# Always On Availability Groups in SQL Server ## Overview - Always On availability groups provide high availability and disaster recovery solutions for SQL Server databases. - They support a failover environment for a set of user databases, known as availability databases. ## Key Components - **Availability Group**: A container for a set of databases that fail over together. - **Availability Replica**: Hosts a copy of each availability database. There are primary and secondary replicas. - **Availability Group Listener**: A server name that directs client connections to the appropriate replica. ## Availability Modes - **Asynchronous-Commit Mode**: Prioritizes performance, allowing some data loss. - **Synchronous-Commit Mode:** Ensures data protection with increased transaction latency. ## Failover Types: - **Automatic Failover**: Occurs without data loss when a synchronized secondary replica takes over. - **Manual Failover**: Initiated by an administrator, also without data loss. - **Forced Failover**: May result in data loss, used for disaster recovery. ## Active Secondary Capabilities: - **Read-Only Access**: Secondary replicas can handle read-only workloads. - **Backup Operations**: Backups can be performed on secondary replicas to offload the primary replica. ## Resources [What is an Always On availability group? - SQL Server Always On | Microsoft Learn](https://learn.microsoft.com/en-us/sql/database-engine/availability-groups/windows/overview-of-always-on-availability-groups-sql-server?view=sql-server-ver16)