Data Guard Broker
The Data Guard broker is a distributed management framework that automates the creation, maintenance, and monitoring of Data Guard configurations. You can use either the Oracle Enterprise Manager graphical user interface (GUI) or the Data Guard command-line interface (DGMGRL) to:
¦ Create and enable Data Guard configurations, including setting up redo transport services and log apply services
¦ Manage an entire Data Guard configuration from any system in the configuration
¦ 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.
In addition, Oracle Enterprise Manager automates and simplifies:
¦ 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
Data Guard Protection Modes
In some situations, a business cannot afford to lose data. In other situations, the availability of the database may be more important than the loss of data. Some applications require maximum database performance and can tolerate some small
amount of data loss. The following descriptions summarize the three distinct modes of data protection.
Maximum protection This protection mode ensures that no data loss will occur if the primary database fails. To provide this level of protection, the redo data needed to recover each transaction must be written to both the local online redo log and to the standby redo log on at least one standby database before the transaction commits. To ensure data loss cannot occur, the primary database shuts down if a fault prevents it from writing its redo stream to the standby redo log of at least one transactionally consistent standby database.
Maximum availability This protection mode provides the highest level of data protection that is possible without compromising the availability of the primary database. Like maximum protection mode, a transaction will not commit until the redo needed to recover that transaction is written to the local online redo log and to the standby redo log of at least one transactionally consistent standby database. Unlike maximum protection mode, the primary database does not shut down if a fault prevents it from writing its redo stream to a remote standby redo log. Instead, the primary database operates in maximum performance mode until the fault is corrected, and all gaps in redo log files are resolved. When all gaps are resolved, the primary database automatically resumes operating in maximum availability mode.
This mode ensures that no data loss will occur if the primary database fails, but only if a second fault does not prevent a complete set of redo data from being sent from the primary database to at least one standby database.
Maximum performance This protection mode (the default) provides the highest level of data protection that is possible without affecting the performance of the primary database. This is accomplished by allowing a transaction to commit as soon as the redo data needed to recover that transaction is written to the local online redo log. The primary database’s redo data stream is also written to at least one standby database, but that redo stream is written asynchronously with respect to the transactions that create the redo data.
When network links with sufficient bandwidth are used, this mode provides a level of data protection that approaches that of maximum availability mode with minimal impact on primary database performance.
The maximum protection and maximum availability modes require that standby redo log files are configured on at least one standby database in the configuration. All three protection modes require that specific log transport attributes be specified on the LOG_ARCHIVE_DEST_n initialization parameter to send redo data to at least one standby database. See Section 5.6 for complete information about the data protection modes.
Summary of Data Guard Benefits
Data Guard offers these benefits:
¦ Disaster recovery, data protection, and high availability
Data Guard provides an efficient and comprehensive disaster recovery and high availability solution. Easy-to-manage switchover and failover capabilities allow role reversals between primary and standby databases, minimizing the downtime of the primary database for planned and unplanned outages.
¦ Complete data protection
Data Guard can ensure no data loss, even in the face of unforeseen disasters. A standby database provides a safeguard against data corruption and user errors. Storage level physical corruptions on the primary database do not propagate to
the standby database. Similarly, logical corruptions or user errors that cause the primary database to be permanently damaged can be resolved. Finally, the redo data is validated when it is applied to the standby database.
¦ Efficient use of system resources
The standby database tables that are updated with redo data received from the primary database can be used for other tasks such as backups, reporting, summations, and queries, thereby reducing the primary database workload necessary to perform these tasks, saving valuable CPU and I/O cycles. With a logical standby database, users can perform normal data manipulation on tables in schemas that are not updated from the primary database. A logical standby database can remain open while the tables are updated from the primary database, and the tables are simultaneously available for read-only access. Finally, additional indexes and materialized views can be created on the maintained tables for better query performance and to suit specific business requirements.
¦ Flexibility in data protection to balance availability against performance requirements
Oracle Data Guard offers maximum protection, maximum availability, and maximum performance modes to help enterprises balance data availability against system performance requirements.
¦ Automatic gap detection and resolution
If connectivity is lost between the primary and one or more standby databases (for example, due to network problems), redo data being generated on the primary database cannot be sent to those standby databases. Once a connection is
reestablished, the missing archived redo log files (referred to as a gap) are automatically detected by Data Guard, which then automatically transmits the missing archived redo log files to the standby databases. The standby databases are synchronized with the primary database, without manual intervention by the DBA.
¦ Centralized and simple management
The Data Guard broker provides a graphical user interface and a command-line interface to automate management and operational tasks across multiple databases in a Data Guard configuration. The broker also monitors all of the systems within a single Data Guard configuration.
¦ Integration with Oracle Database
Data Guard is a feature of Oracle Database Enterprise Edition and does not require separate installation.
¦ Automatic role transitions
When fast-start failover is enabled, the Data Guard broker automatically fails over to a synchronized standby site in the event of a disaster at the primary site, requiring no intervention by the DBA. In addition, applications are automatically
notified of the role transition.