FAN callouts are server-side executables that Oracle Restart executes immediately when high availability events occur.
You can use FAN callouts to automate the following activities when events occur, such as:
To use FAN callouts, place an executable in the directory grid_home/racg/usrco on both the primary and the standby database servers. If you are using scripts, then set the shell as the first line of the executable. The following is an example file for the grid_home/racg/usrco/callout.sh callout:
#! /bin/ksh
FAN_LOGFILE= [your path name]/admin/log/`hostname`_uptime.log
echo $* "reported="`date` >> $FAN_LOGFILE &
Note: Oracle has proved options where a script or utility or application (called server side callout), if placed in $GRID_HOME/racg/usrco directory, will be executed automatically.
References: Expert Oracle RAC Performance Diagnostics and Tuning (2014), page 506
https://docs.oracle.com/cd/E18283_01/server.112/e17120/restart001.htm