one component fails. In addition, single points of failure (SPOF) are avoided. The term high availability is defined by IEEE and states that the availability of all resources of a computer system lies in the detection of faulty components.
Since high availability has a direct influence on the economic success of companies, computer systems, clusters and servers are mainly specified in this availability class and, under certain circumstances, are also implemented as an availability network. They are characterized by having an extremely low downtime, being available even in the event of a fault, not accepting any loss of data, being supported by recovery procedures, detecting faults and having fault tolerance and transparency.