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.

Dataguard maintains these standby as exact copy of primary database. So if , in any case your primary database becomes unavailable we can switch standby database role to primary database and we can minimize time and data loss.

With dataguard , We can even improve performance of primary database by offloading backups or reporting tasks to standby database which are resource-intensive tasks.

Types of Standby Databases : 

There are mainly three types of standby databases available.

1.  Physical Standby Database : Physical Standby database is exact physical copy of primary database , Disk structures are same. Data guard maintains physical standby database by applying redo to the standby database.

With 11g onward physical standby database stays in open mode with read only access so we can use it for query and reporting purpose.

2. Logical Standby Database : Logical Standby Database is logically identical copy of primary database. Physical disk structures may differ here. Data guard maintains logical standby database by applying sql to standby database.

We can use logical standby database for query and reporting purpose along with data protection. Also you have to facility to upgrade oracle database software and patch sets along with data protection with help of logical standby database.

3. Snapshot Standby Database : With Oracle 11g release 1 , a new type of standby database introduced : a snapshot standby where physical standby database will be opened in read-write mode temporarily. When a physical standby is converted into a snapshot standby , an implicit guaranteed restore point is created for the standby database. Flashback database is used to facilitate this functionality. When it is in snapshot standby mode , it still receives all redo data applied to primary but that will not be applied to standby but it is archived into the archivelogs at the standby database. And when it is converted back to a physical standby , all changes we have made to snapshot standby while it was in read-write mode are discarded and all archive logs will applied from the restore point ,making physical standby database transactionally consistent with primary database.

Note : Switchover or Failover operations are not permitted on as snapshot standby database until the database is converted back to a physical standby.

Snapshot standby database is usable with application load testing tools, such as Real Application Testing, to ensure proper application functionality and performance prior to application upgrades take place on the production system and for troubleshooting application-related issues that are dependent on production data sets.

Types of Data Guard Services : 

1. Redo Transport Service : Transmission of redo information from primary database to one or more standby database is controlled by this service.

2. Redo/SQL apply Service : It maintains how data is applied to standby database , by one of following way.

  1. Redo Apply : Uses media recovery to keep the primary and physical standby databases synchronized.
  2. SQL Apply : Reconstitutes SQL statements from the redo received from the primary database and executes the SQL statements against the logical standby database.
  3. Role Management Service : A database can be in one of two mode , Primary or Standby . Dataguard maintains role management service in conjunction with  RTS and apply service to change role dynamically as a planned transition , we can say switchover or as a result of database failure due to failover operation.

Role Transitions in Dataguard :

Two types of role transitions oracle dataguard supports.

  1. Switchover : Switchover is a result of planned role reversal. We can check switchover for hardware or OS maintenance.
  2. Failover : Failover is result of unplanned role reversal . Here zero or minimal data loss may occur depending on choice of data protection mode we use.

Stay tuned for More articles on Oracle DataGuard

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