Demonstrates a how replication request_only mode supports Data Guard.

The purpose of the request_only mode is to allow a recovery appliance to request backups from the second Recovery Appliance of a pair, in order to fill gaps in its backups, such as after an offline period. Figure 15-3 shows two data centers for local and remote, where the primary site is local RA-x and the standby site is remote RA-y. Backups are taken at primary and standby databases, each to their respective local Recovery Appliances. In request_only mode, replication traffic does not occur from RA-x to RA-y. Dataguard and redo logs keeps the remote database in sync with the local database.

The replication servers are configured for bi-direction replication. When using add_replication_server, RA-y gets a protection policy as normal. However, add_replication_server for RA-x has the protection policy with REQUEST_ONLY=TRUE.

When a switchover from the primary (local) to the secondary (remote) has happened, replication traffic does not occur from RA-x to RA-y. However, the RA-x catalog is kept in sync with the RA-y backups. When RA-x is offline, standby backups continue to be sent to RA-y with full recoverability to both data centers. When RA-x comes back online, RA-x requests missing backups from RA-y in order to sync with the current state.