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Oracle Data Guard
 
 
The main difference between physical and logical standby databases is the manner in which log apply services apply the archived redo data.
 
For physical standby databases, Data Guard uses Redo Apply technology, which applies redo data on the standby database using standard recovery techniques of an Oracle database, as shown in Figure (A)
 
For logical standby databases, Data Guard uses SQL Apply technology, which first transforms the received redo data into SQL statements and then executes the generated SQL statements on the logical standby database, as shown in Figure (B)

Oracle provides three ways to manage a Data Guard environment:

1. SQL*Plus and SQL Statements

Using SQL*Plus and SQL commands to manage Data Guard environment.The following SQL statement initiates a switchover operation:

SQL> alter database commit to switchover to physical standby;

2. Data Guard Broker GUI Interface (Data Guard Manager)

Data Guard Manger is a GUI version of Data Guard broker interface that allows you to automate many of the tasks involved in configuring and monitoring a Data Guard environment.

3. Data Guard Broker Command-Line Interface (CLI)

It is an alternative interface to using the Data Guard Manger. It is useful if you want to use the broker from batch programs or scripts. You can perform most of the activities required to manage and monitor the Data Guard environment using the CLI.

Oracle Data Guard
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 production database. Then, 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, minimizing the downtime associated with the outage. Data Guard can be used with traditional backup, restoration, and cluster techniques to provide a high level of data protection and data availability.
 
Protection Modes
 
In Oracle 10g we see an exciting new approach to Data Guard management whereby we write the current redo log to a "standby redo log", allowing complete recovery in cases of catastrophic instance failure.
 
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 Services
 
Redo Apply (A)
SQL Apply (B)
 
Oracle Data Guard

A Data Guard configuration consists of one production database and one or more 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 they can communicate with each other. For example, you can have a standby database on the same system as the production database, along with two standby databases on other systems at remote locations.

You can manage primary and standby databases using the SQL command-line interfaces or the Data Guard broker interfaces, including a command-line interface (DGMGRL) and a graphical user interface that is integrated in Oracle Enterprise Manager.

 

Primary Database

 

A Data Guard configuration contains one production database, also referred to as the primary database, that functions in the primary role. This is the database that is accessed by most of your applications.

The primary database can be either a single-instance Oracle database or an Oracle Real Application Clusters database.

 

Standby Database

 

A standby database is a transactionally consistent copy of the primary database. Using a backup copy of the primary database, you can create up to nine standby databases and incorporate them in a Data Guard configuration. Once created, Data Guard automatically maintains each standby database by transmitting redo data from the primary database and then applying the redo to the standby database.

Similar to a primary database, a standby database can be either a single-instance Oracle database or an Oracle Real Application Clusters database.