Manually Fail Over a Database Mirroring Session (Transact-SQL)
Applies To: SQL Server 2016
When the mirrored database is synchronized (that is, when the database is in the SYNCHRONIZED state), the database owner can initiate manual failover to the mirror server. Manual failover can be initiated only from the principal server.
Connect to the principal server.
Set the database context to the master database:
Issue the following statement on the principal server:
ALTER DATABASE database_name SET PARTNER FAILOVER, where database_name is the mirrored database.
This initiates an immediate transition of the mirror server to the principal role.
On the former principal, clients are disconnected from the database and in-flight transactions are rolled back.