FAN is a high-availability notification mechanism that Oracle RAC uses to notify other processes about configuration and service level information that includes service status changes, such as UP or DOWN events. The Oracle client drivers and Oracle connection pools respond to FAN events and take immediate action. FAN UP and DOWN events can apply to instances, services, and nodes.
Oracle connection pools, for example, use FAN to receive very fast detection of failures, to balance connections following failures, and to balance connections again after the failed components are repaired. So, when a service at an instance starts, the FAN event is used immediately to route work to that resource. When a service at an instance or node fails, the FAN event is used immediately to interrupt applications to recover.
Importance of FAN
Applications can waste time in many critical ways:
Waiting for TCP/IP timeouts when a node fails without closing sockets, and for every subsequent connection while that IP address is down.
Attempting to connect when services are down.
Not connecting when services resume.
Processing the last result at the client when the server goes down.
Attempting to execute work on sub-optimal nodes.
When a node fails without closing sockets, all sessions that are blocked in an I/O wait (read or write) wait for tcp_keepalive. This wait status is the typical condition for an application connected by a socket. Sessions processing the last result are even worse off, not receiving an interrupt until the next data is requested. Using FAN events eliminates applications waiting on TCP timeouts, time wasted processing the last result at the client after a failure has occurred, and time wasted executing work on slow, hung, or dead nodes.
For cluster configuration changes, the Oracle RAC high availability framework publishes a FAN event immediately when a state change occurs in the cluster. Instead of waiting for the application to time out against the database and detect a problem, applications can receive FAN events and react immediately. With FAN, in-flight transactions are immediately terminated and the client notified when the instance fails.
FAN also publishes load balancing advisory events. Applications can take advantage of the load balancing advisory FAN events to direct work requests to the instance in the cluster that is currently providing the best service quality.
You can take advantage of FAN events in the following three ways:
Your application can use FAN without programmatic changes if you use an integrated Oracle client. The integrated clients for FAN events include Oracle JDBC Universal Connection Pool, ODP.NET connection pool, OCI session pool, Oracle WebLogic Server Active Gridlink for Oracle RAC, and OCI and ODP.NET clients. This includes applications that use Application Continuity or Transaction Guard. The integrated Oracle clients must be Oracle Database 10g release 2 (10.2) or later to take advantage of the FAN high-availability events. The pooled clients can also take advantage of the load balancing advisory FAN events.
Applications can use FAN programmatically by using the JDBC and Oracle RAC FAN application programming interface (API) or by using callbacks with OCI and ODP.NET to subscribe to FAN events and to execute event handling actions upon the receipt of an event.
You can implement FAN with server-side callouts on your database tier.
If you use one of the integrated clients listed in item 1 of the preceding list, then, for DOWN events, the disruption to the application is minimized because the FAN-aware client terminates the sessions to the failed instance or node before they are reused. Incomplete transactions are terminated and the application user is immediately notified. Application users who request connections are directed to available instances only.
For UP events, when services and instances are started, new connections are created so that the application can immediately take advantage of the extra hardware resources or additional capacity.
FAN callouts are server-side scripts or executables that run whenever a FAN event is generated. You can design and build callouts to do many things. For example:
Log status information
Page DBAs or to open support tickets when resources fail to start
Automatically start dependent external applications that must be co-located with a service
Change resource plans or shut down services when the number of available instances for a policy-managed database decreases, for example, if nodes fail
Automate the fail back of a service to preferred instances for administrator-managed databases, if needed
FAN events are published using Oracle Notification Service and Oracle Streams Advanced Queuing, the latter being continued for backward compatibility to previous Oracle Database releases. The publication mechanisms are automatically configured as part of your Oracle RAC installation.
Oracle Net Services listeners and Global Data Services (GDS) are integrated with FAN events, enabling the listener and GDS to immediately de-register services provided by the failed instance and to avoid erroneously sending connection requests to failed instances.
If you specify the connection load balancing goal CLB_GOAL_SHORT for the service, then the listener uses the load balancing advisory when the listener balances the connection loads. When load balancing advisory is enabled, the metrics used for the listener are finer grained.