There are two common ways to build online backup databases: Rman auxiliary and Rman backup set recovery.
(1)
When the environment configuration is OK, a Rman command can be used to complete the initialization synchronization of the standby database, which is relatively simple.

# Land
rman target sys/Oracle123@orcl_pri auxiliary sys/Oracle123@orcl_dg
# Start to build auxiliary database
RMAN>duplicate target database for standby nofilenamecheck from active database;

(two)
The recovery steps of Rman backup set are relatively more, but not very complicated.
(1) planning database parameters

(2) main database parameter configuration (do not consider failover, only configure the least parameters)

alter system set log_archive_config = 'dg_config=(orcl,orcl_s)';
alter system set log_archive_dest_2 = 'SERVICE=orcl_dg ASYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=orcl_s' scope=both;
alter system set log_archive_dest_state_2 = ENABLE;
alter system set standby_file_management = auto;
# Generate pfile as the parameter file for standby database startup
create pfile=/home/oracle/pfile.ora' from spfile;

(3) the standby database directly modifies the pfile.ora parameter and starts to the nomount state

*.db_unique_name='center_s'
*.log_archive_dest_1='location=/oradata/log_dg'
*.log_archive_dest_2='SERVICE=orcl_pri ASYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=orcl'
*.db_file_name_convert='/oradata/orcl','/oradata/orcl_dg'
*.log_file_name_convert='/oradata/log','/oradata/log_dg'
#  Start database
create spfile from pfile='/home/oracle/pfile.ora'
startup nomount

(4) configuration of tnsnames.ora and listener.ora
(5) backup the main database and upload it to the standby database

# Full library
RMAN> run {
 allocate channel t1 type disk;
 backup database FORMAT '/oradata/backup/data_%T_%s'; 
 release channel t1;
}
# Control file backup
RMAN> backup current controlfile for standby format '/oradata/backup/control.bak';

(5) backup database recovery

# Restore control file
RMAN> RESTORE STANDBY CONTROLFILE FROM '/oradata/backup/control.bak';

# Start database to mount state
RMAN> sql 'alter database mount'

# Recover database
RMAN> catalog start with '/oradata/backup/';
RMAN> restore database;
RMAN> recover database;
# Error reported in recovery, can be ignored
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of recover command at 10/17/2017 09:33:30
RMAN-06054: media recovery requesting unknown archived log for thread 1 with sequence xx  and starting SCN of xxxx

(6) create standby log file for real-time data synchronization

alter system set standby_file_management = manual;
ALTER DATABASE ADD standby LOGFILE group  11 '/oradata/log_dg/std11.log' size 500m ;
ALTER DATABASE ADD standby LOGFILE group  12 '/oradata/log_dg/std12.log' size 500m ;
ALTER DATABASE ADD standby LOGFILE group  13 '/oradata/log_dg/std13.log' size 500m ;
ALTER DATABASE ADD standby LOGFILE group  14 '/oradata/log_dg/std14.log' size 500m ;

(7) start real-time log application and open database

select open_mode ,flashback_on,current_scn from v$database;
# Application log
alter database recover managed standby database disconnect from session;
# View the application status, switch logs in the main database, and view the sequence changes
select max(sequence#),applied,archived from v$archived_log group by applied,archived;
# Open database
alter database recover managed standby database cancel;
alter database open;
alter database recover managed standby database disconnect from session using current logfile;

Oracle DG is one of the most commonly used database architectures. You must be able to skillfully build a DG environment and master the DG failover /switchover and recovery methods.