Microsoft® SQL Server 2012 Unleashed

Playing Roles and Switching Roles

A role corresponds to what a server is doing at a particular point in time. There are three possible roles:

Witness role—If a server is playing a witness role, it is essentially standing alongside both partners of a database mirror configuration and is used to settle all arguments. It is getting together with any one of the other servers and forming a quorum to come up with decisions. The decision that it will participate in is whether to fail over. That is it. As mentioned before, the witness server can be any edition of a SQL Server (even SQL Server Express, the free version).

Principal role—If a server is ...