How to set parallelism for a data guard recovery process

If you need to manually set the apply-process DOP (Degree of Parallelism), you can do this by setting the property ApplyParallel=num where num is the number of parallel processes. 

In my setup, I have three databases: 

•  A primary database (hdal) 
• A physical standby database (hdal_stb) which serves as a "cascading" database 
• A terminal standby database (hdal_tstb) which receives redo from the cascading standby database  

Logged in on any one of the participating servers of your data guard configuration, start dgmgrl: 

dgmgrl / as sysdba
Connected to "hdal_stb"
Connected as SYSDBA.
DGMGRL> show configuration;

Configuration - DGConfig1

  Protection Mode: MaxPerformance
  Members:
  hdal      - Primary database
    hdal_stb  - Physical standby database
      hdal_tstb - Physical standby database (receiving current redo)

DGMGRL> edit database 'hdal_stb' set property ApplyParallel=4;

In the database's alert log, we can se that Oracle is cancelling the ongoing recovery process:

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL
2020-08-10T09:26:24.249847+02:00
MRP0: Background Media Recovery cancelled with status 16037
Managed Standby Recovery not using Real Time Apply
Recovery interrupted!
Recovered data files to a consistent state at change 27754456
2020-08-10T09:26:24.510262+02:00
ORA-16037: user requested cancel of managed recovery operation
2020-08-10T09:26:24.617519+02:00
MRP0: Background Media Recovery process shutdown (hdal)
2020-08-10T09:26:25.250904+02:00
Managed Standby Recovery Canceled (hdal)

And then starting it again:

Completed: ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT PARALLEL 4 NODELAY
2020-08-10T09:26:25.284547+02:00
Attempt to start background Managed Standby Recovery process (hdal)
Starting background process MRP0
2020-08-10T09:26:25.306150+02:00
MRP0 started with pid=77, OS id=115854
2020-08-10T09:26:25.311038+02:00
MRP0: Background Managed Standby Recovery process started (hdal)
2020-08-10T09:26:30.339894+02:00
 Started logmerger process
2020-08-10T09:26:30.378497+02:00
Managed Standby Recovery starting Real Time Apply
2020-08-10T09:26:30.515607+02:00
Parallel Media Recovery started with 4 slaves
2020-08-10T09:26:30.741846+02:00
Media Recovery Waiting for thread 1 sequence 8299 (in transit)

Verify the new setting by looking at the properties (output truncated for brevity):

DGMGRL> show database verbose hdal_stb

Database - hdal_stb

  Role:               PHYSICAL STANDBY
  Intended State:     APPLY-ON
  Transport Lag:      0 seconds (computed 1 second ago)
  Apply Lag:          0 seconds (computed 1 second ago)
  Average Apply Rate: 0 Byte/s
  Active Apply Rate:  0 Byte/s
  Maximum Apply Rate: 0 Byte/s
  Real Time Query:    OFF
  Instance(s):
    hdal

  Properties:
    DGConnectIdentifier             = 'hdal_stb'
    LogXptMode                      = 'ASYNC'
    ApplyParallel                   = '4'

The change will be propagated to the other members of the data guard configuration. The data guard broker logfile, located in $ORACLE_BASE/diag/rdbms/$UNIQUE_NAME/$ORACLE_SID/trace, shows:

08/10/2020 09:26:24
Forwarding EDIT_RES_PROP operation to member hdal for processing
08/10/2020 09:26:31
Updated broker configuration file available, loading from "$ORACLE_HOME/dbs/dr2hdal_stb.dat"
08/10/2020 09:42:35
Forwarding MON_PROPERTY operation to member hdal_tstb for processing

The cascading database hdal_tstb simply confirms that the broker configuration has been updated:

08/10/2020 09:26:31
Updated broker configuration file available, loading from "$ORACLE_HOME/dbs/dr2hdal_tstb.dat"

The primary database is also verifying the change, although a bit more detailed:

08/10/2020 09:26:24
EDIT DATABASE hdal_stb SET PROPERTY applyparallel = 4
08/10/2020 09:26:31
EDIT INSTANCE hdal ON DATABASE hdal_stb SET PROPERTY applyparallel = 4 completed successfully

Documentation for dgmgrl version 19c can be found here