DataGuard

Oracle Database 11g Release 1 (11.1) was the first version to include the Active Data Guard feature.

Read-Only with Apply (READ ONLY WITH APPLY) - The database is open for queries while continuous redo application (Real-Time Query) occurs.

DML Redirection (New in 19c): Occasional Data Manipulation Language (DML) operations (inserts, updates, deletes) are automatically and transparently redirected from the standby session to the primary database for execution. 

Automatic Block Repair: Detects and fixes corrupted data blocks on the primary or standby database by fetching a good copy of the block from the other database in the configuration.

Buffer Cache Preservation after role transition (New in 19c): Maintains the buffer cache state during a role switch, reducing performance impact after a switchover/failover. 

Before 11g the Active DataGuard was not available.

Mounted - Database is typically closed for user access, though redo is applied. Can be opened read-only, but redo apply must be stopped. 

No DML operations allowed. 

Requires manual intervention or a failover. 

Buffer cache is typically cleared or must be repopulated.