Database mirroring is a feature of SQL Server that allows an
administrator to create a copy of the database that is always up to
date. If the principal server fails or needs to be taken offline for
maintenance the mirror server can be quickly set up to take on the
workload. Mirroring provides high availability for your databases
video demonstrates setting up Database Mirroring using TSQL
* also shows a manual failover
To support automatic failover, a database mirroring session must be configured in high-safety mode and also possess a third server instance, known as the witness. The witness is an optional instance of SQL Server that enables the mirror server in a high-safety mode session to recognize whether to initiate an automatic failover. Unlike the two partners, the witness does not serve the database. Supporting automatic failover is the only role of the witness
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