How to manually register missing logsequences on a standby database

I have previously documented how to identify gaps in your standby database's log sequence. See these posts:

How to check if your physical standby database is applying logs or not
How to find out if your standby database lags behind the primary database
How to find the last archivelog received and applied in a standby database

After a successful rescue operation of my standby database, I had a 3-day lag behind the primary. It's easy to identified these using the data guard broker command below:

show database "prod_stby" RecvQEntries

Output from this command was (abbreviated):

STANDBY_RECEIVE_QUEUE
              STATUS     RESETLOGS_ID  THREAD   LOG_SEQ       TIME_GENERATED       TIME_COMPLETED        FIRST_CHANGE#         NEXT_CHANGE#       SIZE (KBs)
         NOT_APPLIED        894886266  1    372460  01/28/2021 16:46:01  01/28/2021 16:47:35        6196037493227        6196037506094            20840
         NOT_APPLIED        894886266  1    372462  01/28/2021 17:02:35  01/28/2021 18:31:30        6196037647447        6196038652945          1350187
         NOT_APPLIED        894886266  1    372463  01/28/2021 18:31:30  01/28/2021 19:09:06        6196038652945        6196039875468          1051704
         .
         .
         .

The logfiles were phyically present in the Flash Recovery Area:

cd /fra/PROD_STBY/archivelog
find . -name "*372108*"
./2021_01_25/o1_mf_1_372108_j0x8j1fc_.arc

There are two ways to inform the standby database about the presence of the logfile:

1. RMAN.
On the standby database:

rman target /

Verify that the standby database does not recognize the archivelog:

list archivelog sequence between 372106 and 372107;

using target database control file instead of recovery catalog
List of Archived Log Copies for database with db_unique_name PROD_STBY
=====================================================================

Key     Thrd Seq     S Low Time
------- ---- ------- - ---------
371883  1    372106  A 25-JAN-21
        Name: /fra/PROD_STBY/archivelog/2021_01_25/o1_mf_1_372106_j0x6x0cj_.arc

The output above confirms that sequence 372106 exists, and that sequence 372107 does not. 

To catalog the missing file:

RMAN> catalog start with '/fra/PROD_STBY/archivelog/2021_01_25/o1_mf_1_372107_j0x7k8lh_.arc';

using target database control file instead of recovery catalog
searching for all files that match the pattern /fra/PROD_STB//archivelog/2021_01_25/o1_mf_1_372107_j0x7k8lh_.arc

List of Files Unknown to the Database
=====================================
File Name: /fra/PROD_STB/archivelog/2021_01_25/o1_mf_1_372107_j0x7k8lh_.arc

Do you really want to catalog the above files (enter YES or NO)? YES
cataloging files...
cataloging done

List of Cataloged Files
=======================
File Name: /fra/PROD_STB/archivelog/2021_01_25/o1_mf_1_372107_j0x7k8lh_.arc

Confirm again, and you'll see that the new files is registered:

RMAN> list archivelog sequence between 372106 and 372107;

List of Archived Log Copies for database with db_unique_name PROD_STBY
=====================================================================

Key     Thrd Seq     S Low Time
------- ---- ------- - ---------
371883  1    372106  A 25-JAN-21
        Name: /fra/PROD_STBY/archivelog/2021_01_25/o1_mf_1_372106_j0x6x0cj_.arc

371956  1    372107  A 25-JAN-21
        Name: /fra/PROD_STBY/archivelog/2021_01_25/o1_mf_1_372107_j0x7k8lh_.arc

If the number of logfiles missing is large, use a shortcut to register them all:

RMAN> catalog start with '/fra/PROD_STBY/archivelog/2021_01_25';

The above command will register all logfiles in the directory /fra/PROD_STBY/archivelog/2021_01_25

 
2. sqlplus:

SQL> alter database register logfile '/fra/PROD_STBY/archivelog/2021_01_25/o1_mf_1_372107_j0x7k8lh_.arc';

If you tail the alert log of the database, you'll see that the standby database quickly picks up the missing logfiles.