11.3. Database mirroring overview

Available in the Standard and Enterprise editions of SQL Server, database mirroring is used to maintain a hot standby copy of a database. Transactions generated on the source (principal) database are sent to the mirror over an encrypted, compressed log stream and applied in either a synchronous or asynchronous manner. You can choose between the two modes to prioritize transaction safety or performance.

If the principal instance fails, an optional witness server can be used to automatically bring the mirror database online with zero data loss. With the necessary connection logic, applications can automatically reconnect to the mirrored database. The combination of synchronous mirroring with automatic failover ...

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