A SQL Server high-availability and disaster recovery solution that enables automatic or manual failover of one or more databases to a secondary replica, with optional readable secondaries.
- Always On Availability Groups (AG) are the primary HA/DR architecture for SQL Server: They are used in both on-premises and cloud environments.
- Supports synchronous commit mode and asynchronous commit mode: Synchronous commit mode enables zero data loss failover, while asynchronous commit mode is designed for geographically distributed replicas.
- Secondary replicas can be configured as readable: This allows reporting queries, backups, and DBCC checks to be offloaded from the primary replica.
- DBAs must monitor AG health metrics including: Synchronization state, redo queue size, log send queue, and failover readiness.
- Common DBA tasks include: Configuring listeners, monitoring replication latency, managing failover policies, and validating recovery point objectives (RPO).
- Always On AGs work in conjunction with Windows Server Failover Clustering (WSFC): They are also supported in containerized and Kubernetes environments.
- Relevant Idera tools: SQL Diagnostic Manager monitors AG replica health, synchronization lag, and failover status in real time.
- Related terms: Failover Cluster, Log Shipping, Database Mirroring (deprecated), High Availability, RPO/RTO.
