ORA-12537: TNS:connection closed - standby error in DGMGRL

Error: ORA-12537: TNS:connection closed

Details: Occurred on a standby database after switchover from primary to standby. Error came up when checking DataGuard broker configuration.

Cause: The mrp process not running on standby
Solution:
Checked if MRP process was running on standby by executing:

ps -ef | grep mrp

Nothing was running.
Performing regular maintenance on standby database fixed the issue

SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP NOMOUNT;
SQL> ALTER DATABASE MOUNT STANDBY DATABASE;
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION; 

Oracle DataGuard switchiver vs. failover

Switchover is a planned role reversal between primary and standby databases in DataGuard configuration. it guarantees no data loss and is typically done for maintenance. There is no need to re-enable either database when the transition occurs.

Failover is not planned and is typically performed in event of primary database failure event, when there is no possibility of recovering primary database in a timely fashion. When failure occurs, standby database is taking over the primary role. Failover may result in data loss, depending on the protection mode enabled at the time failover occurs.

If a DataGuard broker is enabled, we can automate the failover, however if DataGuard broker is not enabled, we will have to perform manual failover.

If a DataGuard broker cannot detect the heartbeat of the primary DB, it will automatically perform the failover.