Ceilometer is the telemetry component. It collects resource measurements and is able to monitor the cluster. Ceilometer was originally designed as a metering system for billing users. As it was being built, there was a realization that it would be useful for more than just billing and turned into a general-purpose telemetry system.
Ceilometer meters measure the resources being used in an OpenStack deployment. When Ceilometer reads a meter, it's called a sample. These samples get recorded on a regular basis. A collection of samples is called a statistic. Telemetry statistics will give insights into how the resources of an OpenStack deployment are being used.
The samples can also be used for alarms. Alarms are nothing but monitors that watch for a certain criterion to be met. These alarms were originally designed for Heat autoscaling. We'll look more at getting statistics and setting alarms in Chapter 8, Telemetry.Let's finish listing out OpenStack components by talking about Heat.