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Using OCI Driver

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Since the RAC system is a multi-instance single database, whenever one of the instances has failed, all the client connections to the failed instance receive error. They can, however, reconnect to the surviving instance on a different node. Applications can also use a callback function through an OCI call to notify clients or users of the failover or control and mask the failover behavior.

TAF enables the application to automatically reconnect to the database if the database instance to which the connection is made goes down. After the reconfiguration of the failed instance is completed, and when the database is available, the user session is re-authenticated and the open cursors are either re-executed (failover_mode=SESSION) or can continue fetching (failover_mode=SELECT). When TAF is utilized, a new connection to the database is automatically obtained by Oracle. This allows users to continue working as if the original connection had never failed.

TAF functionality is achieved through the use of the Oracle provided client side network libraries, the Oracle call interface (OCI). The client needs to use the Oracle Net OCI libraries to take advantage of TAF. With OCI callback functions, OCI API applications can monitor the failure status of the Oracle Instance.

There are seven possible failover events in the OCI API that can be captured in the application code.  They are

* FO_BEGIN = 1 is used to indicate that a lost connection has been detected during and a fail over is starting.

* FO_END = 2 is used to indicate that a successful fail over occurred.

* FO_ABORT = 3 is used to indicate that a fail over was not successful and there is no retry option.

* FO_REAUTH = 4 is used to indicate that a users handle has been re-authenticated.

* FO_ERROR = 5 is used to indicate that a fail over was temporarily unsuccessful.  This gives the application the opportunity to fail gracefully or to retry fail over. This retry is usually done by issuing a SLEEP() and then retry by returning the value FO_RETRY

* FO_RETRY = 6 is used to tell OCI to retry failover

* FO_EVENT_UNKNOWN = 7 is used to indicate a bad failover event

To implement TAF in Real Application Clusters, Client Applications need to use the JDBC OCI instead of PL/SQL packages. While RAC supports both thin JDBC and JDBC OCI, TAF is only supported with JDBC OCI.

Oracle TechNet can provide a sample as how to use the TAF using OCI driver.  The link is:


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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