Comment by zaneb for Ceilometer (in the past, now it's actually Aodh) calls whatever URL you ask it to. The URL that the Heat autoscaling resources provide (via the signal_url attribute) is for the resource signal API.

The user typically passes the URL of the Heat resource signal API to Aodh, and Aodh then posts to that URL when an alarm is triggered. Here's an (outdated) example: http://git.openstack.org/cgit/openstack/heat-templates/tree/hot/autoscaling.yaml#n126