System Architecture

High-Level Architecture

Architecture summary

An overall summary of Ceilometer’s logical architecture.

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:

  1. polling agent - daemon designed to poll OpenStack services and build Meters.
  2. notification agent - daemon designed to listen to notifications on message queue, convert them to Events and Samples, and apply pipeline actions.
  3. (optional) collector - daemon designed to gather and record event and metering data created by notification and polling agents (if using Gnocchi or full-fidelity storage).
  4. (optional) api - service to query and view data recorded by collector in internal full-fidelity database (if enabled).

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.

Ceilometer+Gnocchi Architecture summary

An overall summary of Ceilometer+Gnocchi’s logical architecture.

Gathering the data

How is data collected?

Collectors and agents

This is a representation of how the collectors and agents gather data from multiple sources.

In a perfect world, each and every project that you want to instrument should send events on the Oslo bus about anything that could be of interest to you. Unfortunately, not all projects have implemented this and you will often need to instrument other tools which may not use the same bus as OpenStack has defined. The Ceilometer project created 2 methods to collect data:

  1. Bus listener agent which takes events generated on the notification bus and transforms them into Ceilometer samples. This is the preferred method of data collection. If you are working on some OpenStack related project and are using the Oslo library, you are kindly invited to come and talk to one of the project members to learn how you could quickly add instrumentation for your project.
  2. Polling agents, which is the less preferred method, will poll some API or other tool to collect information at a regular interval. Where the option exists to gather the same data by consuming notifications, then the polling approach is less preferred due to the load it can impose on the API services.

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 local hypervisor or remote APIs (public REST APIs exposed by services and host-level SNMP/IPMI daemons).

Notification Agents: Listening for data

Notification agents

Notification agents consuming messages from services.

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 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 topic. Event matching can also work using wildcards e.g. compute.instance.*.

Similarly, if enabled, notifications are converted into Events which can be filtered based on event_type declared by other services.

Polling Agents: Asking for data

Polling agents

Polling agents querying services for data.

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 pipeline for processing.

Please notice that there’s an optional config called shuffle_time_before_polling_task in ceilometer.conf. Enable this by setting an integer greater than zero to shuffle agents to start polling task, so as to add some random jitter to the time of sending requests to nova or other components to avoid large number of requests in short time. Additionally, there is an option to stream samples to minimise latency (at the expense of load) by setting batch_polled_samples to False in ceilometer.conf.

Processing the data

Pipeline Manager

Ceilometer pipeline

The assembly of components making the Ceilometer pipeline.

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.

Transforming the data

Transformer example

Example of aggregation of multiple cpu time usage samples in a single cpu percentage sample.

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.

Publishing the data


This figure shows how a sample can be published to multiple destinations.

Currently, processed data can be published using 4 different transports: notifier, a notification based publisher which pushes samples to a message queue which can be consumed by the collector or an external system; udp, which publishes samples using UDP packets; and kafka, which publishes data to a Kafka message queue to be consumed by any system that supports Kafka.

Storing the data

Collector Service

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, or http.

Supported databases

Storage model

An overview of the Ceilometer storage model.

Since the beginning of the project, a plugin model has been put in place to allow for various types of database backends to be used. A list of supported backends can be found in the Choosing a database backend - Legacy section of the documentation for more details.

In the Juno and Kilo release cycle, Ceilometer’s database was divided into three separate connections: alarm, event, and metering. This allows deployers to either continue storing all data within a single database or to divide the data into their own databases, tailored for its purpose. For example, a deployer could choose to store alarms in an SQL backend while storing events and metering data in a NoSQL backend.

Ceilometer’s storage service is designed to handle use cases where full-fidelity of the data is required (e.g. auditing). To handle responsive, long-term data queries, solutions that strip away some of the data’s resolution, such as Gnocchi, are recommended.


As of Liberty, alarming support, and subsequently its database, is handled by Aodh.


We do not guarantee that we won’t change the DB schema, so it is highly recommended to access the database through the API and not use direct queries.

Accessing the data

API Service

If the collected data from polling and notification agents are stored in Ceilometer’s database(s) (see the section Supported databases), it is possible that the schema of these database(s) may evolve over time. For this reasons, we offer a REST API and recommend that you access the collected data via the API rather than by accessing the underlying database directly.

If the way in which you wish to access your data is not yet supported by the API, please contact us with your feedback, so that we can improve the API accordingly.

data access model

This is a representation of how to access data stored by Ceilometer

The list of currently built in meters is available in the developer documentation, and it is also relatively easy to add your own (and eventually contribute it).

Ceilometer is part of OpenStack, but is not tied to OpenStack’s definition of “users” and “tenants.” The “source” field of each sample refers to the authority defining the user and tenant associated with the sample. Deployers can define custom sources through a configuration file, and then create agents to collect samples for new meters using those sources. This means that you can collect data for applications running on top of OpenStack, such as a PaaS or SaaS layer, and use the same tools for metering your entire cloud.

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.