An automatic failover is a switching role that causes the mirror server to take over the role of the principal server and bring its copy of the database online as the principal database. It involves high-safety mode and requires the presence of the mirror server and a witness server. The database should be synchronized and the witness must be associated to the mirror server. The job of the witness is to confirm whether a given partner server is up and operating. If the connection of the mirror server to the principal server is lost but the witness server is still connected to the principal server, the mirror server will not initiate a failover.
