Here’s the summary:
Always try to put the observer(s) on an external site. If you don’t have any, put it where the primary database is, and have one ready on the secondary site after the role transition.
Don’t put the observer together with the standby database!
Configure multiple observers for high availability, and use the PreferredObserverHosts Data Guard member property to ensure you never run the observer where the standby database is.
The importance of Fast-Start Failover in an Oracle Data Guard configuration
Oracle 19c Data Guard Broker – Managing the Observer
Oracle 19c High Availability Overview and Best Practices – Configure Fast-Start Failover
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