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Basic Data Guard Architecture
Primary and standby databases (database roles)
In a standby environment, one database is a primary database while all others
are standby databases. Standby databases are either physical or logical standby
databases.
Query the database_role column in v$database in order to find out the role of a
database.
A standby database environment is meant for disastrous failures. It protects the
data even if a machine is wrecked beyond recognition. Therefore, it makes no
sense to put a primary and a standby database on the same hardware.
A logical standby turns redo into inserts, updates and deletes, while a physical
standby directly applies redo to its datafiles..
Primary database
In a standby environment, exactly one database is a primary database. All the
other databases are standby databases.
Logical standby database
The logical standby database is new with Oracle 9iR2.
The key advantage for logical standby databases is that they’re opened
read/write, even while they’re in applied mode. That is, they can be used to
generate reports and the like. It is indeed a fully functional database. Also,
additional indexes, materialized views and so on can be created.
However (this being a disadvantage) not all datatypes are supported.
Oracle’s log apply services use the primary database’s redo log, transforms them
into SQL statements and replays them on the logical standby database.
Physical standby database
A physical standby database is a byte for byte exact copy of the primary
database. This also means that rowids stay the same in a physical standby
database environment.
The log apply service uses the primary database’s redo log to recover the
physical database.
A physical standby database might be opened read only, however, the received
logs are in this case not applied. When the logs are applied, the database is
not accessible (it is then in a managed recovery state).
Database Synchronization Options
Data Guard can be configured to run with varying synchronization modes
indicating the potential for data loss:
Maximum Protection
Maximum protection ensures the highest level of data availability for the
primary database. In maximum protection mode, redo log records are synchronously
sent by LGWR to the standby database. Primary database changes are not committed
until it has been confirmed that the data is available on at least one standby
database.
Maximum Availability
Maximum availability provides the second highest level of data availability. As
with its maximum reliability counterpart, redo data is synchronously transmitted
from the primary database to the standby database by LGWR. The standby database
may temporarily lag behind, or diverge, from the primary database without
negatively impacting the production environment. If the standby database becomes
unavailable for any reason, the protection mode is temporarily lowered to
maximum performance until the problem has been corrected.
Maximum Performance
Maximum performance is the default protection mode. It offers lower data
availability and higher performance than its counterparts.
Redo log data is asynchronously shipped to the standby database by either LGWR
or ARCH. The commit operation on the primary database is not contingent upon the
data being received by the standby server.
If all of the standby servers become unavailable, processing will continue on
the primary database.
Data Guard Broker
Oracle’s Data Guard Broker is the management framework that is used to
administer a Data Guard environment. An Oracle background server process called
DMON is started on every site that is managed by the broker.
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