The figure shows Oracle Database with Oracle Data Guard architecture.

The production database is connected over the network to the physical standby database site and the logical standby database site (the standby databases may be at the same or different sites). The Oracle Data Guard broker communicates with the production database, the physical standby database, and the logical standby database.

The production database transmits redo data (either synchronously or asynchronously) to redo log files at the physical standby database. Then, the redo data is applied from the logs to the physical standby database, which backs up the redo data to physical media.

At the logical standby database, the redo data is transformed into SQL statements, which are applied to the logical standby database. The logical standby database may contain additional indexes and materialized views. Clients are connected to the logical standby database and can work with its data.

At the snapshot standby database redo data is received, but it is not applied until the snapshot standby database is reconverted to a physical standby database. The figure shows users making local updates to the snapshot standby database. These updates are discarded when the snapshot database is reconverted to a physical standby database.