Oracle Dataguard provides data protection,high availability ad distater recovery solutions.in this setup u can have more that one standby database for a primary database.The standby database can be working as primary database if primary database is in planned or unplanned outage.
Dataguard contains following necessary items :
Primary Database : Dataguard contains one primary database that is in primary role in enterprise level.All related application connected with this database.For a single primary database one or more than one standby database can be possible.
Standby Database: The standby database is the exact replica of primary database.Standby database always getting archive log from primary datbase and apply it.
Type of standby database:
Physical Standby database:
Physical standby database is the exact replica (block-for-block) of primary database.All database objects tables ,indexes are same.The physical standby database is directly applying the redo data recieved from primary database by redo apply service.
Logical Standby database:
The logical standby database is also the replica of primary database but this is not block to block basis this is identical to primary on logical structures like tablespace ,tables ,indexes etc.This is getting sync with primary database by sql apply service which transforms the redo data in sql format recieved from primary datbase and apply this sql statements.
Logical standby has some additional features like it can be used for reporting purpose alongwith the disaster recovery solutions.Others feature are that we can upgrade or apply the patching with almost no downtime.
Snapshot standby database:
A snapshot standby database is completely different than physical and logical standby database.This standby database can be updated and check any rela time scenario in standby and after all testing you can discard all the changes easily. It does not directly apply the recieved redo from primary.To apply the redo it first we need to discard all changes made lcoally then convert it to physical standby mode.