Database Mirroring Witness Catalog Views (Transact-SQL)
In a database mirroring session, automatic failover requires a witness server. Ideally, the witness resides on a separate computer from both the principal and mirror servers. The witness does not serve the database. Instead, it monitors the status of the principal and mirror servers. If the principal server fails, the witness may initiate automatic failover to the witness server. For more information, see Database Mirroring Witness.
For information about the catalog view for database mirroring witnesses, see sys.database_mirroring_witnesses (Transact-SQL).