ssh_machine

This package provides and implementation of the launch.Machine class that can be used to launch processes on remote machines via SSH. It is intended to be used together with the multi-machine-launching branch in my fork of the launch repository: https://github.com/pjreed/launch/tree/multi-machine-launching

Prerequisites

This has been tested in ROS 2 Foxy on Ubuntu 22.04. It also requires the python3-asyncssh package.

Example Usage

Create a workspace, or clone these repos into an existing one:

# Set up the workspace
. /opt/ros/foxy/setup.bash
mkdir -p ssh_machine_ws/src
cd ssh_machine_ws/src
git clone -b multi-machine-launching https://github.com/pjreed/launch.git
git clone https://github.com/pjreed/ssh_machine.git

# Install dependencies
rosdep install . -y --from-paths -i

# Build it
cd ..
colcon build --symlink-install --allow-overriding launch launch_testing launch_testing_ament_cmake launch_xml launch_yaml
. install/setup.bash

# Run the demo; edit ssh_machine/launch/pub_sub.launch.py to use a machine you can access
ros2 launch ssh_machine pub_sub.launch.py

How It Works

In my fork of the launch repository, the launch.actions.ExecuteProcess class has been modified to accept a machine= argument that defines how the process should be launched. That repo also contains a launch.LocalMachine class that implements the previous functionality and will launch a process on the local host; this is the default behavior if no machine is specified.

This repository defines an ssh_machine.SshMachine class that uses the asyncssh library to launch processes on a remote host. It works by creating an SSH channel to the configured host, then running processes in the background; when it is told to shut down, it kills all background processes and then exits. It takes two arguments; a hostname that specifies the host name (or IP) of the remote host, and an env that specifies a script that should be sourced to configure the environment (for example, /opt/ros/foxy/setup.bash).

Limitations

This is a naive implementation that needs to be made more robust for practical use. Currently:

• The paths to ROS nodes are resolved before the command is sent over the SSH channel, meaning nodes must be installed on both the local and remote host and they must be in the same path.
  • A side effect of this is that you must have the same ROS distrubtion installed on every host; for example, if you try to launch a node with the package demo_nodes_cpp and executable talker on a remote host, if the local host is using ROS Foxy, then it will be resolved to /opt/ros/foxy/lib/demo_nodes_cpp/talker before it is executed on the remote host.
• The ssh connection is not very customizable; it will connect as the user who is running the launch script, and it will not prompt the user for a password, so you must have public key authentication correctly configured.
• Aside from specifying a script to source, the environment can't be configured. It should be possible to configure the environment so that it is possible to, for example, change the DDS implementation or domain ID.