ComposableNodeContainer and respawn=True

[simulacrus]

Bug report

There is a non-working combination of a non-empty parameter "composable_node_descriptions" and "respawn=True" in the ComposableNodeContainer class.
Required Info:

• Operating System:
  • Ubuntu 20.04
• Installation type:
  • apt
• Version or commit hash:
  • foxy
• DDS implementation:
  rmw_cyclonedds_cpp

Steps to reproduce issue

ros2 launch ./<launch_name>

import launch
from launch_ros.actions import ComposableNodeContainer
from launch_ros.descriptions import ComposableNode


def generate_launch_description():
    """Generate launch description with multiple components."""
    container = ComposableNodeContainer(
            name='my_container',
            namespace='',
            package='rclcpp_components',
            executable='component_container',
            composable_node_descriptions=[
                ComposableNode(
                    package='composition',
                    plugin='composition::Talker',
                    name='talker'),
                ComposableNode(
                    package='composition',
                    plugin='composition::Listener',
                    name='listener')
            ],
            output='screen',
            respawn=True
    )

    return launch.LaunchDescription([container])

Expected behavior

After the container is killed, it respawns with its components

Actual behavior

After the container is killed, it respawns without components.

Additional information

---

Feature request

Feature description

Implementation considerations

[mjcarroll]

This was probably an oversight in the implementation. I think that the correct thing is happening in that respawn applies to the container itself. As the container doesn't track what has been loaded/unloade, this could get quite tricky (especially if things are changing at runtime).

I would suggest that we maybe add a respawn flag to the ComposableNode description to indicate that it should be respawned with the container, but this will introduce more bookkeeping that doesn't currently exist.

[simulacrus]

@mjcarroll Is there a working solution for the container auto-restart task? I tried to do it through RegisterEventHandler and it didn't work.

import launch
from launch_ros.actions import ComposableNodeContainer, LoadComposableNodes
from launch_ros.descriptions import ComposableNode
from launch.event_handlers import (OnExecutionComplete, OnProcessExit,
                                OnProcessIO, OnProcessStart, OnShutdown)
from launch.actions import RegisterEventHandler, LogInfo
def generate_launch_description():
    """Generate launch description with multiple components."""
    comps =[
                ComposableNode(
                    package='composition',
                    plugin='composition::Talker',
                    name='talker'),
                ComposableNode(
                    package='composition',
                    plugin='composition::Listener',
                    name='listener')
            ]
    container = ComposableNodeContainer(
            name='my_container',
            namespace='',
            package='rclcpp_components',
            executable='component_container',
            output='screen',
            respawn=True
    )
    event = RegisterEventHandler(
                OnProcessStart(
                    target_action=container,
                    on_start=[LogInfo(msg='Container start'),LoadComposableNodes(composable_node_descriptions=comps, target_container=container)]
                ))

    return launch.LaunchDescription([container, event])

[mjcarroll]

Is there a working solution for the container auto-restart task?

Not that I'm aware of, I will take a look as well.

[simulacrus]

@mjcarroll I found a working way for this task

import launch
from launch_ros.actions import ComposableNodeContainer, LoadComposableNodes
from launch_ros.descriptions import ComposableNode
from launch.event_handlers import (OnExecutionComplete, OnProcessExit,
                                OnProcessIO, OnProcessStart, OnShutdown)
from launch.actions import RegisterEventHandler, LogInfo, OpaqueFunction

import time

def func(context, *args, **kwargs):
	comps =[
                ComposableNode(
                    package='composition',
                    plugin='composition::Talker',
                    name='talker'),
                ComposableNode(
                    package='composition',
                    plugin='composition::Listener',
                    name='listener')
            ]
	time.sleep(1) # <--- lmao
	return [LoadComposableNodes(composable_node_descriptions=comps, target_container='my_container')]
	
	
def generate_launch_description():

	container = ComposableNodeContainer(
		name='my_container',
		namespace='',
		package='rclcpp_components',
		executable='component_container',
		output='screen',
		respawn=True
	)
	event = RegisterEventHandler(
		OnProcessStart(
			target_action=container,
			on_start=[LogInfo(msg='Container start'),OpaqueFunction(function=func)]
		))

	return launch.LaunchDescription([event, container])

[mjcarroll]

@mjcarroll I found a working way for this task

Great! Would you like to contribute an example to the documentation so that others may easily reference?

[simulacrus]

@mjcarroll The code example above is more of a hack than a reference working example. This solution works, but the LoadComposableNodes class gives a warning "there are now at least 2 nodes with the name ...". It looks like I'm doing something wrong in terms of launch_ros architecture

[tonynajjar]

I came to create exactly the same issue, glad to see I'm not the only one needing this. I'll try out the hack 😄

[tonynajjar]

Some feedback about the proposed workaround, it worked well for me until I ran it for a simulation. Basically I set use_sim_true globally from a parent launch file using SetParameter("use_sim_time", True) and for some reason that does not propagate into the Actions in on_start from OnProcessStart. Created separate issue #373

[tonynajjar]

this could get quite tricky (especially if things are changing at runtime).

@mjcarroll I can see how it can get tricky if things are changing at runtime, however do you see a straightforward implementation for respawning the container and loading what was first inputted in composable_node_descriptions? I assume that would already unblock 90% of applications. If you have some guidance there I could make a PR

[tonynajjar]

@mjcarroll any updates/insights here? I'm willing to do the contribution here as I need this pretty urgently but I need some guidance. @clalancette Maybe you have an idea?

[mjcarroll]

Yeah @tonynajjar sorry to leave you in the lurch here.

I think that your initial idea of respawning with the description that was used at launch is generally the safest choice.

To prevent any sort of surprise side effects, I think we should make this an opt in flag on the ComposableNodeContainer such that you have respawn=True and respawn_components=True would cause the desired behavior here.

We could consider making that the default further down the road, but it could be surprising right now.

[tonynajjar]

Sure, can you give me some guidance on how to implement the logic behind respawn_components? I'm not very familiar with this package and it seems pretty complex with many abstractions.

As far as I see the respawning logic of the process is in the launch repo and there we don't have any access to the loaded components. Vice-versa, in launch_ros where we do have access to the loaded components, we don't have any respawning logic 🤔.

[Aposhian]

What is a case in which you would want respawn_components=False? Just to slowly roll in this feature? It seems like it will primarily just break people who have hacks for this specific issue.

[TheMarksniper]

Any updates on this? I think respawn_components could be very useful flag to have when we are launching nodes from other developers.
In my example i am trying to use depthai oakd camera, that sometimes crashes when ping misses and respawn_components would be i think the correct solution