How to fix a DataGuard issue - logs do not ship from primary to standby

One way to check if logs get shipped and applied from primary to standby is by running the following query on both primary and standby databases:

select al.thrd "Thread",
almax "Last Seq Received",
lhmax "Last Seq Applied"
from (select thread# thrd, max(sequence#) almax
from v$archived_log
where resetlogs_change#=(select resetlogs_change# from v$database)
group by thread#) al,
(select thread# thrd, max(sequence#) lhmax
from v$log_history
where first_time=(select max(first_time) from v$log_history)
group by thread#) lh
where al.thrd = lh.thrd;

If there is a discrepancy between primary and standby, one way to fix it is by rolling forward the physical standby using RMAN incremental backups of the primary. Here is a document on how to do that.
On Standby:

1. Retrieve the current SCN number to be used for backing up database later on

   
   SQL> SELECT to_char(current_scn, '999999999999999999') FROM V$DATABASE;
   
   TO_CHAR(CURRENT_SCN
   -------------------
          XXXXXXXXXXX
   

2. Cancel the managed recovery on standby

   SQL>ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;
   

Primary:
Connect to RMAN

rman

RMAN> connect target /

RMAN> BACKUP DEVICE TYPE DISK INCREMENTAL FROM SCN XXXXXXXX DATABASE FORMAT '/tmp/DBNameStandby_%U' tag 'DBNAMESTANDBY';

Standby:
Connect standby via RMAN and then connect to the catalog:

rman 

RMAN>connect target /

RMAN>connect catalog username/pwd@catalogname

RMAN>catalog start with '/tmp/DBNameStandby';

RMAN>REPORT SCHEMA;


RMAN>STARTUP FORCE NOMOUNT;
RMAN>RESTORE STANDBY CONTROLFILE FROM TAG 'DBNAMESTANDBY';
RMAN>ALTER DATABASE MOUNT;
RMAN>RECOVER DATABASE NOREDO;

If error occurs "ORA-19573: cannot obtain exclusive enqueue for datafile 5" when executing RECOVER DATABASE NOREDO; Run ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL; via sqlplus on Standby
And finally run the following on the standby database via sqlplus:

SQL>ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE DISCONNECT FROM SESSION;