DatabaseMirror

DatabaseMirror is a high-availability solution for Microsoft SQL Server databases. It provides a near real-time copy of a production database on a separate server, ensuring data redundancy and minimizing downtime in case of hardware or software failures. The primary server, known as the principal, is the active database that handles all read and write operations. The secondary server, known as the mirror, is a passive copy that receives transaction logs from the principal and applies them to its own database. In the event of a failure on the principal server, the mirror can be promoted to become the new principal, allowing for quick recovery and minimal data loss.

DatabaseMirror operates in one of three modes: high safety, high performance, or high availability. High safety mode ensures that all transactions are committed on both the principal and mirror before acknowledging the client, providing the highest level of data protection. High performance mode acknowledges transactions to the client as soon as they are committed on the principal, potentially leading to some data loss in case of a failure. High availability mode is a compromise between the two, acknowledging transactions after they are committed on the principal and before they are applied to the mirror.

DatabaseMirror is a synchronous replication technology, meaning that the mirror server must acknowledge receipt and application of transaction logs before the principal can proceed. This ensures that the mirror is always up-to-date with the principal, but it may introduce some latency in write operations. It is important to note that DatabaseMirror does not provide automatic failover; manual intervention is required to promote the mirror to principal.

DatabaseMirror was introduced in SQL Server 2005 and has been deprecated in SQL Server 2012, with AlwaysOn Availability Groups being recommended as the successor technology for high-availability and disaster recovery solutions.