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 mirror server.
database_name sysname Name of the two copies of the database in the database mirroring session.
principal_server_name sysname Name of partner server whose copy of the database is currently the principal database.
mirror_server_name sysname Name of the partner server whose copy of the database is currently the mirror database.
safety_sequence_number int Update sequence number for changes to safety_level.
role_sequence_number int Update sequence number for changes to principal/mirror roles played by the mirroring partners.
mirroring_guid uniqueidentifier Identifier of the mirroring partnership.
family_guid uniqueidentifier Identifier of the backup family for the database. Used for detecting matching restore states.
is_suspended bit Database mirroring is suspended.
is_suspended_sequence_number int Sequence number for setting is_suspended.
5 = The partners are synchronized. Failover is potentially possible. For information about the requirements for failover see, Role Switching During a Database Mirroring Session (SQL Server).