Fix docs to suit merged compute/central agents concept
DocImpact: Fix agents-related docs to describe polling agent Related-Blueprint: merge-compute-central-agents Change-Id: If377ac49bf75c0698f4af6be021e8354240f4e42
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Dina Belova
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@ -94,8 +94,7 @@ Each of Ceilometer's services are designed to scale horizontally. Additional
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workers and nodes can added depending on the expected load. Ceilometer offers
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five core services:
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1. polling agents - compute and central agent daemons designed to poll
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OpenStack services.
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1. polling agent - daemon designed to poll OpenStack services.
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2. notification agent - daemon designed to listen to message queue.
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3. collector - daemon designed to gather and record event and metering data
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created by notification and polling agents.
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@ -133,14 +132,17 @@ methods to collect data:
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prefer this compared to a polling agent method as resilience (high
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availability) will not be a problem with this method.
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3. :term:`Polling agents` which is the least preferred method, that will poll
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some API or other tool to collect information at a regular interval.
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some API or other tool to collect information at a regular interval.
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This method is least preferred due to the inherent difficulty in making such
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a component resilient.
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The first method is supported by the ceilometer-notification agent, which
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monitors the message queues for notifications and for metering data coming
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from the "push" agents. Methods 2 and 3 rely on the ceilometer-compute-agent
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and ceilometer-central-agent respectively.
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from the "push" agents. Methods 2 and 3 rely on the ceilometer-polling agent,
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which behaves differently depending on the resources it's collecting data
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about. Polling agent can be configured either to poll local hypervisor or
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remote APIs (public REST APIs exposed by services and/or host-level SNMP
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daemons).
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How to access collected data?
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-----------------------------
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@ -366,16 +368,15 @@ Polling
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Metering data comes from two sources: through notifications built into
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the existing OpenStack components and by polling the infrastructure
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(such as via libvirt). Polling for compute resources is handled by an
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agent running on the compute node (where communication with the
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hypervisor is more efficient). The compute agent daemon is configured
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to run one or more *pollster* plugins using the
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``ceilometer.poll.compute`` namespace. Polling for resources not tied
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to the compute node is handled by the central agent. The central
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agent daemon is configured to run one or more *pollster* plugins using
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the ``ceilometer.poll.central`` namespace.
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(such as via libvirt). Polling for compute resources is handled by a
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polling agent running on the compute node (where communication with the
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hypervisor is more efficient). Polling for resources not tied
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to the compute node is handled by the agent running on the cloud
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controller node via services APIs. The polling agent daemon is configured
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to run one or more *pollster* plugins using either the
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``ceilometer.poll.compute`` and/or ``ceilometer.poll.central`` namespaces.
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The agents periodically asks each pollster for instances of
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The agents periodically ask each pollster for instances of
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``Sample`` objects. The agent framework then publishes the Samples using
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the publishers defined in the pipeline configuration. For example,
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the ``notifier`` publisher converts the Sample to metering messages, which it
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@ -24,34 +24,41 @@ has not yet been covered by an existing plugin.
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Agents
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======
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The compute agent runs on each compute node to poll for resource
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Polling agent might be run either on central cloud management nodes or on the
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compute nodes (where direct hypervisor polling is quite logical).
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The agent running on each compute node polls for compute resources
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usage. Each metric collected is tagged with the resource ID (such as
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an instance) and the owner, including tenant and user IDs. The metrics
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are then reported to the collector via the message bus. More detailed
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information follows.
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The compute agent is implemented in ``ceilometer/compute/manager.py``. As
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you will see in the manager, the computeagent loads all plugins defined in
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the namespace ``ceilometer.poll.compute``, then periodically calls their
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:func:`get_samples` method.
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The agent running on the cloud central management node polls other types of
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resources from a management server (usually using OpenStack services API to
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collect this data).
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The central agent polls other types of resources from a management server.
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The central agent is defined in ``ceilometer/central/manager.py``. It loads
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plugins from the ``ceilometer.poll.central`` namespace and polls them by
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calling their :func:`get_samples` method.
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The polling agent is implemented in ``ceilometer/agent/manager.py``. As
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you will see in the manager, the agent loads all plugins defined in
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the namespace ``ceilometer.poll.agent``, then periodically calls their
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:func:`get_samples` method.
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Plugins
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=======
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An agent can support multiple plugins to retrieve different
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A polling agent can support multiple plugins to retrieve different
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information and send them to the collector. As stated above, an agent
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will automatically activate all plugins of a given class. For example,
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the compute agent will load all plugins of class
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``ceilometer.poll.compute``. This will load, among others, the
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:class:`ceilometer.compute.pollsters.CPUPollster`, which is defined in
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the file ``ceilometer/compute/pollsters.py`` as well as the
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:class:`ceilometer.compute.notifications.InstanceNotifications` plugin
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which is defined in the file ``ceilometer/compute/notifications.py``
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will automatically activate all possible plugins if no additional information
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about what to poll was passed. Previously we had separated compute and
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central agents with different namespaces with plugins (pollsters) defined
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within. Currently we keep separated namespaces - ``ceilometer.poll.compute``
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and ``ceilometer.poll.central`` for quick separation of what to poll depending
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on where is polling agent running. This will load, among others, the
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:class:`ceilometer.compute.pollsters.cpu.CPUPollster`, which is defined in
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the folder ``ceilometer/compute/pollsters``.
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Notifications mechanism uses plugins as well, for instance
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:class:`ceilometer.compute.notifications.instance.InstanceNotifications` plugin
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which is defined in the ``ceilometer/compute/notifications`` folder.
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We are using these two existing plugins as examples as the first one provides
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an example of how to interact when you need to retrieve information from an
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@ -62,24 +69,45 @@ Pollster
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--------
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Compute plugins are defined as subclasses of the
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:class:`ceilometer.compute.plugin.ComputePollster` class as defined in
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the ``ceilometer/compute/plugin.py`` file. Pollsters must implement one
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:class:`ceilometer.compute.BaseComputePollster` class as defined in
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the ``ceilometer/compute/__init__.py`` file. Pollsters must implement one
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method: ``get_samples(self, manager, context)``, which returns a
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sequence of ``Sample`` objects as defined in the
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``ceilometer/sample.py`` file.
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In the ``CPUPollster`` plugin, the ``get_samples`` method is implemented as a loop
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which, for each instances running on the local host, retrieves the cpu_time
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from the hypervisor and sends back two ``Sample`` objects. The first one, named
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"cpu", is of type "cumulative", meaning that between two polls, its value is
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not reset, or in other word that the cpu value is always provided as a duration
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that continuously increases since the creation of the instance. The second one,
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named "cpu_util", is of type "gauge", meaning that its value is the percentage
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of cpu utilization.
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In the ``CPUPollster`` plugin, the ``get_samples`` method is implemented as a
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loop which, for each instances running on the local host, retrieves the
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cpu_time from the hypervisor and sends back two ``Sample`` objects. The first
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one, named "cpu", is of type "cumulative", meaning that between two polls, its
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value is not reset while the instance remains active, or in other words that
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the CPU value is always provided as a duration that continuously increases
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since the creation of the instance. The second one, named "cpu_util", is of
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type "gauge", meaning that its value is the percentage of cpu utilization.
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Note that the ``LOG`` method is only used as a debugging tool and does not
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participate in the actual metering activity.
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There is the way to specify either namespace(s) with pollsters or just
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list of concrete pollsters to use, or even both of these parameters on the
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polling agent start via CLI parameter:
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ceilometer-polling --polling-namespaces central compute
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This command will basically make polling agent to load all plugins from the
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central and compute namespaces and poll everything it can. If you need to load
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only some of the pollsters, you can use ``pollster-list`` option:
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ceilometer-polling --pollster-list image image.size storage.*
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If both of these options are passed, the polling agent will load only those
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pollsters specified in the pollster list, that can be loaded from the selected
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namespaces.
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.. note::
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Agents coordination cannot be used in case of pollster-list option usage.
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This allows to avoid both samples duplication and their lost.
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Notifications
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-------------
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@ -45,10 +45,10 @@
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A ceilometer is a device that uses a laser or other light
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source to determine the height of a cloud base.
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central agent
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Software service running on a central management node within the
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OpenStack infrastructure measuring usage and sending the results
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to the :term:`collector`.
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polling agent
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Software service running either on a central management node within the
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OpenStack infrastructure or compute node measuring usage and sending the
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results to the :term:`collector`.
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collector
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Software service running on the OpenStack infrastructure
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@ -56,11 +56,6 @@
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samples from the ceilometer agent and recording the results
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in the database.
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compute agent
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Software service running on a compute node within the OpenStack
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infrastructure measuring usage and sending the results to the
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:term:`collector`.
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notification agent
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The different OpenStack services emit several notifications about the
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various types of events. The notification agent consumes them from
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@ -18,13 +18,22 @@
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Installing and Running the Development Version
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================================================
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Ceilometer has several daemons. The basic are: :term:`compute agent` runs on
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the Nova compute node(s) while the :term:`central agent`, :term:`collector`
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and :term:`notification agent` run on the cloud's management node(s).
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Ceilometer has several daemons. The basic are: :term:`polling agent` running
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either on the Nova compute node(s) or :term:`polling agent` running on the
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central management node(s), :term:`collector`
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and :term:`notification agent` running on the cloud's management node(s).
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In a development environment created by devstack_, these services are
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typically running on the same server. They do not have to be, though, so some
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of the instructions below are duplicated. Skip the steps you have already done.
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.. note::
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In fact, previously Ceilometer had separated compute and central agents, and
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their support is implemented in devstack_ right now, not one agent variant.
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For now we do have deprecated cmd sripts emulating old compute/central
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behaviour using namespaces option passed to polling agent, which will be
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maintained for a transitional period.
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.. _devstack: http://www.devstack.org/
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Configuring Devstack
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