Database Reference
In-Depth Information
Database mirroring has been deprecated in SQL Server 2012. AlwaysOn Availability Groups are a great replacement
for database mirroring; however, it requires the Enterprise Edition of SQL Server to work. Microsoft did not announce
any replacement for database mirroring in the Standard edition of SQL Server as of the writing of this topic.
Database mirroring still works in SQL Server 2014. Even though it is not known when mirroring will be removed
from the product; historically, Microsoft has kept deprecated features available for at least three major releases.
Hopefully, SQL Server will have high availability technology that replaces database mirroring and works in the
Standard Edition of SQL Server by the time database mirroring is removed from the engine.
■
Note
you can read more about database mirroring at:
http://technet.microsoft.com/en-us/library/ms189852.aspx
AlwaysOn Availability Groups
AlwaysOn Availability Groups
were introduced in the Enterprise Edition of SQL Server 2012. You can think of
AlwaysOn Availability Groups as enhanced database mirroring with a set of new, useful features. AlwaysOn works
in a similar manner to database mirroring under the hood; therefore, all of the performance and data safety
considerations from the Database Mirroring section of this chapter apply here.
AlwaysOn Availability Groups require and rely on the Windows Server Failover Clustering Cluster. While this
makes the AlwaysOn infrastructure and setup more complicated as compared to database mirroring, it also simplifies
the deployment of client applications. They can connect to the AlwaysOn Availability Group through the
listener
,
which virtualizes a SQL Server instance in a similar way as the SQL Server Failover Cluster.
The AlwaysOn Availability Group consists of one primary node with read/write access and up to four secondary
nodes in SQL Server 2012, and up to eight secondary nodes in SQL Server 2014. The three nodes in the availability
group can use synchronous commit. Two nodes support automatic failover. Figure
31-7
shows an example of an
AlwaysOn Availability Group configuration.
