The oracle data guard forms an extension of the oracle relational database management system (RDBMS). It mainly aids in the establishing and maintaining of secondary databases “standby databases” as an alternative or supplementary repositories to production of “primary databases”. Oracle provides with both graphical user interface (GUI) and command–line tools for managing data guard configurations.

Data guard provides both physical standby and logical standby sites. Oracle corporation makes Data guard available only as a bundled feature included within its “enterprise edition” of the oracle RDBMS. With appropriate set-up of Data guard operations, database administrators can facilitate failovers or switchovers to alternative hosts in the same or alternative locations.

For the purpose of Data guard, each Oracle database functions either in a primary database role or in a standby database role-with the ability to transition from one role to another

Physical Standby (Redo Apply)

A physical standby database replicates the exact contents of its primary database across the Oracle Net network layer. The data in the database will be exactly the same as in the primary database while the relative physical storage locations can differ. Physical standby databases can function either in managed-recovery mode or in read only mode, but not in both modes at the same time. This can only occur in a case when the database is at oracle database 11.1 or higher and the active data guard options is licensed. The standby makes use of “Redo Apply” technology. Physical standby databases have the same DBID identifiers as their primary equivalents.

Logical Standby (SQL Apply)

Logical standby databases converts the Redo generated at the primary database into data and SQL and then re-apply those SQL transactions on the logical standby. Thus this makes the physical structures of an organizations to be different from the primary database. Users are able to read from the logical standby database while the changes are being applied and if the Guard is set to standby, they are able write to tables in the logical standby database that are not being maintained by SQL Apply. Unfortunately there are a number of unsupported objects and unsupported data types hence making physical standby appropriate in such a case.

Active Data Guard

This is able to allow read-only access on the physical standby node at the same time as applying archived transactions from the primary node. Also it features Automatic Block Repair and fast incremental backup on physical standby.

Advantages

  • Through the use of standby redo log files, data guard can minimize data loss.
  • Data guard provides high availability for a database system. It can also reduce the human intervention required to switch between databases at disaster-recovery (failover) or upgrade/maintenance (switchover) time
  • It supports heterogeneous configurations in which the primary and standby systems may have different CPU architectures, operating systems, operating-system binaries or oracle database binary.

Disadvantages

  • The same release of Oracle Database Enterprise Edition must be installed on the primary database and all standby databases, except during rolling database upgrades using logical standby databases.
  • If the network link connecting primary and standby is over-subscribed the redo logs are ot shipped in chronological order, which can result in large gaps appearing in the available redo at the standby. Such a condition results in the standby being behind the primary

Oracle data guard is available only as a feature of Oracle Database Enterprise Edition.

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