Chapter 13. Real Application Cluster
(RAC) / TAF Support
Transparent Application Failover (TAF) is a protocol within
Oracle whereby, if a connection to a database node fails, it can be
re-established against an alternative node.
Figure 13.1. ora_rac.png
Once a broken connection has failed-over, an application can
continue without any special action on its part. However, TAF does
not restore all facets of a connection. For instance, TAF does
restore active transactions - any active transactions are rolled
back at the time of failure because TAF cannot preserve active
transactions after failover: the application instead receives an
error message until a ROLLBACK is submitted;
restore session properties set using 'ALTER SESSION';
maintain the state of server-side program variables, such as
PL/SQL package states.
If any of these situations apply to a failed connection, an
application may need to take action following failover to return
the connection fully to the desired state. In order to do this, the
application may request to be notified when failover has occurred.
The OpenLink Lite Driver for Oracle 10g and the OpenLink Generic
Multi-Tier Client provide this facility through TAF event
The Oracle 10g Lite Driver and Multi-Tier Agent also allow
configuration of the failover retry interval and the maximum number
of failover retries, in the event that failover is not successful
on the first attempt.