Oracle 11g Data Guard Concepts

Oracle 11g DataGuard

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Concepts:-

• The databases in a Data Guard configuration are connected by Oracle Net and may be dispersed geographically.
• As of Oracle Database 11g release 1 (11.1), a physical standby database can receive and apply redo while it is open for read-only access. A physical standby database can therefore be used concurrently for data protection and reporting.
• A logical standby database can be used for other business purposes in addition to disaster recovery requirements. This allows users to access a logical standby database for queries and reporting purposes at any time. Also, using a logical standby database, you can upgrade Oracle Database software and patch sets with almost no downtime. Thus, a logical standby database can be used concurrently for data protection, reporting, and database upgrades.
• A snapshot standby database is a fully updatable standby database that is created by converting a physical standby database into a snapshot standby database.
• Like a physical or logical standby database, a snapshot standby database receives and archives redo data from a primary database. Unlike a physical or logical standby database, a snapshot standby database does not apply the redo data that it receives. The redo data received by a snapshot standby database is not applied until the snapshot standby is converted back into a physical standby database, after first discarding any local updates made to the snapshot standby database.
• A snapshot standby database is best used in scenarios that require a temporary, updatable snapshot of a physical standby database. Note that because redo data received by a snapshot standby database is not applied until it is converted back into a physical standby, the time needed to recover from a primary database failure is directly proportional to the amount of redo data that needs to be applied.
• The redo data transmitted from the primary database is written to the standby redo log on the standby database. 
• For physical standby databases, Data Guard uses Redo Apply technology, which applies redo data on the standby database using standard recovery techniques of an Oracle database.
• For logical standby databases, Data Guard uses SQL Apply technology, which first transforms the received redo data into SQL statements and then executes the generated SQL statements on the logical standby database.
• A switchover is a role reversal between the primary database and one of its standby databases. A switchover ensures no data loss. This is typically done for planned maintenance of the primary system. During a switchover, the primary database transitions to a standby role, and the standby database transitions to the primary role.
• A failover is when the primary database is unavailable. Failover is performed only in the event of a failure of the primary database, and the failover results in a transition of a standby database to the primary role. The database administrator can configure Data Guard to ensure no data loss