Optimize Distributed Workload - Expert Oracle RAC Performance Diagnostics and Tuning - page 489

Figure 15-2. FAN event notification overview
FAN and ONS
ONS allows users to send SMS (short message system) messages, e-mails, voice notifications, and fax messages in an
easy to access manner. Oracle Clusterware uses ONS to send notifications about the state of the database instances to
mid-tier applications that use this information for load balancing and for fast failure detection.
ONS is a daemon process that communicates with other ONS daemons on other nodes informing it of the current
state of the database components on the database server. For example, if a listener or node or service is down, a DOWN
event is triggered by the EVMD process, which is then sent by the ONS daemon to the ONS daemon process on other
nodes, including all clients and application servers participating in the network. Only nodes or client machines that
have the ONS daemon running and have registered with each other will receive such notification. Once the ONS on
the client machines receive this notification, the application (if using Oracle provided API [Application Programming
Interface]) will determine, based on the notification, which nodes and instances have had a state change and
appropriately send a new connection request. ONS helps the application to understand state changes proactively
instead of the traditional reactive method.
ONS Configuration
ONS is installed and configured as part of the Oracle Grid Infrastructure (GI) installation. Execution of the root.sh file
installation will create and start the ONS daemon process on all nodes participating in the cluster:
[[oracle@ssky1l1p1 ~]$ ps -ef | grep ons
oracle 5677 1 0 Feb03 ? 00:00:00 /u01/app/12.1.0.1/grid/opmn/bin/ons -d
oracle 5678 5677 0 Feb03 ? 00:00:02 /u01/app/12.1.0.1/grid/opmn/bin/ons -d