.. _architecture:

=====================
 System Architecture
=====================

.. index::
   single: agent; architecture
   double: compute agent; architecture
   double: collector; architecture
   double: data store; architecture
   double: database; architecture
   double: API; architecture

High-Level Architecture
=======================

.. The source for the following diagram can be found at: https://docs.google.com/presentation/d/1XiOiaq9zI_DIpxY1tlkysg9VAEw2r8aYob0bjG71pNg/edit?usp=sharing

.. figure:: ./ceilo-arch.png
   :width: 100%
   :align: center
   :alt: 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).

Data normalised and collected by Ceilometer can be sent to various targets.
Gnocchi_ was developed to capture measurement data in a time series database to
optimise storage and querying. Gnocchi is intended to replace the existing
metering database interface. Additionally, Aodh_ is the alarming service which
can be send notifications when user defined rules are broken. Lastly, Panko_ is
the event storage project designed to capture document-oriented data such as
logs and system event actions.

.. _Gnocchi: http://docs.openstack.org/developer/gnocchi/
.. _Aodh: http://docs.openstack.org/developer/aodh
.. _Panko: http://docs.openstack.org/developer/panko


Gathering the data
==================

How is data collected?
----------------------

.. figure:: ./1-agents.png
   :width: 100%
   :align: center
   :alt: Collectors and agents

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

The Ceilometer project created 2 methods to collect data:

1. :term:`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. :term:`Polling agents`, which is the less preferred method, will poll
   some API or other tool to collect information at a regular interval.
   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 the local hypervisor or remote APIs (public REST APIs exposed by
services and host-level SNMP/IPMI daemons).

Notification Agents: Listening for data
---------------------------------------

.. index::
      double: notifications; architecture

.. figure:: ./2-1-collection-notification.png
   :width: 100%
   :align: center
   :alt: 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 ``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:

Polling Agents: Asking for data
-------------------------------

.. index::
      double: polling; architecture

.. figure:: ./2-2-collection-poll.png
   :width: 100%
   :align: center
   :alt: 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 :ref:`Pipeline-Configuration` for details.
The agent framework then passes the samples to the notification agent for processing.


Processing the data
===================

.. _multi-publisher:

Pipeline Manager
----------------

.. figure:: ./3-Pipeline.png
   :width: 100%
   :align: center
   :alt: 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
---------------------

.. figure:: ./4-Transformer.png
   :width: 100%
   :align: center
   :alt: 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
-------------------

.. figure:: ./5-multi-publish.png
   :width: 100%
   :align: center
   :alt: Multi-publish

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

Currently, processed data can be published using 5 different transports:

1. direct, which publishes samples to a configured dispatcher directly, default
   is database dispatcher;
2. notifier, a notification based publisher which pushes samples to a message queue
   which can be consumed by the collector or an external system;
3. udp, which publishes samples using UDP packets;
4. http, which targets a REST interface;
5. 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, gnocchi or http.

More details on database and Gnocchi targets can be found in the 
:ref:`choosing_db_backend` guide.



Accessing the data
==================

API Service
-----------

If the collected data from polling and notification agents are stored in Ceilometer's
database(s) (see the section :ref:`choosing_db_backend`), a REST API is available
to access the collected data rather than by accessing the underlying database directly.

.. figure:: ./2-accessmodel.png
   :width: 100%
   :align: center
   :alt: data access model

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

Moreover, end users can also
:ref:`send their own application specific data <user-defined-data>` into the
database through the REST API for a various set of use cases.

.. _send their own application centric data: ./webapi/v2.html#user-defined-data
