How to reinstate the old Primary as a Standby after Failover in #Oracle

You have done a failover to your Standby database so it becomes the new Primary. It may be possible to convert the old Primary into a Standby database now instead of having to do a time consuming duplicate again. The old Primary must have been running in flashback mode before the failover. The playground:

DGMGRL> show configuration;

Configuration - myconf

  Protection Mode: MaxAvailability
  Members:
  prima - Primary database
    physt - Physical standby database

Fast-Start Failover:  (more...)