A ceilometer is a device that uses a laser or other light
source to determine the height of a cloud base.
http callback
HTTP callback is used for calling a predefined URL, whenever an
alarm has been set off. The payload of the request contains
all the details of why the alarm was triggered.
log
Logging is one of the alarm actions that is useful mostly for debugging,
it stores the alarms in a log file.
project
The OpenStack tenant or project.
resource
The OpenStack entity being metered (e.g. instance, volume, image, etc).