ORA-16516: current state is invalid for the attempted operation

Problem: when I am trying to do a switch over using DG broker and I am facing the issue. 

DGMGRL> switchover to 'BHU_A';

Performing switchover NOW, please wait...

Error: ORA-16516: current state is invalid for the attempted operation

Failed.

Unable to switchover, primary database is still "BHU_B"

DGMGRL>

Reason

After analyzing I found the apply has been stopped in the standby database and noted that we should have a zero lag before proceeding with the switchover

DGMGRL> show database verbose 'BHU_A';

Database - BHU_A

  Role:            PHYSICAL STANDBY

  Intended State:  APPLY-OFF

  Transport Lag:   0 seconds

  Apply Lag:       24 minutes 26 seconds

  Real Time Query: OFF

  Instance(s):

    BHU_1 (apply instance)

    BHU_2

<Removed some lines of the output>

Enabling the Apply On process using the DG Broker

DGMGRL> edit database 'BHU_A' set state='APPLY-ON';

Succeeded.

DGMGRL> show database verbose 'BHU_A';

Database - BHU_A

  Role:            PHYSICAL STANDBY

  Intended State:  APPLY-ON

  Transport Lag:   0 seconds

  Apply Lag:       0 seconds

  Real Time Query: ON

  Instance(s):

    BHU_1 (apply instance)

    BHU_2

<Removed some lines of the output>

Starting the Switchover process again after checking the LAG.

Note: since I have write note for the above error message only. If you are performing the switchover process, a lot of check needs to be performed.

DGMGRL> switchover to 'BHU_A'

Performing switchover NOW, please wait...

New primary database "BHU_A" is opening...

Operation requires shutdown of instance "BHU_1" on database "BHU_B"

Shutting down instance "BHU_1"...

ORACLE instance shut down.

Operation requires startup of instance "BHU_1" on database "BHU_B"

Starting instance "BHU_1"...

ORACLE instance started.

Database mounted.

Switchover succeeded, new primary is "BHU_A"

DGMGRL>

I hope this document helps you to solve the issue.