A database engine is the underlying system that a database uses to function. Many different technologies rely on internal "engines," which are the fundamental building blocks on which they operate.
Hot standby is a redundant method in which one system runs simultaneously with an 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 systems have identical data.
Hot standby is also known as hot spare, especially at the component level, such as a hard drive in a disk array.
Hot standby also is 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 of device or system failure. Hot standby also describes the ability of a software or hardware component to connect to a server and run read-only queries while in standby or recovery mode. Additionally, it describes the ability of a server to continually answer queries while maintaining open connections for users during recovery to normal operations.
Examples of hot standby components include audio/visual switches, network printers, computers and hard drives, which are often considered redundant. Hot standby frequently refers to an immediate backup for a critical component, without which the entire system would fail. The switchover occurs manually or automatically, but normally, some means of error detection is involved. Furthermore, a hot standby component is designed to significantly reduce the time required for a failed system to return to normal operations ensuring not to provide 100 percent system availability.
A hot standby system may be located close to the primary system, in the same building, city, another state or even 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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