It is the process of copying data (transaction logs) to a second storage location in real time and keeping the two locations synchronized to provide active backup and redundancy in case of primary server failure or disaster and increasing availability. It was introduced into database software with Microsoft SQL Server 2005. In case the primary server fails, the standby server takes over as primary server using a witness server that determines the quorum in case of switchover. It is based on TCP endpoints and only committed transaction data is sent to the mirror, not the rolled-back transaction data.
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