What is the difference between Switchover and Failover in Oracle Dataguard?

A switchover means just switching roles between the primary database and standby db.
nswitchover, the primary database chnaged to a standby role, and the standby database changed to the primary role.
This is typically done for planned maintenance of the primary db server.

A failover is when the primary database fails and one of the standby databases is 
transitioned to take over the primary role. Failover is performed only in the event 
of a catastrophic failure of the primary database, and there is no possibility of 
recovering the primary database in a timely manner. Failover may or may not result 
in data loss depending on the protection mode in effect at the time of the failover.

How to enable ArchiveLog mode in Oracle Database RAC Environment?

Steps to enable Archivelog mode in RAC environment:

The following steps need to be taken to enable archive logging in a RAC database environment:

1. Shutdown immediate all database instances
$ srvctl stop database -d <db_unique_name>

2. Startup database in mount mode
$ srvctl start database -d <db_unique_name> -o mount

3. Enable archive logging
$ sqlplus / as sysdba
sql> alter database archivelog;
sql> exit;

4. Stop database
$ srvctl stop database -d <db_unique_name>

5.Start all database instances
$ srvctl start database -d <db_unique_name>

6.Verify archiving is enabled/disabled 
sql> archive log list;

How to mirror database Privileges of a Oracle database user to another database user?

Script for Mirroring Privileges of a user:

Scenario: A Database schema named "TEST2" is created and business wants to grant the privileges for the user same as already existing Database user "TEST1".

Run the below queries as SYS or any Privileged user to get the DDL commands in Oracle for TEST1 user.

SELECT DBMS_METADATA.GET_GRANTED_DDL('ROLE_GRANT','TEST1') FROM DUAL;

 SELECT DBMS_METADATA.GET_GRANTED_DDL('SYSTEM_GRANT','TEST1') FROM DUAL;

 SELECT DBMS_METADATA.GET_GRANTED_DDL('OBJECT_GRANT','TEST1') FROM DUAL;


 The output will be a sequence of grant commands related to privileges assigned to TEST1 user. Now replace the string TEST1 with new user name "TEST2" and run the commands.

 This will grant all privileges same as TEST1 user to TEST2 user in Oracle Database.