Oracle Data Guard is the management, monitoring, and automation software infrastructure that creates, maintains, and monitors one or more standby databases to protect enterprise data from failures, disasters, errors, and corruptions. 

Data Guard maintains these standby databases as transactionally consistent copies of the production database. These standby databases can be located at remote disaster recovery sites thousands of miles away from the production data center, or they may be located in the same city, same campus, or even in the same building. If the production database becomes unavailable because of a planned or an unplanned outage, Data Guard can switch any standby database to the production role, thus minimizing the downtime associated with the outage, and preventing any data loss.

Available as a feature of the Enterprise Edition of the Oracle Database, Data Guard can be used in combination with other Oracle High Availability (HA) solutions such as Real Application Clusters (RAC) and Recovery Manager (RMAN), to provide a high level of data protection and data availability that is unprecedented in the industry.

The following diagram presents a hi-level overview of Oracle Data Guard.

Overview of Oracle Data Guard Functional Components

Data Guard Configuration

A Data Guard configuration consists of one production (or primary) database and up to nine standby databases. The databases in a Data Guard configuration are connected by Oracle Net and may be dispersed geographically. There are no restrictions on where the databases are located, provided that they can communicate with each other. However, for disaster recovery, it is recommended that the standby databases are hosted at sites that are geographically separated from the primary site.

Redo Apply and SQL Apply

A standby database is initially created from a backup copy of the primary database. Once created, Data Guard automatically maintains the standby database as a transactionally consistent copy of the primary database by transmitting primary database redo data to the standby system and then applying the redo logs to the standby database.

Data Guard provides two methods to apply this redo data to the standby database and keep it transactionally consistent with the primary, and these methods correspond to the two types of standby databases supported by Data Guard.

• Redo Apply, used for physical standby databases
• SQL Apply, used for logical standby databases

A 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. The database schema, including indexes, are the same. The Redo Apply technology applies redo data on the physical standby database using standard Oracle media recovery techniques. 

A 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 SQL apply technology keeps the logical standby database synchronized with the primary database by transforming the data in the redo logs received from the primary database into SQL statements and then executing the SQL statements on the standby database. This makes it possible for the logical standby database to be accessed for queries and reporting purposes at the same time the SQL is being applied to it. Thus, a logical standby database can be used concurrently for data protection and reporting.

Role Management

Using Data Guard, the role of a database can be switched from a primary role to a standby role and vice versa, ensuring no data loss in the process, and minimizing downtime. There are two kinds of role transitions - a switchover and a failover. A switchover is a role reversal between the primary database and one of its standby databases. 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 an irreversible transition of a standby database to the primary role. This is only done in the event of a catastrophic failure of the primary database, which is assumed to be lost and to be used again in the Data Guard configuration, it must be re-instantiated as a standby from the new primary.

Data Guard Protection Modes

In some situations, a business cannot afford to lose data at any cost. In other situations, some applications require maximum database performance and can tolerate a potential loss of data. Data Guard provides three distinct modes of data protection to satisfy these varied requirements:

• Maximum Protection—This mode offers the highest level of data protection. Data is synchronously transmitted to the standby database from the primary database and transactions are not committed on the primary database unless the redo data is available on at least one standby database configured in this mode. If the last standby database configured in this mode becomes unavailable, processing stops on the primary database. This mode ensures no-data-loss.
  
  

• Maximum Availability—This mode is similar to the maximum protection mode, including zero data loss. However, if a standby database becomes unavailable (for example, because of network connectivity problems), processing continues on the primary database. When the fault is corrected, the standby database is automatically resynchronized with the primary database.
  
  

• Maximum Performance—This mode offers slightly less data protection on the primary database, but higher performance than maximum availability mode. In this mode, as the primary database processes transactions, redo data is asynchronously shipped to the standby database. The commit operation of the primary database does not wait for the standby database to acknowledge receipt of redo data before completing write operations on the primary database. If any standby destination becomes unavailable, processing continues on the primary database and there is little effect on primary database performance.
  

Data Guard Broker

The Oracle Data Guard Broker is a distributed management framework that automates and centralizes the creation, maintenance, and monitoring of Data Guard configurations. All management operations can be performed either through Oracle Enterprise Manager, which uses the Broker, or through the Broker's specialized command-line interface (DGMGRL).

Data Guard Architecture Diagram

The following diagram shows an overview of the Oracle Data Guard architecture.

What's New in Oracle Data Guard 10g?

Data Guard in Oracle Database 10g has been developed with the following goals in mind.

• Ease of use
• Tight integration with new 10g HA features such as flashback database
• Comprehensiveness in terms of features & functionality

This section will highlight some of the key new features of Oracle Data Guard 10g. For details into these features, please refer to the Oracle Data Guard 10g documentation.

Click here for a presentation on the Oracle Data Guard 10g new features.

General New Features

Real Time Apply

With this feature, redo data can be applied on the standby database (whether Redo Apply or SQL Apply) as soon as they have written to a Standby Redo Log (SRL). Prior releases of Data Guard require this redo data to be archived at the standby database in the form of archivelogs before they can be applied. 

The Real Time Apply feature allows standby databases to be closely synchronized with the primary database, enabling up-to-date and real-time reporting (especially for Data Guard SQL Apply). This also enables faster switchover and failover times, which in turn reduces planned and unplanned downtime for the business.

The impact of a disaster is often measured in terms of Recovery Point Objective (RPO - i.e. how much data can a business afford to lose in the event of a disaster) and Recovery Time Objective (RTO - i.e. how much time a business can afford to be down in the event of a disaster). With Oracle Data Guard, when Maximum Protection is used in combination with Real Time Apply, businesses get the benefits of both zero data loss as well as minimal downtime in the event of a disaster and this makes Oracle Data Guard the only solution available today with the best RPO and RTO benefits for a business.

Integration with Flashback Database

Data Guard in 10g has been integrated with the Flashback family of features to bring the Flashback feature benefits to a Data Guard configuration.

One such benefit is human error protection. In Oracle9i, administrators may configure Data Guard with an apply delay to protect standby databases from possible logical data corruptions that occurred on the primary database. The side-effects of such delays are that any reporting that gets done on the standby database is done on old data, and switchover/failover gets delayed because the accumulated logs have to be applied first. In Data Guard 10g, with the Real Time Apply feature, such delayed-reporting or delayed-switchover/failover issues do not exist, and - if logical corruptions do land up affecting both the primary and standby database, the administrator may decide to use Flashback Database on both the primary and standby databases to quickly revert the databases to an earlier point-in-time to back out such user errors. 

Another benefit that such integration provides is during failovers. In releases prior to 10g, following any failover operation, the old primary database must be recreated (as a new standby database) from a backup of the new primary database, if the administrator intends to bring it back in the Data Guard configuration. This may be an issue when the database sizes are fairly large, and the primary/standby databases are hundreds/thousands of miles away. However, in Data Guard 10g, after the primary server fault is repaired, the primary database may simply be brought up in mounted mode, "flashed back" (using flashback database) to the SCN at which the failover occurred, and then brought back as a standby database in the Data Guard configuration. No reinstantiation is required.

SQL Apply New Features

Zero Downtime Instantiation

Logical standby database can now be created from an online backup of the primary database, without shutting down or quiescing the primary database, as was the case in prior releases. No shutdown of the primary system implies production downtime is eliminated, and no quiesce implies no waiting for quiescing to take effect and no dependence on Resource Manager.

Rolling Upgrades

Oracle Database 10g supports database software upgrades (from Oracle Database 10g Patchset 1 onwards) in a rolling fashion, with near zero database downtime, by using Data Guard SQL Apply. The steps involve upgrading the logical standby database to the next release, running in a mixed mode to test and validate the upgrade, doing a role reversal by switching over to the upgraded database, and then finally upgrading the old primary database. While running in a mixed mode for testing purpose, the upgrade can be aborted and the software downgraded, without data loss. For additional data protection during these steps, a second standby database may be used.

By supporting rolling upgrades with minimal downtimes, Data Guard reduces the large maintenance windows typical of many administrative tasks, and enables the 24x7 operation of the business.

Additional Datatypes

SQL Apply now supports the following additional data types.

• NCLOB
• LONG
• LONG RAW
• BINARY_FLOAT
• BINARY_DOUBLE
• IOT-s (without overflows and without LOB columns)

This support for additional datatypes allows logical standby databases to recover and protect a wider variety of data, thus increasing the overall database protection and recovery options for Data Guard.

Enterprise Manager / Data Guard Broker New Features

RAC Support

It is now possible to use the Data Guard Broker, and the Broker's Command Line Interface (DGMGRL), as well as Enterprise Manager, to create and manage Data Guard configurations that contain RAC primary and RAC standby databases. In Oracle9i, such administration is possible only through SQL*Plus. In Data Guard 10g, Data Guard Broker interfaces with Oracle Clusterware such that it has control over critical operations during specific Data Guard state transitions, such as switchovers, failovers, protection mode changes and state changes.

Simplified Browser-based Interface

Administration of a Data Guard configuration can be done through the new streamlined browser-based HTML interface of Enterprise Manager, that enables complete standby database lifecycle management. The focus of such streamlined administration is on:

• Ease of use.
• Management based on best practices.
• Pre-built integration with other HA features.

Data Guard Benefits

1. Disaster recovery 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.
   
   
2. Complete data protection
   
   With its standby databases, Data Guard ensures 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 at the time it is applied to the standby database.
   
   
3. Efficient utilization of system resources
   
   The standby database tables that are updated with redo logs received from the primary database can be used for other tasks such as backup operations, 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 operations 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.
   
   
4. Flexibility in data protection to balance availability against performance requirements
   
   Oracle Data Guard offers the maximum protection, maximum availability, and maximum performance modes to help enterprises balance data availability against system performance requirements.
   
   
5. Automatic gap detection and resolution
   
   If connectivity is lost between the primary and one or more standby databases (e.g. because of network problems), redo data being generated on the primary database cannot be sent to those standby databases. Once connectivity is re-established, the missing archivelog sequence (or the gap) is automatically detected by Data Guard and the necessary archivelogs are automatically transmitted to the standby databases. The standby databases are resynchronized with the primary database, with no manual intervention by the administrator. 
   
   
6. Centralized and simple management
   
   The Data Guard Broker automates the management and operational tasks across the multiple databases in a Data Guard configuration. The Broker also monitors all of the systems within a single Data Guard configuration. Administrators may use either Oracle Enterprise Manager or the Broker's own specialized command-line interface (DGMGRL) to take advantage of this integrated management framework.
   
   
7. Integrated with Oracle database
   
   Oracle Data Guard is available as a fully integrated feature of the Oracle Database (Enterprise Edition) at no extra cost.