What is the difference between Always On Availability Groups and regular replication?

Always On Availability Groups (AG) and traditional replication (e.g., transactional replication in SQL Server) are different technologies for ensuring high availability and fault tolerance, but with different goals and mechanisms.

Always On Availability Groups:

• Allow creating database groups that are replicated between multiple SQL Server instances.
• Support automatic failover between primary and secondary replicas.
• Replication can be synchronous or asynchronous, ensuring minimal data loss.
• Used for high availability and Disaster Recovery.
• Enable reading data from secondary replicas (read-only).

Traditional replication (transactional):

• Designed for spreading data changes between databases.
• Can be unidirectional or bidirectional.
• Does not support automatic failover.
• Used for read scaling, data integration, and load distribution.
• Replication may have delays, affecting data currency.

Key differences:

• Always On AGs focus on high availability and fault tolerance with automatic failover, while traditional replication focuses on data distribution and scaling.
• AGs operate at the database group level, replication at the level of individual tables or databases.
• AGs support synchronous replication with consistency guarantees, while traditional replication is often asynchronous.

Thus, the choice between them depends on requirements for fault tolerance, scaling, and system architecture.