Always On Availability Groups (SQL Server)

The Always On Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012, Always On Availability Groups maximizes the availability of a set of user databases for an enterprise. An availability group supports a failover environment for a discrete set of user databases, known as availability databases, that fail over together. An availability group supports a set of read-write primary databases and one to eight sets of corresponding secondary databases. Optionally, secondary databases can be made available for read-only access and/or some backup operations.

An availability group fails over at the level of an availability replica. Failovers are not caused by database issues such as a database becoming suspect due to a loss of a data file, deletion of a database, or corruption of a transaction log.


Always On Availability Groups provides a rich set of options that improve database availability and that enable improved resource use. The key components are as follows:

Terms and Definitions

availability group
A container for a set of databases, availability databases, that fail over together.

availability database
A database that belongs to an availability group. For each availability database, the availability group maintains a single read-write copy (the primary database) and one to eight read-only copies (secondary databases).

primary database
The read-write copy of an availability database.

secondary database
A read-only copy of an availability database.

availability replica
An instantiation of an availability group that is hosted by a specific instance of SQL Server and maintains a local copy of each availability database that belongs to the availability group. Two types of availability replicas exist: a single primary replica and one to eight secondary replicas.

primary replica
The availability replica that makes the primary databases available for read-write connections from clients and, also, sends transaction log records for each primary database to every secondary replica.

secondary replica
An availability replica that maintains a secondary copy of each availability database, and serves as a potential failover targets for the availability group. Optionally, a secondary replica can support read-only access to secondary databases can support creating backups on secondary databases.

availability group listener
A server name to which clients can connect in order to access a database in a primary or secondary replica of an Always On availability group. Availability group listeners direct incoming connections to the primary replica or to a read-only secondary replica.


For more information, see Overview of AlwaysOn Availability Groups;SQL Serve;.

Interoperability and Coexistence with Other Database Engine Features

Always On Availability Groups can be used with the following features or components of SQL Server:


For information about restrictions and limitations for using other features with Always On Availability Groups, see Always On Availability Groups: Interoperability;SQL Server;.

Related Tasks

Related Content

See Also

Overview of Always On Availability Groups;SQL Server;
Prerequisites, Restrictions, and Recommendations for AlwaysOn Availability Groups (SQL Server)
Configuration of a Server Instance for Always On Availability Groups;SQL Server;
Creation and Configuration of Availability Groups;SQL Server;
Administration of an Availability Group;SQL Server;
Monitoring of Availability Groups (SQL Server)
Overview of Transact-SQL Statements for Always On Availability Groups;SQL Server;
Overview of PowerShell Cmdlets for AlwaysOn Availability Groups;SQL Server;