ROS 2: Multi-Node Communication

This tutorial demonstrates how to set up ROS 2 multi-node communication using a centralized Discovery Server architecture in OSMO. You will learn how to orchestrate multiple ROS 2 nodes across different containers to communicate in a distributed environment, which is essential for building scalable robotics applications.

This tutorial is based on the official ROS 2 Discovery Server tutorial .

The complete workflow example is available here .

Architecture

This workflow orchestrates three tasks running in parallel to demonstrate ROS 2 inter-node communication:

🌐 Discovery Server

A FastDDS Discovery Server that facilitates node discovery and message routing between containers.

📢 Publisher Node

A ROS 2 node that publishes “Hello World” messages to a topic at regular intervals.

👂 Subscriber Node

A ROS 2 node that subscribes to the topic and receives messages from the talker.

Building the Workflow

Let’s examine the key components of the ROS 2 communication workflow:

 workflow:
   groups:
   - name: nodes
     tasks:
     - name: discovery-server
       image: osrf/ros:humble-desktop-full
       args:
       - /tmp/client.sh
       command:
       - /bin/bash
       files:
       - contents: |-
           #!/bin/bash
           source /opt/ros/humble/setup.bash
           fastdds discovery --server-id 0 -p 11811
         path: /tmp/client.sh
     - name: publisher-node
       image: osrf/ros:humble-desktop-full
       args:
       - /tmp/entry.sh
       command:
       - bash
       files:
       - contents: |-
           apt-get update
           apt-get install -y dnsutils

           # (1)
           DISCOVERY_SERVER_IP=$(nslookup {{host:discovery-server}} | grep -oP \
               'Address: \K\d[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}')

           export ROS_DISCOVERY_SERVER=$DISCOVERY_SERVER_IP:11811

           # Set up environment variables
           source /opt/ros/humble/setup.bash

           # (2)
           ros2 run demo_nodes_cpp talker --ros-args --remap __node:=talker_discovery_server
         path: /tmp/entry.sh
       lead: true
     - name: subscriber-node
       image: osrf/ros:humble-desktop-full
       args:
       - /tmp/entry.sh
       command:
       - bash
       files:
       - contents: |-
           apt-get update
           apt-get install -y dnsutils

           DISCOVERY_SERVER_IP=$(nslookup {{host:discovery-server}} | grep -oP \
               'Address: \K\d[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}')

           export ROS_DISCOVERY_SERVER=$DISCOVERY_SERVER_IP:11811

           # Set up environment variables
           source /opt/ros/humble/setup.bash

           # (3)
           ros2 run demo_nodes_cpp listener --ros-args --remap __node:=listener_discovery_server
         path: /tmp/entry.sh
   name: ros-comms
   resources:
     default:
       cpu: 1
       memory: 1Gi
       storage: 1Gi
   timeout:
     exec_timeout: 15m

1. The discovery server IP is resolved using DNS lookup. The IP is required in order to set the ROS_DISCOVERY_SERVER environment variable to communicate through the discovery server. This is also done in the subscriber-node.

2. The talker node publishes simple String messages to the topic /chatter.

3. The listener node subscribes to the topic /chatter and prints the messages to the console.

Understanding the Logs

After submitting the workflow, once all the tasks are running, you should see that the publisher node is publishing messages to the topic through the discovery server.

Publisher Node logs:

[INFO] [talker_discovery_server]: Publishing: 'Hello World: 1'
[INFO] [talker_discovery_server]: Publishing: 'Hello World: 2'
[INFO] [talker_discovery_server]: Publishing: 'Hello World: 3'

Since the subscriber node is subscribed to the topic and configured to listen for messages from the discovery server, it will only print the messages to the console if it receives the messages from the publisher node.

Subscriber Node logs:

[INFO] [listener_discovery_server]: I heard: [Hello World: 1]
[INFO] [listener_discovery_server]: I heard: [Hello World: 2]
[INFO] [listener_discovery_server]: I heard: [Hello World: 3]

If the subscriber node prints these messages, it means the subscriber node is able to receive the messages from the publisher node through the discovery server, even if the two ROS nodes are running on different machines in different networks.

Note

This workflow has a timeout of 15 minutes. If the workflow runs for longer than that, it will be terminated by the service automatically.