Database Theory

Delaying the application of logs to a physical standby

A standby database automatically applies redo logs when they arrive from the primary database. But in some cases, we want to create a time lag between the archiving of a redo log at the primary site, and the application of the log at the standby site.

Modify the LOG_ARCHIVE_DEST_n initialization parameter on the primary database to set a delay for the standby database.

The following is an example of how to add a 1-hour delay:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_2='SERVICE=stdby_srvc DELAY=60';

The DELAY attribute is expressed in minutes.

The archived redo logs are still automatically copied from the primary site to the standby site, but the logs are not immediately applied to the standby database. The logs are applied when the specified time interval expires.
 
11g Snapshot Standby Database

Oracle 11g introduces the Snapshot Standby database which essentially is an updateable standby database which has been created from a physical standby database.

We can convert a physical standby database to a snapshot standby database, do some kind of testing on a database which is a read write copy of the current primary or production database and then finally revert it to it’s earlier state as a physical standby database.

While the snapshot standby database is open in read-write mode, redo is being received from the primary database, but is not applied.

After converting it back to a physical standby database, it is resynchronized with the primary by applying the accumalated redo data which was earlier shipped from the primary database but not applied.

Using a snapshot standby, we are able to do real time application testing using near real time production data. Very often we are required to do production clones for the purpose of testing. But using snapshot standby databases we can meet the same requirement sparing the effort,time,resources and disk space.

Reference:

http://gavinsoorma.com/2009/07/11g-snapshot-standby-database/

11g Oracle Active Data Guard

Oracle Active Data Guard enables read-only access to a physical standby database for queries, sorting, reporting, web-based access, etc., while continuously applying changes received from the production database. 


Oracle Active Data Guard also enables the use of fast incremental backups when offloading backups to a standby database, and can provide additional benefits of high availability and disaster protection against planned or unplanned outages at the production site.