1 Oracle Data Guard Broker Concepts

1.1 Overview of Oracle Data Guard and the Broker

Oracle Data Guard ensures high availability, data protection, and disaster recovery for enterprise data. Data Guard provides a comprehensive set of services that create, maintain, manage, and monitor one or more standby databases to enable production Oracle databases to survive disasters and data corruptions. Data Guard maintains these standby databases as transactionally consistent copies of the primary database. If the primary 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. Data Guard can be used with traditional backup, recovery, and cluster techniques, as well as the Flashback Database feature to provide a high level of data protection and data availability.

1.2 Benefits of Data Guard Broker

The broker's interfaces improve usability and centralize management and monitoring of the Data Guard configuration. Available as a feature of the Enterprise Edition and Personal Edition of the Oracle database, the broker is also integrated with the Oracle database and Oracle Enterprise Manager. These broker attributes result in the following benefits:

Disaster protection:  By automating many of the manual tasks required to configure and monitor a Data Guard configuration, the broker enhances the high availability, data protection, and disaster protection capabilities that are inherent in Oracle Data Guard. Access is possible through a client to any system in the Data Guard configuration, eliminating any single point of failure. If the primary database fails, the broker automates the process for any one of the standby databases to replace the primary database and take over production processing. The database availability that Data Guard provides makes it easier to protect your data.

Higher availability and scalability with Oracle Real Application Clusters (RAC) Databases: While Oracle Data Guard broker enhances disaster protection by maintaining transactionally consistent copies of the primary database, Data Guard, configured with Oracle high availability solutions such as Oracle Real Application Clusters (RAC) databases, further enhances the availability and scalability of any given copy of that database. The intrasite high availability of an Oracle RAC database complements the intersite protection that is provided by Data Guard broker.

Consider that you have a cluster system hosting a primary Oracle RAC database comprised of multiple instances sharing access to that database. Further consider that an unplanned failure has occurred. From a Data Guard broker perspective, the primary database remains available as long as at least one instance of the clustered database continues to be available for transporting redo data to the standby databases. Oracle Clusterware manages the availability of instances of an Oracle RAC database. Cluster Ready Services (CRS), a subset of Oracle Clusterware, works to rapidly recover failed instances to keep the primary database available. If CRS is unable to recover a failed instance, the broker continues to run automatically with one less instance. If the last instance of the primary database fails, the broker provides a way to fail over to a specified standby database. If the last instance of the primary database fails, and fast-start failover is enabled, the broker can continue to provide high availability by automatically failing over to a pre-determined standby database.

The broker is integrated with CRS so that database role changes occur smoothly and seamlessly. This is especially apparent in the case of a planned role switchover (for example, when a physical standby database is directed to take over the primary role while the former primary database assumes the role of standby). The broker and CRS work together to temporarily suspend service availability on the primary database, accomplish the actual role change for both databases during which CRS works with the broker to properly restart the instances as necessary, and then start services defined on the new primary database. The broker manages the underlying Data Guard configuration and its database roles while CRS manages service availability that depends upon those roles. Applications that rely on CRS for managing service availability will see only a temporary suspension of service as the role change occurs in the Data Guard configuration.

Note that while CRS helps to maintain the availability of the individual instances of an Oracle RAC database, the broker coordinates actions that maintain one or more physical or logical copies of the database across multiple geographically dispersed locations to provide disaster protection. Together, the broker and Oracle Clusterware provide a strong foundation for Oracle's high-availability architecture.

Automated creation of a Data Guard configuration: The broker helps you to logically define and create a Data Guard configuration consisting of a primary database and (physical or logical, snapshot, RAC or non-RAC) standby databases. The broker automatically communicates between the databases in a Data Guard configuration using Oracle Net Services. The databases can be local or remote, connected by a LAN or geographically dispersed over a WAN.

Oracle Enterprise Manager provides a wizard that automates the complex tasks involved in creating a broker configuration, including:

• Adding an existing standby database, or a new standby database created from existing backups taken through Enterprise Manager

• Configuring the standby control file, server parameter file, and datafiles

• Initializing communication with the standby databases

• Creating standby redo log files

• Enabling Flashback Database if you plan to use fast-start failover

Although DGMGRL cannot automatically create a new standby database, you can use DGMGRL commands to configure and monitor an existing standby database, including those created using Enterprise Manager.

Easy configuration of additional standby databases: After you create a Data Guard configuration consisting of a primary and a standby database, you can add up to eight new or existing, physical, snapshot, or logical standby databases to each Data Guard configuration. Oracle Enterprise Manager provides an Add Standby Database wizard to guide you through the process of adding more databases. It also makes all Oracle Net Services configuration changes necessary to support redo transport services and log apply services across the configuration.

Simplified, centralized, and extended management: You can issue commands to manage many aspects of the broker configuration. These include:

• Simplify the management of all components of the configuration, including the primary and standby databases, redo transport services, and log apply services.

• Coordinate database state transitions and update database properties dynamically with the broker recording the changes in a broker configuration file that includes profiles of all the databases in the configuration. The broker propagates the changes to all databases in the configuration and their server parameter files.

• Simplify the control of the configuration protection modes (to maximize protection, to maximize availability, or to maximize performance).

• Invoke the Enterprise Manager verify operation to ensure that redo transport services and log apply services are configured and functioning properly.

Simplified switchover and failover operations: The broker simplifies switchovers and failovers by allowing you to invoke them using a single key click in Oracle Enterprise Manager or a single command at the DGMGRL command-line interface (referred to in this documentation as manual failover). For lights-out administration, you can enable fast-start failover to allow the broker to determine if a failover is necessary and to initiate the failover to a pre-specified target standby database automatically, with no need for DBA intervention. Fast-start failover can be configured to occur with no data loss or with a configurable amount of data loss.

Fast-start failover allows you to increase availability with less need for manual intervention, thereby reducing management costs. Manual failover gives you control over exactly when a failover occurs and to which target standby database. Regardless of the method you choose, the broker coordinates the role transition on all databases in the configuration. Once failover is complete, the broker posts the DB_DOWN event to notify applications that the new primary is available.

Note that you can use the DBMS_DG PL/SQL package to enable an application to initiate a fast-start failover when it encounters specific conditions. See Section 4.7.3 for more information.

Only one command is required to initiate complex role changes for switchover or failover operations across all databases in the configuration. The broker automates switchover and failover to a specified standby database in the broker configuration. Enterprise Manager enables you to select a new primary database from a set of viable standby databases (enabled and running, with normal status). The DGMGRL SWITCHOVER and FAILOVER commands only require you to specify the target standby database before automatically initiating and completing the many steps in switchover or failover operations across the multiple databases in the configuration.

Built-in monitoring and alert and control mechanisms: The broker provides built-in validation that monitors the health of all of the databases in the configuration. From any system in the configuration connected to any database, you can capture diagnostic information and detect obvious and subtle problems quickly with centralized monitoring, testing, and performance tools. Both Enterprise Manager and DGMGRL retrieve a complete configuration view of the progress of redo transport services on the primary database and the progress of Redo Apply or SQL Apply on the standby database.

The ability to monitor local and remote databases and respond to events is significantly enhanced by the broker's health check mechanism and tight integration with the Oracle Enterprise Manager event management system.

Transparent to application: Use of the broker is possible for any database because the broker works transparently with applications; no application code changes are required to accommodate a configuration that you manage with the broker.

1.3 Data Guard Broker Management Model

The broker simplifies the management of a Data Guard environment by performing operations on the following logical objects:

• Configuration of databases

• A single database

The broker supports one or more Data Guard configurations, each of which includes a profile for the primary database and each standby database. A supported broker configuration consists of:

• A configuration object, which is a named collection of database profiles. A database profile is a description of a database object including its current state, current status, and properties. The configuration object profiles one primary database and its standby databases that can include a mix of physical, snapshot, and logical standby databases. The databases of a given configuration are typically distributed across multiple host systems.

• Database objects, corresponding to primary or standby databases. The broker uses a database object's profile to manage and control the state of a single database on a given system. The database object may be comprised of one or more instance objects if this is an Oracle RAC database.

• Instance objects. The broker treats a database as a collection of one or more named instances. The broker automatically discovers the instances and associates them with their database.

Figure 1-1 shows the relationship of these objects.

Figure 1-1 Relationship of Objects Managed by the Data Guard Broker

Description of "Figure 1-1 Relationship of Objects Managed by the Data Guard Broker"

1.4 Data Guard Broker Components

The Oracle Data Guard broker consists of the following components:

• Oracle Enterprise Manager

• Data Guard Command-Line Interface (DGMGRL)

• Data Guard Monitor

Oracle Enterprise Manager and the Data Guard command-line interface (DGMGRL) are the broker client interfaces that help you define and manage a configuration consisting of a collection of primary and standby databases. DGMGRL also includes commands to create an observer, a process that facilitates fast-start failover. Section 1.5 describes these interfaces in more detail.

The Data Guard monitor is the broker server-side component that is integrated with the Oracle database. Data Guard monitor is composed of several processes, including the DMON process, and broker configuration files that allow you to control the databases of that configuration, modify their behavior at runtime, monitor the overall health of the configuration, and provide notification of other operational characteristics. Section 1.6 describes the Data Guard monitor in more detail.

Figure 1-2 shows these components of the broker.

Figure 1-2 Oracle Data Guard Broker

Description of "Figure 1-2 Oracle Data Guard Broker"

1.5 Data Guard Broker User Interfaces

You can use either of the broker's user interfaces to create a broker configuration and to control and monitor the configuration. The following sections describe the broker's user interfaces:

• Oracle Enterprise Manager

• Data Guard Command-Line Interface (DGMGRL)

1.6 Data Guard Monitor

The configuration, control, and monitoring functions of the broker are implemented by server-side software and configuration files that are maintained for each database that the broker manages. The software is called the Data Guard monitor.

The following sections describe how the Data Guard monitor interacts with the Oracle database and with remote Data Guard monitors to manage the broker configuration.

1.6.1 Data Guard Monitor (DMON) Process

The Data Guard monitor process (DMON) is an Oracle background process that runs for every database instance that is managed by the broker. When you start the Data Guard broker, a DMON process is created.

See Also:

Section 3.3 for information on starting the broker

Whether you use Oracle Enterprise Manager or DGMGRL to manage a database, the DMON process is the server-side component that interacts with the local database and the DMON processes of the other databases to perform the requested function. The DMON process is also responsible for monitoring the health of the broker configuration and for ensuring that every database has a consistent description of the configuration.

See Also:

Oracle Database Concepts for more information about the memory structures and processes that are used with an Oracle database

Figure 1-3 shows the broker's DMON process as one of several background processes that constitute an instance of the Oracle database. Each database instance shown in the figure has its own DMON process.

Figure 1-3 Databases With Broker (DMON) Processes

Description of "Figure 1-3 Databases With Broker (DMON) Processes"

The zigzag arrow in the center of Figure 1-3 represents the two-way Oracle Net Services communication channel that exists between the DMON processes of two databases in the same broker configuration.

This two-way communication channel is used to pass requests between databases and to monitor the health of all of the databases in the broker configuration.