The overall goals of FAN are to enable end-to-end, lights-out recovery of applications and load balancing based on real transaction performance. Applications use the FAN high availability (HA) events to achieve very fast detection of failures, balancing of connection pools following failures, and distribution of connections again when the failed components are repaired.
The FAN events carrying load balancing advice help connection pools consistently deliver connections to available instances that provide the best service. FAN HA is integrated with the JDBC-thin and OCI drivers. FAN HA and FAN load balancing are both integrated with the JDBC Universal Connection Pool (and the deprecated Implicit Connection Cache), the OCI session pools, the ODP.NET connection pool, and Oracle WebLogic Server Active GridLink for Oracle RAC.
Due to the integration with FAN, Oracle integrated clients are more aware of the current status of an Oracle RAC cluster. This prevents client connections from waiting or trying to connect to instances or services that are no longer available. When instances start, Oracle RAC uses FAN to notify the connection pool so that the connection pool can create connections to the recently started instance and take advantage of the additional resources that this instance provides.
Oracle client drivers that are integrated with FAN can:
Remove terminated connections immediately when a service is declared DOWN at an instance, and immediately when nodes are declared DOWN
Report errors to clients immediately when Oracle Database detects the NOT RESTARTING state, instead of making the client wait while the service repeatedly attempts to restart
Oracle connection pools that are integrated with FAN can:
Balance connections across all of the Oracle RAC instances when a service starts; this is preferable to directing the sessions that are defined for the connection pool to the first Oracle RAC instance that supports the service
Balance work requests at run time using load balancing advisory events
The use of client drivers or connection pools and FAN requires that you properly configure the Oracle Notification Service to deliver the FAN events to the clients. In addition, for load balancing, configure database connection load balancing across all of the instances that provide the services used by the connection pool. Oracle recommends that you configure both client-side and server-side load balancing with Oracle Net Services. If you use DBCA to create your database, then both client-side and server-side load balancing are configured by default.
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