Each of Ceilometer’s services are designed to scale horizontally. Additional workers and nodes can be added depending on the expected load. Ceilometer offers three core services, the data agents designed to work independently from collection, but also designed to work together as a complete solution:
As Ceilometer has grown to capture more data, it became apparent that data storage would need to be optimised. To address this, Gnocchi (resource metering as a service) was developed to capture the data in a time series database to optimise storage and querying. Gnocchi is intended to replace the existing metering database interface.
The Ceilometer project created 2 methods to collect data:
The first method is supported by the ceilometer-notification agent, which monitors the message queues for notifications. Polling agents can be configured either to poll the local hypervisor or remote APIs (public REST APIs exposed by services and host-level SNMP/IPMI daemons).
The heart of the system is the notification daemon (agent-notification) which monitors the message bus for data being provided by other OpenStack components such as Nova, Glance, Cinder, Neutron, Swift, Keystone, and Heat, as well as Ceilometer internal communication.
The notification daemon loads one or more listener plugins, using the namespace ceilometer.notification. Each plugin can listen to any topics, but by default it will listen to notifications.info. The listeners grab messages off the defined topics and redistributes them to the appropriate plugins(endpoints) to be processed into Events and Samples.
Sample-oriented plugins provide a method to list the event types they’re interested in and a callback for processing messages accordingly. The registered name of the callback is used to enable or disable it using the pipeline of the notification daemon. The incoming messages are filtered based on their event type value before being passed to the callback so the plugin only receives events it has expressed an interest in seeing. For example, a callback asking for compute.instance.create.end events under ceilometer.compute.notifications would be invoked for those notification events on the nova exchange using the notifications.info topic. Event matching can also work using wildcards e.g. compute.instance.*.
Polling for compute resources is handled by a polling agent running on the compute node (where communication with the hypervisor is more efficient), often referred to as the compute-agent. Polling via service APIs for non-compute resources is handled by an agent running on a cloud controller node, often referred to the central-agent. A single agent can fulfill both roles in an all-in-one deployment. Conversely, multiple instances of an agent may be deployed, in which case the workload is shared. The polling agent daemon is configured to run one or more pollster plugins using either the ceilometer.poll.compute and/or ceilometer.poll.central namespaces.
The agents periodically ask each pollster for instances of Sample objects. The frequency of polling is controlled via the pipeline configuration. See Pipelines for details. The agent framework then passes the samples to the notification agent for processing.
Ceilometer offers the ability to take data gathered by the agents, manipulate it, and publish it in various combinations via multiple pipelines. This functionality is handled by the notification agents.
The data gathered from the polling and notifications agents contains a wealth of data and if combined with historical or temporal context, can be used to derive even more data. Ceilometer offers various transformers which can be used to manipulate data in the pipeline.
Currently, processed data can be published using 5 different transports: 1. direct, which publishes samples to a configured database dispatcher directly; 2. notifier, a notification based publisher which pushes samples to a message queue
which can be consumed by the collector or an external system;
The collector daemon gathers the processed event and metering data captured by the notification and polling agents. It validates the incoming data and (if the signature is valid) then writes the messages to a declared target: database, file, gnocchi or http.
More details on database and Gnocchi targets can be found in the Choosing a database backend guide.
If the collected data from polling and notification agents are stored in Ceilometer’s database(s) (see the section Choosing a database backend), a REST API is available to access the collected data rather than by accessing the underlying database directly.
Moreover, end users can also send their own application specific data into the database through the REST API for a various set of use cases.