Oracle Data Guard: Introduction
Oracle Data Guard ensures high availability, data protection, and disaster recovery for enterprise data. Data Guard provides a comprehensive set of services that create, maintain, manage, and monitor one or more standby databases to enable production Oracle databases to survive disasters and data corruptions.
In Data Guard Configuration, production database is known as primary database and standby database is synchronized copy of primary database.
You can create one to 30 standby database from one production database.
Type of Standby Database:
There are three type of standby databases. Those are -
1. Physical Standby
2. Logical Standby
3. Snapshot Standby
1. Physical Standby Database:
Physical standby database is identical to the primary database on a block-for-block basis. The database schema, including indexes, are the same. It is synchronized with the primary database through application of redo data received from primary database.
Physical standby database can be used concurrently for data protection and reporting purpose.
2. Logical Standby Database:
Logical standby database shares the same schema definition.
Logical standby database is kept synchronized with the primary database by transferring data in the redo received from primary database into SQL statements and then executing the SQL statements.
Logical standby database can be used concurrently for data protection, reporting and database upgrades.
3. Snapshot Standby Database:
Snapshot standby database is fully update-able standby database. It is created by converting a physical standby database.
Snapshot standby database can be used for updates, but those updates are discarded before the snapshot standby database is converted back into a physical standby database.
Snapshot standby database can be used for testing.
Architecture :
Data Guard Services:
Data Guard provides three type of services:
1. Redo Transport Services
2. Apply Services
>> Redo Apply
>> SQL Apply
3. Role Management Services
1. Redo Transport Services:
Control the automated transfer of redo data from the production database to one
or more archival destinations.
2. Apply Services:
Control when and how data is applied to the standby database.
Redo apply technology is used for physical standby databases. Redo data is applied on standby database by using Oracle Media Recovery.
SQL apply technology is used for logical standby databases. The received redo data is first transformed into SQL statements, and then the generated SQL statements are executed on the logical standby database.
3. Role Management Services:
Role management services operate in conjunction with redo transport services and apply services to change roles of database ( Primary or Standby) dynamically as a planned transition ( called switchover operation ) or as a result of database failure due to the failover operation.
switchover:
The switchover feature enables you to switch the role of primary database to one available standby databases. The chosen standby database becomes primary database and the original primary database then becomes a standby database.
failover:
You invoke failover when catastrophic failure occurs on the primary database. During a failover operation, failed primary database is removed from the Data Guard environment, and a standby database assumes the primary database role.
You invoke failover operation on a standby database that you want to fail over to the primary database role.
You can also enable fast-start failover, which allows Data Guard to automatically and quickly fail over to a previously chosen synchronized standby database.
Note:
Database that are disabled after a role transition are not removed from the broker configuration, but they are disabled in the sense that the database is no longer managed by the DG broker. To re-enable broker management of these databases, you must re-instate or re-create the databases.
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