Once the standby database is setup using Data Guard and works properly, you may want to test switchover, or perform switchover to reduce primary database downtime during OS upgrades or hardware upgrades. (To set up a Physical Standby database, please refer to my OraFAQ blog “Oracle 10g – Manually Create a Physical Standby Database Using Data Guard”.)

A switchover allows the primary database to switch roles with its standby database. There is no data loss during a switchover. You can switch back to the original Primary database later by performing another switchover.

In case of primary database failure, you will need to perform failover to transition the standby database to the primary role. After a failover, the original primary database can no longer participate in the Data Guard configuration. So if the original Primary database is still accessible, you should always consider a switchover first.

This document only talks about switchover involving physical standby database. In this example, the original primary data is called PRIM and the original standby database is called STAN.

I. Before Switchover:

1. As I always recommend, test the Switchover first on your testing systems before working on Production.

2. Verify the primary database instance is open and the standby database instance is mounted.

3. Verify there are no active users connected to the databases.

4. Make sure the last redo data transmitted from the Primary database was applied on the standby database. Issue the following commands on Primary database and Standby database to find out:
SQL>select sequence#, applied from v$archvied_log;
Perform SWITCH LOGFILE if necessary.

In order to apply redo data to the standby database as soon as it is received, use Real-time apply.

II. Quick Switchover Steps

1. Initiate the switchover on the primary database PRIM:
SQL>connect /@PRIM as sysdba
SQL> ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY WITH SESSION SHUTDOWN;

2. After step 1 finishes, Switch the original physical standby db STAN to primary role;
Open another prompt and connect to SQLPLUS:
SQL>connect /@STAN as sysdba
SQL> ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY;

3. Immediately after issuing command in step 2, shut down and restart the former primary instance PRIM:
SQL>SHUTDOWN IMMEDIATE;
SQL>STARTUP MOUNT;

4. After step 3 completes:
- If you are using Oracle Database 10g release 1, you will have to Shut down and restart the new primary database STAN.
SQL>SHUTDOWN IMMEDIATE;
SQL>STARTUP;

- If you are using Oracle Database 10g release 2, you can open the new Primary database STAN:
SQL>ALTER DATABASE OPEN;

STAN is now transitioned to the primary database role.

5. On the new primary database STAN, perform a SWITCH LOGFILE to start sending redo data to the standby database PRIM.
SQL>ALTER SYSTEM SWITCH LOGFILE;

Reference:
1. Oracle Data Guard Concepts and Administration 10g Release 2 (10.2), B14239-04;
2. Switchover and Failover Best Practices: Oracle Data Guard 10g release 2 by Oracle.

About Author
Haili Jiao works as Oracle DBA at Baltimore, Maryland. She is an Oracle 10g Certified Professional and 9i Certified Associate.