Database mirroring was introduced with Microsoft SQL Server 2005 technology that can be used to design high-availability and high-performance solutions for database redundancy.

In database mirroring, transaction log records are sent directly from the principal database to the mirror database. This helps to keep the mirror database up to date with the principal database, with no loss of committed data. If the principal server fails, the mirror server automatically becomes the new principal server and recovers the principal database using a witness server under high-availability mode. We will discuss these modes later. Fundamentally to summarize there are three jargon to understand – Principal database is the active live database that supports all the commands, Mirror is the hot standby and witness which allows for a quorum in case of automatic switch over.