Activating a Oracle Standby Database

In the event of a disaster on the primary machine, you need to make the standby database active as soon as possible. It is assumed that the primary database server is not available and that the standby database needs to be activated as the primary database immediately. The standby database will need to be activated in its current state - no other redo from the primary database will be applied as it is assumed to be down The steps involved are as followed:

1. If the standby database is in managed recovery mode, it will need to be cancelled and the instance shutdown using the IMMEDIATE option:

SQL> alter database recover managed standby database cancel;

Media recovery complete.

SQL> shutdown immediate;

2. Startup the instance and then MOUNT the DB in standby mode

SQL> connect / as sysdba
  Connected to an idle instance.

  SQL> startup nomount
  ORACLE instance started.

  Total System Global Area  252777660 bytes
  Fixed Size                   451772 bytes
  Variable Size             218103808 bytes
  Database Buffers           33554432 bytes
  Redo Buffers                 667648 bytes

  SQL> alter database mount standby database;

  Database altered
3. If possible, archive the current redo logs on the primary database and transfer them (along with any remaining archived redo logs) to the standby machine. It is assumed that the primary database and host is not available.
4. Apply the primary database's archived logs by recovering the standby database.

SQL> recover standby database until cancel;
5. Cancel the recovery and activate the standby database. A CANCEL will need to be performed as this is Incomplete Recovery. In issuing the ALTER DATABASE ACTIVATE STANDBY DATABASE command, the standby bit in the control file is reset. This process takes the database back to nomount mode.
SQL> alter database activate standby database;
Database altered.
This process can take several minutes to complete, depending on the speed of the server.
6. Shutdown and restart the new primary database. This step ensures that all file headers are reset and clears all buffers. Remember that in the previous step when activating the standby database, that command puts the database back into nomount mode.

SQL> shutdown immediate
  ORA-01507: database not mounted

  ORACLE instance shut down.

  SQL> startup open
  ORACLE instance started.

  Total System Global Area  252777660 bytes
  Fixed Size                   451772 bytes
  Variable Size             218103808 bytes
  Database Buffers           33554432 bytes
  Redo Buffers                 667648 bytes
  Database mounted.
  Database opened.
You cannot copy online redo logs from the primary to the standby in a failover scenario.
7. Shutdown the standby instance and backup if possible, then open the database for normal use. Users can now connect to the "new" primary database.
The standby bit in the controlfile is now set, so you can never go back - the standby is now your primary database. Since the redo log sequence was reset when the standby was opened, it is a good idea to take a full backup at this point.