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I wouldn't call Aodh and Panko extensions of Ceilometer. Panko uses Ceilometer as a data source, but could use any other compatible telemetry tool. Aodh doesn't even interface with Ceilometer. Of course, they all belong to OpenStack's telemetry family, and developers probably cooperate closely.

Aodh manages and triggers alarms. For sample-based alarms, it gets the necessary data from a samples database, which is usually Gnocchi these days. For example, here is a snippet from /etc/ceilometer/pipeline.yaml on my cloud, which shows how Ceilometer is configured to send its samples to Gnocchi:

sinks:
    - name: meter_sink
      publishers:
          - gnocchi://?archive_policy=low&filter_project=gnocchi_swift

Panko sits right next to Gnocchi, but is optimized to store event data rather than samples. Ceilometer's event processing is normally configured in event_pipeline.yaml, and it is there that the event data destination is configured. It could be Panko, but my cloud (Devstack) uses Gnocchi for events.

To get data for event-based alarms, I suppose Aodh can use either Gnocchi or Panko.