Merge "[OVN] Add documentation about the OVN agent"

doc/source/ovn/index.rst

@@ -13,3 +13,4 @@ OVN Driver
    dhcp_opts.rst
    ml2ovn_trace.rst
    faq/index.rst
+   ovn_agent.rst

doc/source/ovn/ovn_agent.rst

@@ -0,0 +1,75 @@
+.. _ovn_agent:
+
+OVN agent
+=========
+
+The OVN agent is a service that could be executed in any node running the
+ovn-controller service. This agent provides additional functionalities not
+provided by OVN; for example, a metadata proxy between the virtual machines
+and the Nova metadata service. This agent will replace the need of
+the OVN metadata agent.
+
+
+OVN and OVS database connectivity
+---------------------------------
+
+The OVN agent can access the local OVS database where the service is
+running. It also has access to the Northbound and Southbound OVN databases. The
+connection strings to these databases are defined in the agent configuration
+file:
+
+.. code-block:: console
+
+    [ovn]
+    ovn_nb_connection = tcp:192.168.10.100:6641
+    ovn_sb_connection = tcp:192.168.10.100:6642
+    [ovs]
+    ovsdb_connection = tcp:127.0.0.1:6640
+
+
+Plugable extensions
+-------------------
+
+The OVN agent provides functionalities via extensions. When the agent is
+started, the ``OVNAgentExtensionManager`` instance loads the configured
+extensions. The extensions are defined in the stevedore entry points, under
+the section "neutron.agent.ovn.extensions". The extensions are defined in the
+agent configuration file in the "extensions" parameter:
+
+.. code-block:: console
+
+    [DEFAULT]
+    extensions = metadata
+
+
+Each extension will inherit from ``OVNAgentExtension``, which provides the API
+for an OVN agent extension. The extensions are loaded in two steps:
+
+* Initialization: this phase involves the call of
+  ``OVNAgentExtension.consume_api`` and ``OVNAgentExtension.initialize`` (in
+  this order). The first one assigns the extension API to the instance. In this
+  case, the OVN agent has a specific instance ``OVNAgentExtensionAPI`` that
+  gives to the extensions the needed access to OVS and OVN databases, using the
+  same IDL instance. The second one is not currently used in the base class;
+  it could be used, for example as in the metadata extension, to spawn the
+  process monitor.
+* Start: in this phase, the OVN and OVS database connections are established
+  and can be accessed. The extension manager will call each extension
+  ``OVNAgentExtension.start`` method.
+
+
+Each extension should define a set of OVS, OVN Northbound and OVN Southbound
+tables to monitor, and a set of events related to these databases. The OVN
+agent will create the corresponding IDL connections using the conjunction of
+these tables and events.
+
+
+Event-driven service
+--------------------
+
+The OVN agent is a ``oslo_service.service.Service`` type class, that is
+launched when the script is executed. Once initialized, the service is waiting
+for new events that will trigger actions. As mentioned in the previous section,
+each extension will subscribe to a set of events from the OVN and OVS
+databases; these events will trigger a set of actions executed on the OVN
+agent.