Database Mirroring

Database Mirroring is a deprecated (but still used) high-availability feature in SQL Server that maintains two copies of a database—one on a principal server and the other on a mirror server. All transactions from the principal are sent to the mirror to maintain a hot standby.

There are two modes:

• High-safety (synchronous): ensures zero data loss; automatic failover with a witness server.
• High-performance (asynchronous): allows data loss but offers faster throughput over long distances.

Though Microsoft recommends Always On Availability Groups for new deployments, Database Mirroring remains in use due to its simplicity and lower licensing complexity. It supports automatic failover and fast recovery, making it suitable for mission-critical workloads in older systems.