Database mirroring is a configuration of three database servers, running on separate computers, that co-operate to maintain copies of the database and transaction log files.

The primary server and mirror server each maintain a copy of the database files and transaction log files. The primary server has the read-write copy of the database, while the mirror server has the read-only copy of the database. The third server, called the arbiter server, is used when it is necessary to determine which of the other two servers can take ownership of the database. Ownership of the database refers to the server that has the read-write copy of the database. The arbiter does not maintain a copy of the database. The configuration of three database servers (the primary, mirror, and arbiter servers) is called a mirroring system, and the primary and mirror servers together are called the partner servers.

Clients connect to the primary server to access the database. Any changes that are made to the database are recorded in the transaction log on the primary server. When the changes are committed, the transaction log pages are sent to the mirror server where they are applied to a mirror copy of the database. The copy of the database on the mirror server can only be accessed in read-only mode while that server is acting as the mirror server.

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