Database mirroring is a new technology in Microsoft® SQL Server™ 2005 database software that can deliver high availability and high performance solutions for database redundancy. In database mirroring, transactions are sent directly from a principal database and server to a mirror database and server whenever the principal database’s transaction log buffer is written to disk. This technique can keep the mirror database nearly up to date with the principal database. You can optionally use a third server, a witness server, to enable automatic failover from the principal server to the mirror server.
The update includes more prescriptive information on the type of supported topologies. It also recommends SQL connection aliasing for failover within a farm.