Hot standby is a redundant method of having one system running simultaneously with another identical primary system. Upon failure of the primary system, the hot standby system immediately takes over replacing the primary system. However, data is still mirrored in real time, thus both the systems are assured of having identical data. Hot standby may also be described as a failover technique to ensure system reliability and security, which is achieved by having a standby device or system ready to take over in the event a device or system fails. Hot standby also describes the ability of a software or hardware component to connect to a server and run read-only queries while it is in standby or recovery mode. Additionally, it describes the ability of a server to continually answer queries meanwhile maintaining open connections for users during recovery to normal operations.This term is also known as hot spare, especially at the component level (such as a hard drive in a disk array).
Examples of hot standby components are audio/visual switches, network printers, computers and hard drives which in many cases are considered redundant. Most often hot standby refers to an immediate backup for a critical component, without which the entire system would fail. The switchover may happen manually or automatically, but normally there is some means of error detection involved. Furthermore, a hot standby component is designed to significantly reduce the time required for a failed system to return to normal operation ensuring not to provide 100 percent availability of the system.A hot standby system may be located close to the primary system, in the same building, city, another state or even in another country. The location of a hot standby is particularly relevant when for example the primary system is located on an earthquake fault line.
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