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In this lesson, we will discuss about, high availability, disaster recovery, old hand-database mirroring, new rock star availability group, availability database, availability replica, availability group listener, readable secondary replicas and AS +BI.
It masks the effects of a hardware or software failure, outrage, planned/unplanned maintenance and maintain the availability of applications so that the perceived downtime for users is maintained. For example: database mirroring.
It is the process, policies and procedure which are related to preparing for recovery or continuation of technology infrastructure which are vital to an organisation after a natural or human -induced disaster. It is a subset of business continuity. Example: database replication.
Old-hand database mirroring:
Database mirroring maintains two copies of a single database which must reside on different instances of SQL Server database Engine (server instances). Typically, these server instances lie on computers in different location. One server instance serves the database to client (the principal server), while the other server instance acts as a hot or warm stand by server (the mirror server), depending upon the configuration and state of the mirroring session.
The feature of database mirroring was first introduced with Microsoft SQL Server 2005.
Why not Database Mirroring?
The feature of database mirroring has now been marked for deprecation which means that this feature will be removed from future release of SQL server. The mirroring database is in a constantly in a restoring state (almost real time), a database snapshot will be required to use the database for reporting requirements. The only possibility is to use Database snapshots to get a consistent view of your database at a given point in time and refresh the database snapshot on a regularly basis.
The failover is done on the Mirroring session level, which means you can only failover 1 database at a given time. Again-if you have more than 1 database, you need different mirroring session and you have to coordinate a multi-database failover at your own.
A DBM can be used to maintain a single standby database, or mirror database, for a corresponding production database that is referred to as the principal database.
New Rockstar availability group:
It was introduced with SQL Server 2012;Always On Availability Groups maximizes the availability of a set (instances where an application is using more than one database) of user database. An availability group support a failover environment for a discrete set of user databases, known as availability database that fail over together as one single unit.
Database availability and database performance are terms that are often confused with one another, and indeed, there are similarities between the two. The major difference lies in the user’s ability to access the database. It is possible to access a database suffering from poor performance, but it is not possible to access a database that is unavailable. If performance suffers to such a great degree that the users of the database cannot perform their job, the database has become, for all intents and purposes, unavailable.
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