Always On Availability Groups#
The Always On Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Always On Availability Groups maximize 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.
Always On on Availability Groups versus on Failover Cluster Instances#
An availability group is composed by a primary availability replica and one to four secondary replicas that synchronize via log-based data movement for data protection without the need for shared storage. This reduces the overall cost of the solution.
An availability group listener on the primary node responds to connection requests to the virtual network name, and redirects each request to the appropriate SQL Server instance. In the event of a failover, the system does not transfer ownership of shared physical resources to another node. WSFC assists SQL Server in the reconfiguration of a secondary replica on another SQL Server instance to become the availability group's primary replica. The availability group virtual network name resource is moved to that instance to complete the failover process.
Always On Terms#
| Name | Definition |
|---|---|
| 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 target for the availability group. Optionally, a secondary replica can support read-only access to secondary databases and 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. |
Table: Always On Terms