1.1.1. Discovery Server Minimal Example

1.1.1.1. Background

By default, ROS 2 uses the Simple Discovery Protocol (SDP), which relies on multicast communication to discover other nodes. While SDP is suitable for many scenarios, it may not be the best choice in complex and large-scale network configurations. This is where the Fast DDS Discovery Server comes into play. Fast DDS offers an alternative discovery mechanism that allows ROS 2 nodes to connect to a Discovery Server. This server acts as a centralized point for node registration and discovery, providing more flexibility and control in managing the network of nodes and is particularly advantageous in scenarios such as:

• Large networks

• Complex networks

• WiFi networks

In this tutorial, we will explore how to set up and use the Fast DDS Discovery Server with ROS 2, enabling robust and dynamic node discovery communication.

1.1.1.2. Prerequisites

It is required to have previously installed Vulcanexus using one of the following installation methods:

• Linux binary installation

• Linux installation from sources

• Docker installation

1.1.1.3. Run this tutorial

Once we finish with the set up of Vulcanexus environment we are ready to start with the talker-listener demo tutorial. In this demo both a talker and a listener nodes from the demo_nodes_cpp ROS 2 package are created: the talker node will publish a “hello world” message every second, while the listener node will listen to these messages. By sourcing Vulcanexus you will get access to the CLI tool fastdds, together with the ROS 2 environment. This tool gives access to the discovery tool, which can be used to launch a discovery server. This server will manage the discovery process for the nodes that connect to it.

Important

Do not forget to source ROS 2 every time you open another session to have access to the ROS 2 environment and the CLI tool fastdds.

1.1.1.3.1. Setup Discovery Server

Start by launching a discovery server with id 0, port 11811 (default port) and listening on all available interfaces. The role of the server is to re-distribute the clients (and servers) discovery information to their known clients (and servers).

fastdds discovery --server-id 0

1.1.1.3.2. Launch Talker Node

Execute the Talker demo to publish on the /chatter topic. When setting the ROS_DISCOVERY_SERVER environment variable you are indicating that ROS node should act as a client that connects to a discovery server to discover other nodes on the network. Use the argument --remap __node:=talker_discovery_server to change the node’s name for this tutorial.

export ROS_DISCOVERY_SERVER=127.0.0.1:11811
ros2 run demo_nodes_cpp talker --ros-args --remap __node:=talker_discovery_server

1.1.1.3.3. Launch Listener Node

Execute the Listener demo to listen to the /chatter topic.

export ROS_DISCOVERY_SERVER=127.0.0.1:11811
ros2 run demo_nodes_cpp listener --ros-args --remap __node:=listener_discovery_server