Always On Availability Groups (SQL Server)

THIS TOPIC APPLIES TO: yesSQL Server (starting with 2016)noAzure SQL DatabasenoAzure SQL Data Warehouse noParallel Data Warehouse

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.

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).

See Also

Overview of Always On Availability Groups (SQL Server)
Prerequisites, Restrictions, and Recommendations for Always On 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 Always On Availability Groups (SQL Server)