Data Guard provides a comprehensive set of services and tools, which help to create, maintain, manage and monitor one or more standby databases, to enable production Oracle databases to survive disasters and any data corruption problems. These standby databases can be located anywhere according to your convenience; they can be in the same building or can be 1000 miles apart. If the production databases become unavailable due to some planned or unplanned outage, Dataguard can switch any standby database to the production role.



At the core of the concept called ?Dataguard? is something called the Data Guard broker. We will also have a peep into this critical component a little later in the article.

Benefits of Dataguard

1.Disaster Recovery and Data Protection

Data Guard provides an efficient and comprehensive disaster recovery mechanism. Because of this Data Guard can ensure that no data is lost even during some unforeseen outages. The 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.

2.Efficient utilization of system resources

Since the standby database is continuously updated with redo data received from the primary database, it can used for other tasks like reporting, summation and queries, thereby reducing the primary database workload, thus saving valuable CPU and I/O cycles.

3.Automatic gap detection and correction

In case the connection between the primary and standby database is lost due to some reason like network outage, the redo data generated by the primary database cannot be sent to the standby database. However once the connection is reestablished, the Dataguard automatically detects the gap, and the missed archived redo logs are transferred to the standby database. Hence the synchronization happens without any manual intervention.

4.Flexibility in Data protection

Oracle Data Guard offers the maximum protection, maximum availability, and maximum performance modes to help enterprises balance data availability against system performance requirements.

5.Integrated with Oracle Database

What's more, Data Guard is available as an integrated feature of the Oracle Database (Enterprise Edition) at no extra cost!!!

Functional Components

Data Guard Configuration

A Data Guard configuration consists of one primary database and a set of standby databases. The databases in the Data Guard configuration are connected by Oracle Net and can be spread across different locations.

A Typical Data Guard Configuration

Redo Apply and SQL Apply

A standby database is created as a transitionally consistent copy of the primary database. Once this is done, the Data Guard automatically takes care of the synchronization of the primary and standby databases.

A standby database can be either a physical standby database or a logical standby database:

Physical standby database

Provides a physically identical copy of the primary database, with on disk database structures that are identical to the primary database on a block-for-block basis. 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.

Logical standby database

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.

Role Transitions

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.

A failover is when the standby database takes over as the primary database. 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 ensure no data loss.

Protection Modes

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.

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.

Data Guard Broker

The Data Guard broker logically groups the primary and standby databases into a broker configuration that allows the broker to manage and monitor them together as an integrated unit. You can manage a broker configuration using either the Oracle Enterprise Manager graphical user interface or the Data Guard command-line interface.