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Add ROS 2 CLI module to Fast DDS Discovery Server Tutorial (#16)
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* Refs #17064: Add ROS 2 CLI module to Fast DDS Discovery Server Tutorial

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>

* Refs #17064: ROS2 repo suggestions: A publisher is a communication interface, not a node.

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>

* Refs #17064: ROS2 repo suggestions: Nits

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>

* Refs #17064: ROS2 repo suggestions: Network graph

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>

* Refs #17064: Applied suggestions

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>

* Refs #17064: Nitpicks

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>

---------

Signed-off-by: Mikel Rico <mikelrico@eprosima.com>
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@MRicoIE2CS @richiprosima
MRicoIE2CS authored and richiprosima committed Feb 28, 2023
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153 changes: 151 additions & 2 deletions source/Tutorials/Advanced/Discovery-Server/Discovery-Server.rst
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Expand Up @@ -51,7 +51,7 @@ The following figure shows the decrease in discovery messages:
:align: center

This architecture reduces the number of messages sent between the server and clients dramatically.
In the following graph, the reduction in network traffic over the discovery phase for the `RMF Clinic demonstration <https://github.com/osrf/rmf_demos>`__ is shown:
In the following graph, the reduction in network traffic over the discovery phase for the `RMF Clinic demonstration <https://github.com/open-rmf/rmf_demos#Clinic-World>`__ is shown:

.. image:: figures/discovery_server_v2_performance.svg
:align: center
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ROS 2 Introspection
-------------------

ROS 2 Command Line Interface (CLI) implements several introspection tools to analyze the behaviour of a ROS 2 network.
These tools (i.e. ``ros2 bag record``, ``ros2 topic list``, etc.) are very helpful to understand a ROS 2 working network.

Most of these tools use DDS simple discovery to exchange topic information with every existing participant (using simple discovery, every participant in the network is connected with each other).
However, the new Discovery Server v2 implements a network traffic reduction scheme that limits the discovery data between participants that do not share a topic.
This means that not every node will receive every topic's discovery data unless it has a reader for that topic.
As most ROS 2 CLIs need a node in the network (some of them rely on a running ROS 2 daemon, and some create their own nodes), using the Discovery Server v2 these nodes will not have all the network information, and thus their functionalities are limited.

The Discovery Server v2 functionality allows every Participant running as a **SUPER_CLIENT**, a kind of **Client** that connects to a **SERVER**, from which it receives all the available discovery information (instead of just what it needs).
In this sense, ROS 2 introspection tools can be configured as **Super Client**, thus being able to discover every entity that is using the Discovery Server protocol within the network.

.. note::

In this section we use the term *Participant* as a DDS entity. Each DDS *Participant* corresponds with a ROS 2 *Context*, a ROS 2 abstraction over DDS.
`Nodes <ROS2Nodes>` are ROS 2 entities that rely on DDS communication interfaces: ``DataWriter`` and ``DataReader``.
Each *Participant* can hold multiple ROS 2 Nodes.
For further details about these concepts, please visit the `Node to Participant mapping design document <http://design.ros2.org/articles/Node_to_Participant_mapping.html>`__


Daemon's related tools
^^^^^^^^^^^^^^^^^^^^^^

The ROS 2 Daemon is used in several ROS 2 CLI introspection tools.
It creates its own Participant to add a ROS 2 Node to the network graph, in order to receive all the data sent.
In order for the ROS 2 CLI to work when using Discovery Server mechanism, the ROS 2 Daemon needs to be
configured as **Super Client**.
Therefore, this section is devoted to explain how to use ROS 2 CLI with ROS 2 Daemon running as a **Super Client**.
This will allow the Daemon to discover the entire Node graph, and to receive every topic and endpoint information.
To do so, a Fast DDS XML configuration file is used to configure the ROS 2 Daemon and CLI tools.

.. warning::
Although it is possible to run the ROS 2 Daemon as a **Server**, this is not recommended since the daemon will stop
after two hours of inactivity, taking the **Server** down with it.

Below you can find a XML configuration profile, which for this tutorial should be saved in the working directory as ```super_client_configuration_file.xml``` file.
This file will configure every new participant using it, as a **Super Client**.

.. code-block:: xml
<?xml version="1.0" encoding="UTF-8" ?>
<dds>
<profiles xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles">
<participant profile_name="super_client_profile" is_default_profile="true">
<rtps>
<builtin>
<discovery_config>
<discoveryProtocol>SUPER_CLIENT</discoveryProtocol>
<discoveryServersList>
<RemoteServer prefix="44.53.00.5f.45.50.52.4f.53.49.4d.41">
<metatrafficUnicastLocatorList>
<locator>
<udpv4>
<address>127.0.0.1</address>
<port>11811</port>
</udpv4>
</locator>
</metatrafficUnicastLocatorList>
</RemoteServer>
</discoveryServersList>
</discovery_config>
</builtin>
</rtps>
</participant>
</profiles>
</dds>
.. note::

Under the *RemoteServer* tag, the *prefix* attribute value should be updated according to the server ID passed on the CLI (see `Fast DDS CLI <https://fast-dds.docs.eprosima.com/en/latest/fastddscli/cli/cli.html#discovery>`__).
The value specified in the shown XML snippet corresponds to an ID of value 0.

First of all, instantiate a Discovery Server using `Fast DDS CLI <https://fast-dds.docs.eprosima.com/en/latest/fastddscli/cli/cli.html#discovery>`__ specifying an ID of value 0.

.. code-block:: console
fastdds discovery -i 0 -l 127.0.0.1 -p 11811
Run a talker and a listener that will discover each other through the Server (notice that ``ROS_DISCOVERY_SERVER`` configuration is the same as the one in ``super_client_configuration_file.xml``).

.. code-block:: console
export ROS_DISCOVERY_SERVER="127.0.0.1:11811"
ros2 run demo_nodes_cpp listener --ros-args --remap __node:=listener
.. code-block:: console
export ROS_DISCOVERY_SERVER="127.0.0.1:11811"
ros2 run demo_nodes_cpp talker --ros-args --remap __node:=talker
Then, instantiate a ROS 2 Daemon using the **Super Client** configuration (remember to source ROS 2 installation in every new terminal).

.. code-block:: console
export FASTRTPS_DEFAULT_PROFILES_FILE=super_client_configuration_file.xml
ros2 daemon stop
ros2 daemon start
ros2 topic list
ros2 node info /talker
ros2 topic info /chatter
ros2 topic echo /chatter
We can also see the Node's Graph using the ROS 2 tool ``rqt_graph`` as follows (you may need to press the refresh button):

.. code-block:: console
export FASTRTPS_DEFAULT_PROFILES_FILE=super_client_configuration_file.xml
rqt_graph
No Daemon tools
^^^^^^^^^^^^^^^

Some ROS 2 CLI tools do not use the ROS 2 Daemon.
In order for these tools to connect with a Discovery Server and receive all the topics information they need to be instantiated as a **Super Client** that connects to the **Server**.

Following the previous configuration, build a simple system with a talker and a listener.
First, run a **Server**:

.. code-block:: console
fastdds discovery -i 0 -l 127.0.0.1 -p 11811
Then, run the talker and listener in separate terminals:

.. code-block:: console
export ROS_DISCOVERY_SERVER="127.0.0.1:11811"
ros2 run demo_nodes_cpp listener --ros-args --remap __node:=listener
.. code-block:: console
export ROS_DISCOVERY_SERVER="127.0.0.1:11811"
ros2 run demo_nodes_cpp talker --ros-args --remap __node:=talker
Continue using the ROS 2 CLI with ``--no-daemon`` option with the new configuration.
New nodes will connect with the existing Server and will know every topic.
Exporting ``ROS_DISCOVERY_SERVER`` is not needed as the ROS 2 tools will be configured through the ``FASTRTPS_DEFAULT_PROFILES_FILE``.

.. code-block:: console
export FASTRTPS_DEFAULT_PROFILES_FILE=super_client_configuration_file.xml
ros2 topic list --no-daemon
ros2 node info /talker --no-daemon --spin-time 2
Compare Fast DDS Discovery Server with Simple Discovery Protocol
----------------------------------------------------------------

In order to compare executing nodes using the Simple Discovery Protocol (the default DDS mechanism for distributed discovery) or the discovery server, two scripts that execute a talker and many listeners and analyze the network traffic during this time are provided.
In order to compare executing nodes using the *Simple Discovery* Protocol (the default DDS mechanism for distributed discovery) or the *Discovery Server*, two scripts that execute a talker and many listeners and analyze the network traffic during this time are provided.
For this experiment, ``tshark`` is required to be installed on your system.
The configuration file is mandatory in order to avoid using intraprocess mode.

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