alwayson

Always On is a brand used by Microsoft to describe a family of high availability and disaster recovery features for SQL Server. It was introduced with SQL Server 2012 to unify and extend database and instance protection. The main components are Always On Failover Cluster Instances (FCI) and Always On Availability Groups (AG).

Failover Cluster Instances provide instance-level high availability by using Windows Server Failover Clustering to move an entire SQL Server instance between nodes in a cluster. Availability Groups, the more widely used feature, replicate user databases between multiple SQL Server instances and allow failover at the database level. An AG consists of a primary replica and one or more secondary replicas, typically up to seven secondaries, which can be located on the same site or across multiple sites. A listener provides a single connection endpoint for applications to reach the primary replica, even if failover occurs.

AGs support both synchronous and asynchronous data movement. Synchronous commit enables automatic failover to a healthy secondary, suitable for high availability within a site, while asynchronous commit is used for disaster recovery across longer distances. Secondary replicas can be configured for read access to offload read workloads, and backups can be taken on secondary replicas in many configurations. AGs require Windows Server Failover Clustering and, historically, higher edition licenses, though later versions introduced broader edition support such as basic availability groups.

In practice, Always On deployments are used to reduce downtime, tolerate site failures, and improve read scalability, often spanning on-premises and cloud environments. See also Windows Server Failover Clustering and SQL Server.