Database mirroring in SQL server

Database mirroring is high accessibility and disaster recovery highlight in SQL server. It is an alternative to improve the accessibility of a databases which supports programmed bomb over with no loss of information. This is essential element for expanding database accessibility. Fundamentally, it is move the database exchanges from one SQL Server database (Principal database) to another SQL Server database (Mirror database) on an alternate server or instance.Mirroring works just with the full recovery model.

Things that are clarify in database mirroring

  1. Advantages of database mirroring
  2. Server jobs of database mirroring
  3. Modes of database mirroring
  4. Server misfortune in database mirroring

Advantages of database mirroring

  1. Increases information assurance
  2. Increases accessibility of a database
  3. Increases the accessibility of the creation database during overhauls

Server Roles of database mirroring

There are three kind of server jobs in database mirroring

  1. Primary server
  2. Mirror server
  3. Witness server

Principal server:

The key server has the dynamic duplicate of the database (alluded to as the central database) and administrations customer demands. The foremost server advances all exchanges to the mirror server before it applies them in the chief database.

Mirror server:

The mirror server has a duplicate of the central database (alluded to as the mirror database) and applies the exchanges sent by the foremost database to keep the mirror database synchronized with the essential database.

Witness server:

The observer server is a discretionary segment of a database mirroring arrangement. Whenever present, an observer server screens the head and mirror servers to guarantee proceeded with availability and investment in the mirror session (alluded to as majority). In the event that either server loses majority, the observer server doles out the chief server job, causing programmed failover from the main server to the mirror server if fundamental. An observer server is required for programmed failover; be that as it may, one observer server can bolster a few mirror sessions since it’s anything but a serious

Modes of database mirroring

There are three kind of server modes in database mirroring

  1. High Availability
  2. High Protection
  3. High Performance

High Availability: –

This High-accessibility mode, runs synchronously. It is jumped into two classes

• With consequently disappointment: In this mode we need three server (Primary, Mirror and witness) server Requires a Witness Server occurrence. The Principal server sends the log cradle to the mirror server, and after that hangs tight for a reaction from the mirror server.

• Without Failure: – rule isn’t accessible the observer and mirror will choose programmed failover. mirror winds up on the web.

High Protection: –

This High-insurance mode, runs synchronously. Need just Primary and mirror server Always submit changes at both the Principal and Mirror. programmed failover is unimaginable.

High Performance: –

High-execution mode, runs non concurrently and the exchange well being set to off. The Principal server does not hang tight for a reaction from the mirror server subsequent to sending the log cradle. The chief server running pleasant and quick however could lose information on the mirror server.

Operating Mode

Transaction safety

Transfer mechanism

Quorum required

Witness server

Failover Type

High Availability





Automatic or Manual

High Protection





Manual only

High Performance





Forced only

Server misfortune in database mirroring

Server can be misfortune three cases in database mirroring

We should take a model server A, B and C

Head Server Lost:

The accompanying situation thinks about what happens when the chief server is lost in a High Availability situation:

MIRROR Server Lost:

On the off chance that the mirror server is lost first, the primary server is viewed as uncovered on the grounds that it can’t send information to the mirror.

WITNESS Server Lost:

At the point when the observer server falls flat, mirroring proceeds yet no programmed failover is conceivable. Loss of only one more server will mean there is no majority, and the foremost database will never again have the option to serve the database.

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