Summary of Physical Data Guard Concepts

Data Guard is unique among Oracle replication solutions in supporting both synchronous
(zero ata loss) and asynchronous (near-zero data loss) configurations.supporting ad-hoc queries, reporting, backups, or test activity.

Types of standby databases:

- physical standby : uses Redo Apply.Physical standby databases provide the best disaster recovery    (DR) protection for the Oracle Database.(block-for-block basis).

- Logical  standby : uses SQL Apply.

Standby roles :

- physical standby : not read and not write.

        - Active Data Guard : enables a physical standby database to be used for read-only applications.

-Snapshot Standby   : enables a physical standby database to be open read-write for testing.
A snapshot standby database receives and archives, but does not apply, redo data from its primary database. Redo data received from the primary database is applied when a snapshot standby database is converted back into a physical standby database, after discarding all local updates to the snapshot standby database.

A standby database is initially created from a backup copy of the primary database.

Data Guard Services:

- Log Transport Services : Data Guard transport services transmit the redo directly from the primary database log buffer to the standby database(s) where it is written to a standby redo log file.

- Log Apply Services : on the standby database read redo records from a standby redo log file, perform continuous Oracle validation to insure
that the redo is not corrupt, and then applies redo changes to the standby database.

- Role Management Services : Change the role of a database from a standby database to a primary database,
or from a primary database to a standby database using either a switchover or a
failover operation.


Data Guard  two types of replication : Data guard is similar repilication without capturing

- synchronous : primary database wait for confirmation from a standby database that redo has been received and written to disk (a standby redo log file).Data Guard Maximum Protection mode.

- asynchronous : Primary without waiting for acknowledgment that redo has been received by the standby database.(Maximum Performance)

Protection Modes :

- Maximum Protection : Zero data loss Double failure protection - SYNC -Signal commit success to the application only after acknowledgement is received from a standby database that redo for that transaction is hardened to disk.

- Maximum Availability : Zero data loss Single failure protection - SYNC -Signal commit success to the application only after acknowledgement is received from a standby database or after NET_TIMEOUT threshold period expires – whichever occurs first.

- Maximum Performance : Potential for minimal data loss - ASYNC - Primary never waits for standby acknowledgment to signal commit success to the application.

Managed Recovery Process (MRP) : A physical standby database applies redo received from the primary using MRP.