76 lines
2.8 KiB
ReStructuredText
76 lines
2.8 KiB
ReStructuredText
.. _ovn_agent:
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OVN agent
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=========
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The OVN agent is a service that could be executed in any node running the
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ovn-controller service. This agent provides additional functionalities not
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provided by OVN; for example, a metadata proxy between the virtual machines
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and the Nova metadata service. This agent will replace the need of
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the OVN metadata agent.
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OVN and OVS database connectivity
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---------------------------------
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The OVN agent can access the local OVS database where the service is
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running. It also has access to the Northbound and Southbound OVN databases. The
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connection strings to these databases are defined in the agent configuration
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file:
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.. code-block:: console
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[ovn]
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ovn_nb_connection = tcp:192.168.10.100:6641
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ovn_sb_connection = tcp:192.168.10.100:6642
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[ovs]
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ovsdb_connection = tcp:127.0.0.1:6640
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Plugable extensions
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-------------------
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The OVN agent provides functionalities via extensions. When the agent is
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started, the ``OVNAgentExtensionManager`` instance loads the configured
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extensions. The extensions are defined in the stevedore entry points, under
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the section "neutron.agent.ovn.extensions". The extensions are defined in the
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agent configuration file in the "extensions" parameter:
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.. code-block:: console
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[DEFAULT]
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extensions = metadata
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Each extension will inherit from ``OVNAgentExtension``, which provides the API
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for an OVN agent extension. The extensions are loaded in two steps:
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* Initialization: this phase involves the call of
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``OVNAgentExtension.consume_api`` and ``OVNAgentExtension.initialize`` (in
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this order). The first one assigns the extension API to the instance. In this
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case, the OVN agent has a specific instance ``OVNAgentExtensionAPI`` that
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gives to the extensions the needed access to OVS and OVN databases, using the
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same IDL instance. The second one is not currently used in the base class;
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it could be used, for example as in the metadata extension, to spawn the
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process monitor.
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* Start: in this phase, the OVN and OVS database connections are established
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and can be accessed. The extension manager will call each extension
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``OVNAgentExtension.start`` method.
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Each extension should define a set of OVS, OVN Northbound and OVN Southbound
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tables to monitor, and a set of events related to these databases. The OVN
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agent will create the corresponding IDL connections using the conjunction of
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these tables and events.
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Event-driven service
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--------------------
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The OVN agent is a ``oslo_service.service.Service`` type class, that is
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launched when the script is executed. Once initialized, the service is waiting
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for new events that will trigger actions. As mentioned in the previous section,
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each extension will subscribe to a set of events from the OVN and OVS
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databases; these events will trigger a set of actions executed on the OVN
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agent.
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