Database Reference
In-Depth Information
Figure 31-7.
AlwaysOn Availability Group
In fact, the availability group can consist of a single primary node only. This behavior helps abstract the
availability group infrastructure from applications. For example, you can set up a single-node availability group
and create listener, virtualizing a SQL Server instance during the initial stage of deployment. After that, system
administrators can start changing connection strings using listener as the server without having to worry about the
availability group infrastructure state while you are adding other nodes there.
Another useful example is changing database options that require single user access, such as enabling READ
COMMITTED SNAPSHOT isolation level. It is impossible to switch the database to
SINGLE_USER
mode with database
mirroring enabled. You can remove database mirroring and reestablish it later, although you will need to check all
connection strings, making sure that the principal server is always specified as the
Server
rather than the
Failover
Partner
. On the contrary, an AlwaysOn Availability Group allows you to remove all secondary nodes without having
to worry about connection strings. While it is still not possible to switch a database that participates in an AlwaysOn
Availability Group to
SINGLE_USER
mode, you can drop the Availability Group, change the database options, and
recreate the Availability Group in a matter of seconds with minimal impact on client applications.
Contrary to database mirroring, which works on a single database scope, AlwaysOn Availability Groups can
include multiple databases. This guarantees that all of the databases in the group will be failed over together and
always have the same primary node. This behavior is helpful when a system requires multiple databases residing on
the same server in order to be operational.
AlwaysOn Availability Groups allow read-only access to secondary nodes and also allow you to perform database
backups from them. Moreover, an application can specify that it only needs read-only access in the connection string
and the AlwaysOn Availability Group routes it to a readable secondary node automatically.
