A physical standby database is kept synchronized with the primary database, though Redo Apply , which recovers the redo data received from the primary database and applies the redo to the physical standby database. A physical standby database can be used for business purposes other than disaster recovery on a limited basis.
Contains the same logical information as the production database, although the physical organization and structure of the data can be different.
The logical standby database is kept synchronized with the primary database though SQL Apply , which transforms the data in the redo received from the primary database into SQL statements and then executing the SQL statements on the standby database. 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. Figure shows a typical Data Guard configuration that contains a primary database that transmits redo data to a standby database.
The standby database is remotely located from the primary database for disaster recovery and backup operations. You can configure the standby database at the same location as the primary database. However, for disaster recovery purposes, Oracle recommends you configure standby databases at remote locations.
Figure shows a t ypical Data Guard configuration in which redo is being applied out of standby redo log files to a standby database. The following sections explain how Data Guard manages the transmission of redo data, the application of redo data, and changes to the database roles:.
Control the automated transfer of redo data from the production database to one or more archival destinations. Apply redo data on the standby database to maintain transactional synchronization with the primary database. Redo data can be applied either from archived redo log files, or, if real-time apply is enabled, directly from the standby redo log files as they are being filled, without requiring the redo data to be archived first at the standby database.
Change the role of a database from a standby database to a primary database, or from a primary database to a standby database using either a switchover or a failover operation. Redo transport services control the automated transfer of redo data from the production database to one or more archival destinations.
A utomatically detect missing or corrupted archived redo log files on a standby system and automatically retrieve replacement archived redo log files from the primary database or another standby database. The redo data transmitted from the primary database is written on the standby system into standby redo log files, if configured, and then archived into archived redo log files. Log apply services automatically apply the redo data on the standby database to maintain consistency with the primary database.
It also allows read-only access to the data. The main difference between physical and logical standby databases is the manner in which log apply services apply the archived redo data:.
F or 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, as shown in Figure F or 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, as shown in Figure An Oracle database operates in one of two roles: primary or standby.
Using Data Guard, you can change the role of a database using either a switchover or a failover operation. 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. The transition occurs without having to re-create either database. A failover is when the primary database is unavailable. Failover is performed only in the event of a catastrophic 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 e nsure no data loss. The role transitions described in this documentation are invoked m anually using SQL statements. The Data Guard broker is a distributed management framework that automates the creation, maintenance, and monitoring of Data Guard configurations.
Create and enable Data Guard configurations, including setting up redo transport services and log apply services. Manage and monitor Data Guard configurations that contain Real Application Clusters primary or standby databases.
Simplify switchovers and failovers by allowing you to invoke them using either a single key click in Oracle Enterprise Manager or a single command in the DGMGRL command-line interface. Enable fast-start failover to fail over automatically when the primary database becomes unavailable. When fast-start failover is enabled, the Data Guard broker determines if a failover is necessary and initiates the failover to the specified target standby database automatically, with no need for DBA intervention and with no loss of data.
Creating a physical or logical standby database from a backup copy of the primary database. Adding new or existing standby databases to an existing Data Guard configuration. Monitoring log apply rates, capturing diagnostic information, and detecting problems quickly with centralized monitoring, testing, and performance tools. Oracle Enterprise Manager, also referred to as Enterprise Manager, provides a web-based interface for viewing, monitoring, and administering primary and standby databases in a Data Guard configuration.
Enterprise Manager's easy-to-use interfaces combined with the broker's centralized management and monitoring of the Data Guard configuration enhance the Data Guard solution for high availability, site protection, and data protection of an enterprise.
From the Enterprise Manager Central Console, all management operations can be performed locally or remotely. You can view home pages for Oracle databases, including primary and standby databases and instances, create or add existing standby databases, start and stop instances, monitor instance performance, view events, schedule jobs, and perform backup and recovery operations.
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