Database mirroring concept provides the failover support to database centric applications. Database mirroring continuously streams transactions from a principal server to a mirror server. This type of configuration can be used in situations with moderately large amounts of data, a network with sufficient bandwidth to support the transaction load, and the need for physically dispersed backup servers.
In SQL Server 2005 database mirroring relies on a new copy-on-write technology . This availability option requires three servers—a principal, a mirror and a witness.
The principal server writes each transaction to the database and then sends a copy of the transaction to a mirror server, which applies the copied transaction as soon as it is received. The witness assists the principal and the mirror to determine whether the other is up and running. If the witness detects that the principal server is no longer available, it instantly designates the mirror server as the new principal server and redirects all client transactions to the new principal server. This failover process has very low latency and can be used to resynchronize the failed principal when it comes back online as the mirror.