To bring one of the standby databases as the new primary database and perform a failover, especially in the case of a catastrophic failure in the primary database where it cannot be restored in a timely manner, the problematic primary database is ignored, and the target standby database takes on the role of the new primary database.

Failover is performed on the standby database that will become the new primary database. After the failover is complete, the standby database will assume the role of the primary database, while the original primary database cannot be used as a standby database unless it is recreated or restored using Flashback Database. Therefore, it is recommended to enable Flashback Database on both the primary and standby databases.

Here are the steps to execute a failover:

Failover Execution Steps:

1. Connect to the Target Standby Database

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DGMGRL> CONNECT sys/sys@STANNET
Connected.

2. Execute the Failover Command

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DGMGRL> FAILOVER TO "STAN";
Performing failover NOW. Please wait...
Operation requires shutdown of instance "STAN" on database "STAN".
Shutting down instance "STAN"...
database not mounted
ORACLE instance shut down.
Operation requires startup of instance "STAN" on database "STAN".
Starting instance "STAN"...
ORACLE instance started.
Database mounted.
Failover succeeded. New primary is "STAN"

3. Confirm Failover Success

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DGMGRL> SHOW CONFIGURATION;
Configuration
  Name:            TEST
  Enabled:         YES
  Protection Mode: MaxPerformance
  Databases:
    PRIM - Physical standby database
    STAN - Primary database
Current status for "TEST":
SUCCESS

4. Verify the Original Primary Database

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DGMGRL> SHOW DATABASE 'PRIM';
Database
  Name:            PRIM
  Role:            PHYSICAL STANDBY
  Enabled:         NO
  Intended State:  ONLINE
  Instance(s):
    PRIM
Current status for "PRIM":
Error: ORA-16795: Database resource guard detects that database reinstantiation is required

Notice that the Data Guard Broker has disabled the old primary database.

Data Guard Broker Commands

Oracle Data Guard provides a set of commands to manage and monitor databases. You can view the available commands using the HELP command:

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DGMGRL> HELP
The following commands are available:
Add:       Add a standby database into the broker configuration
Connect:   Connect to an Oracle instance
Create:    Create a broker configuration
Disable:   Disable broker control of a configuration or database
Edit:      Edit a configuration, database, or instance
Enable:    Enable broker control of a configuration or database
Exit:      Exit the program
Failover:  Change a standby database to be the primary database
Help:      Display description and syntax for a given command
Quit:      Exit the program
Rem:       Comment to be ignored by DGMGRL
Remove:    Remove a configuration, database, or instance
Show:      Display information of a configuration, database, or instance
Shutdown:  Shut down a currently running Oracle instance
Startup:   Start an Oracle database instance
Switchover: Switch roles between the primary database and a standby database

To learn the syntax for a specific command, use HELP followed by the command name:

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DGMGRL> HELP CREATE
Create a broker configuration
Syntax:
  CREATE CONFIGURATION <configuration_name> AS
    PRIMARY DATABASE IS <database_name>
    CONNECT IDENTIFIER IS <connect_identifier>;

By following these steps and commands, you can efficiently manage the failover process and handle the database roles in Oracle Data Guard.