Ceilometer API and central agent - are they mandatory ? Can compute agent directly talk to Collector ?

For clarity, let's break this into two questions:

Are the API and central agent mandatory?

Assuming you want to rely on notifications only and don't care to poll the various service api's for data (see http://docs.openstack.org/developer/ceilometer/architecture.html#how-is-data-collected (link here) to understand the difference) it's perfectly fine to leave out ceilometer-agent-central. By not installing the API, however, you will lose the ability to:

• see resource usage in the Horizon dashboard (Horizon relies on the Ceilometer API)
• query the data store for meter statistics
• create/delete alarms

The alternative to leaving ceilometer-agent-central shut off is to configure it to poll some number of service endpoints (or none) in the /etc/ceilometer/pipeline.yaml file.

Can compute agent directly talk to collector?

No. The collector listens only to the "internal" Ceilometer bus. Without the notification agent to push information to the Ceilometer bus there will be no information for the collector to pass on to the database, or whatever the configured target is.

But note that your error is actually from ceilometer-agent-compute. This agent plays a special role in the collection of data: it sits on the compute node and queries Nova for data that is otherwise not available to Ceilometer, then pushes it to the aforementioned Ceilometer bus. In this case, you might re-state your first question to "Is the ceilometer-compute-agent mandatory?" The answer again depends on what data you are trying to collect. Take a look at the http://docs.openstack.org/developer/ceilometer/measurements.html#compute-nova (list of Ceilometer measurements) and note the "origin" column: the ceilometer-agent-compute produces all Nova measurements where the origin is "p". If one of those measurements is needed, then you'll have to run the compute agent.

If you want to understand more about the flow of notifications you can review the http://docs.openstack.org/developer/ceilometer/architecture.html#handling-notifications (brief description) provided in the Ceilometer developer documentation.

In a setup we use, we have both ceilometer API and central agent running on the controller.

Here is an example of my ceilometer.conf on compute hosts (if it may help)

[DEFAULT]
glance_control_exchange=glance
log_dir=/var/log/ceilometer
use_syslog=False
notification_topics=notifications,glance_notifications
rpc_backend=ceilometer.openstack.common.rpc.impl_kombu
rabbit_host=172.29.0.1
rabbit_port=5672
rabbit_hosts=172.29.0.1:5672
rabbit_userid=guest
rabbit_password=yourrabbitpassword
rabbit_virtual_host=/
rabbit_ha_queues=False
metering_secret=f18somesecretab1
[alarm]
[api]
[collector]
[database]
[dispatcher_file]
[event]
[keystone_authtoken]
[matchmaker_redis]
[matchmaker_ring]
[notification]
[publisher]
[publisher_rpc]
[rpc_notifier2]
[service_credentials]
os_auth_url=http://10.41.0.82:35357/v2.0
metering_secret=f18somesecretab1
os_tenant_name=services
os_username=ceilometer
os_password=92898ospassword821
os_auth_region=RegionOne
[ssl]
[vmware]