Transparent Application Failover TAF in Oracle RAC
Transparent Application Failover TAF is an automatic database session re-establishment or reconnection characteristic built into Oracle Call Interface (OCI) in Oracle Network. It is principally aimed at Real Application Cluster RAC environments to re-establish database sessions in case of cluster node failure or instance breakdown, but the utility as such is completely self-determining of RAC and may be used for single instance as well as Data Guard environments. TAF does not work with the JDBC Thin driver/client, since that driver is not on top of OCI.
The following is a sample configuration of a Net service name configured with Transparent Application Failover:
(ADDRESS LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = racserver1)(PORT = 1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = racserver2)(PORT = 1521))
(CONNECT DATA =
(SERVICE NAME = RAC. oracpro.com)
(FAILOVER MODE =
(TYPE = select)
(METHOD = basic)
(RETRIES = 15)
(DELAY = 8)
The former Net service name definition instructs Oracle Net to do the following:
If you want to deploy session failover (or somewhat re-connection to a current instance). As long as FAILOVER MODE is present, it is not essential to unambiguously request reconnection by adding (FAILOVER=ON) to the DESCRIPTION section.
Attempt to robotically and evidently re-execution SELECT statements that were in progress at the time the database connection was interrupted.
Wait eight seconds before each attempt to reconnect (DELAY).
Retry connection re-establishment at most 15 times, such that a connection must be re-connected within three minutes. After the expiration of the reconnection interval (DELAY times RETRIES), Oracle Net indicates the error “ORA-03113: end-of-file on communication channel” to the client, if the motive of the disconnection was a node or instance failure/break-down. If the session was disconnected and the reconnection interval expires, the error “ORA-00028: your session has been killed” is reported. If a session effort to run SQL statements after one of these errors, it acquires the error “ORA-03114: not connected to ORACLE”. At this point, it might attempt to start a new database session without the assistance of TAF.
In a RAC environment, one would typically add the parameter (LOAD BALANCE=ON) to the DESCRIPTION section. Even lacking this parameter, sessions are distributed across available RAC instances.
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