I am trying to understand collectd notifications but could not understand how exactly it works.
What plugins raise the notifications, what plugin receives the notifications and how are they handled? And how these notifications can be integrated in system like nagios and/or graphite.
I tried to google any sample depicting usage, but could not find one.
This is quite well explained on the wiki: https://collectd.org/wiki/index.php/Notifications_and_thresholds.
In short words: some plugins produce notifications, e.g. the 'df' plugin, and some consume them like the 'syslog' plugin. There's currently no "nagios consumer", but you can use the shipped 'collectd-nagios' helper as an interface to nagios using 'nrpe'.