Oracle Data Guard
Encyclopedia
The software which Oracle Corporation
markets as Oracle Data Guard forms an extension to the Oracle
RDBMS
. It aids in establishing and maintaining secondary "standby databases" as alternative/supplementary repositories to production "primary databases".
Oracle Corporation provides both GUI
and command-line tools for managing Data Guard configurations.
Data Guard supports 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 appropriately set-up Data Guard operations, DBA
s can facilitate failover
s or switchover
s to alternative hosts in the same or alternative locations.
. While the physical storage locations can be different, the data in the database will be exactly the same as the primary database. It can function either in managed Recovery mode or in read-only mode, but not in both modes at the same time unless the databases are at Oracle Database 11.1 or higher and the Active Data Guard option is licensed (See below).
Unfortunately there are a number of unsupported objects (ie: tables or sequences owned by SYS, tables that use table compression, tables that underlie a materialized view or Global temporary tables (GTTs)) and unsupported data types (ie: Datatypes BFILE, ROWID, and UROWID, user-defined TYPEs, Multimedia data types like Oracle Spatial, ORDDICOM, and Oracle Text
Collections (e.g. nested tables, VARRAYs), SecureFile LOBs, OBJECT RELATIONAL XMLTypes and BINARY XML). Physical standby may be appropriate in such a case.
extends Oracle Data Guard physical standby functionality in Oracle 11g configurations. It allows read-only access on the standby node at the same time as archiving transactions from the primary node.
s and send them to the standby database,
where the RFS (remote file server) background process within the Oracle instance performs the task of receiving archived redo-logs originating from the primary database.
Alternatively, a supplementary mechanism may transfer the archived redo logs. On the standby database a Fetch Archive Log (FAL) client monitors for gaps in the sequence of received logs. If it finds a gap, it may invoke one or more Fetch Archive Log (FAL) servers to run on the primary database to forward the missing item(s).
Once the archived redo logs have arrived, other processes (such as an ARCH (Archiver process), an MRP (Managed Recovery Process), and/or an LSP (Logical Standby Process)) may set about applying the log contents to the standby database.
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.
If the network link connecting primary and standby is over-subscribed, the redo logs are not applied 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.
Data Guard restricts use to like platform and Oracle RDBMS versions.
Active Data Guard is an added cost option bundled within "Enterprise Edition" of the Oracle RDBMS and allows purely read-only access.
Substantial costs savings are possible using Oracle Standard Edition (SE), Oracle Standard Edition One (SE1) or even Oracle Express (XE) with use of a third-party products to provide high availability
for a database system.
Oracle Corporation
Oracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...
markets as Oracle Data Guard forms an extension to the Oracle
Oracle Database
The Oracle Database is an object-relational database management system produced and marketed by Oracle Corporation....
RDBMS
Database management system
A database management system is a software package with computer programs that control the creation, maintenance, and use of a database. It allows organizations to conveniently develop databases for various applications by database administrators and other specialists. A database is an integrated...
. It aids in establishing and maintaining secondary "standby databases" as alternative/supplementary repositories to production "primary databases".
Oracle Corporation provides both GUI
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
and command-line tools for managing Data Guard configurations.
Data Guard supports both physical standby and logical standby sites. Oracle Corporation makes Data Guard available only as a bundled
Product bundling
Product bundling is a marketing strategy that involves offering several products for sale as one combined product. This strategy is very common in the software business , in the cable television industry Product bundling is a marketing strategy that involves offering several products for sale as...
feature included within its "Enterprise Edition" of the Oracle RDBMS.
With appropriately set-up Data Guard operations, DBA
Database administrator
A database administrator is a person responsible for the design, implementation, maintenance and repair of an organization's database. They are also known by the titles Database Coordinator or Database Programmer, and is closely related to the Database Analyst, Database Modeller, Programmer...
s can facilitate failover
Failover
In computing, failover is automatic switching to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active application, server, system, or network...
s or switchover
Switchover
Switchover is the manual switch from one system to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active server, system, or network, or to perform system maintenance, such as installing patches, and upgrading software or...
s to alternative hosts in the same or alternative locations.
Physical standby (Redo Apply)
A Physical standby database replicates the exact contents of its primary database across the Oracle Net network layerNetwork layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
. While the physical storage locations can be different, the data in the database will be exactly the same as the primary database. It can function either in managed Recovery mode or in read-only mode, but not in both modes at the same time unless the databases are at Oracle Database 11.1 or higher and the Active Data Guard option is licensed (See below).
Logical standby (SQL Apply)
Logical standby databases convert the redo generated at the Primary database into data and SQL and then re-apply those SQL transactions on the Logical standby, thus physical structures and organization will be different from the Primary database. Users can read from logical standby databases while the changes are being applied and, if the GUARD is set to STANDBY (ALTER DATABASE GUARD STANDBY;), write to tables in the Logical standby database that are not being maintained by SQL Apply.Unfortunately there are a number of unsupported objects (ie: tables or sequences owned by SYS, tables that use table compression, tables that underlie a materialized view or Global temporary tables (GTTs)) and unsupported data types (ie: Datatypes BFILE, ROWID, and UROWID, user-defined TYPEs, Multimedia data types like Oracle Spatial, ORDDICOM, and Oracle Text
Collections (e.g. nested tables, VARRAYs), SecureFile LOBs, OBJECT RELATIONAL XMLTypes and BINARY XML). Physical standby may be appropriate in such a case.
Active Data Guard
The "Oracle Active Data Guard" option, an extra-cost facility,extends Oracle Data Guard physical standby functionality in Oracle 11g configurations. It allows read-only access on the standby node at the same time as archiving transactions from the primary node.
Server-side functionality
LNS (log-write network-server) and ARCH (archiver) processes running on the primary database select archived redo logRedo log
In the Oracle RDBMS environment, redo logs comprise files in a proprietary format which log a history of all changes made to the database. Each redo log file consists of redo records...
s and send them to the standby database,
where the RFS (remote file server) background process within the Oracle instance performs the task of receiving archived redo-logs originating from the primary database.
Alternatively, a supplementary mechanism may transfer the archived redo logs. On the standby database a Fetch Archive Log (FAL) client monitors for gaps in the sequence of received logs. If it finds a gap, it may invoke one or more Fetch Archive Log (FAL) servers to run on the primary database to forward the missing item(s).
Once the archived redo logs have arrived, other processes (such as an ARCH (Archiver process), an MRP (Managed Recovery Process), and/or an LSP (Logical Standby Process)) may set about applying the log contents to the standby database.
Client-side access
The Data Guard Broker subsystem can aid in the setup, management and monitoring of Data Guard configurations.Advantages
Data Guard provides high availabilityHigh availability
High availability is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period....
for a database system. It can also reduce the human intervention required to switch between databases at disaster-recovery
Disaster recovery
Disaster recovery is the process, policies and procedures related to preparing for recovery or continuation of technology infrastructure critical to an organization after a natural or human-induced disaster. Disaster recovery is a subset of business continuity...
("failover") or upgrade
Upgrade
The term upgrade refers to the replacement of a product with a newer version of the same product. It is most often used in computing and consumer electronics, generally meaning a replacement of hardware, software or firmware with a newer or better version, in order to bring the system up to date...
/maintenance
Preventive maintenance
Preventive maintenance has the following meanings:#The care and servicing by personnel for the purpose of maintaining equipment and facilities in satisfactory operating condition by providing for systematic inspection, detection, and correction of incipient failures either before they occur or...
("switchover") time.
Disadvantages
Version 11.2.0.1.0 and earlier is somewhat unreliable and buggy in its physical standby incarnation.If the network link connecting primary and standby is over-subscribed, the redo logs are not applied 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.
Data Guard restricts use to like platform and Oracle RDBMS versions.
Active Data Guard is an added cost option bundled within "Enterprise Edition" of the Oracle RDBMS and allows purely read-only access.
Substantial costs savings are possible using Oracle Standard Edition (SE), Oracle Standard Edition One (SE1) or even Oracle Express (XE) with use of a third-party products to provide high availability
High availability
High availability is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period....
for a database system.