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disaster recovery protection while the original primary is repaired.
Applications well suited to Fast-Start Failover
Data Guard is typically used to maintain a hot standby system in a data center
remotely located from the primary production site for the purposes of data
protection and disaster recovery. Beginning with Oracle9i, Data Guard has
enabled zero data loss protection and database failover times that can be
completed in minutes assuming an administrator is immediately available to
execute the failover.
However, there is a class of applications that are extremely sensitive to downtime.
Manufacturing applications where any downtime translates into lost production,
trading systems where downtime results in lost business, online web retailers where
downtime directly impacts revenue generation, to name a few. These applications
cannot tolerate the minutes of downtime that a manual failover may require. They
also cannot afford exposure to additional delay if an administrator is not
immediately available to execute failover when needed.
Fast-Start Failover eliminates the uncertainty of a process that requires manual
intervention and automatically executes a zero data loss failover within seconds of
an outage being detected.
AUTOMATIC CLIENT FAILOVER
There are numerous approaches to configuring client failover. Prior to Fast-Start
Failover, however, accommodations needed to be made for manual intervention
required to execute database failover. Fast-Start Failover eliminates this
requirement by making database failover automatic. In addition, Data Guard 10g
Release 2 also includes a new DB_ROLE_CHANGE system event and DB_DOWN
event that make it possible to quickly notify clients that a failover has occurred so
that they are automatically redirected to the new primary database. These events
are described below, along with a sample implementation of automatic client
failover.
Data Guard Role Transition Events
In Oracle Database 10g Release 2, Data Guard role changes cause events to be
posted in an effort to help the administrator automate post role change tasks and
notify applications when a database it is connected to is no longer operating in the
primary/production role. Specifically, a system event, DB_ROLE_CHANGE, and a
FAN (Fast Application Notification) event, DB_DOWN, will be posted. This is in
Fast-Start Failover – Oracle Data Guard 10g Release 2 Page 16
addition to the other HA features already provided by RAC and will be available
for the single instance case as well.
DB_ROLE_CHANGE Event
Whenever a database transitions from one role to another, a DB_ROLE_CHANGE
system event is fired. This is much like the STARTUP system event, except it fires
only after a role change. Administrators can develop triggers that execute on this
event to manage post role change tasks. The event fires when the database opens
for the first time after the role transition regardless of its new role, i.e., regardless
of whether the role change caused it to open for the first time as a primary
database or as a logical standby or as a physical standby, in read-only mode.
The DB_ROLE_CHANGE system event may be used to manage/automate post
role change tasks. Typical tasks include starting a service / services on the new
primary, changing the naming services (e.g. LDAP/OID changes) or connection
descriptors so clients will reconnect to the new primary, starting third party
applications, adding temporary tablespaces, etc. This is a flexible option allowing
the administrator to automate any actions that can be accomplished via database
triggers.
DB_DOWN Event
In addition, when a failover operation is coordinated through the Data Guard
Broker, a DB_DOWN event is posted for the failed primary database. This is a
FAN event to notify OCI clients of the failure of the primary database. Further, if
the connection was TAF-enabled (Transparent Application Failover), the
application could automatically failover to the new primary database.
The DB_DOWN event is posted on successful completion of the failover
operation and is posted by the database that is operating in the new primary
database role on behalf of the old primary. In other words, the event notifies
subscribers of this event that the database that used to be operating in the primary
role is now down or unavailable. In Oracle Database 10g Release 2, only OCI
clients that meet the following requirements are subscribers of this event:
• The environment is created in OCI_EVENTS mode.
• The application is linked with a thread library.
• The service to which the application connects is enabled for events.
Application Notification
　
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