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Hi,
I would like know, if there is a way to launch set of ros2 nodes in specific order using launch.py file Say, I have following ros2 nodes; node_1, node_2, node_3 and node_4. I would like launch node_1 only after node_2, node_3 and node_4 are launched.
Any example or code snippets are highly appreciated!!
If you only need to specify the order, then launch will already respect the order you specify nodes in. But if you need to have node1 wait for until nodes 2, 3, and 4 are present and running then read on.
I'm not sure exactly what criteria you have for "when a node starts", so I'm just going to use the built-in events. If you require more specific start-up conditions, you may have to write your own event, event handler, and event emitter.
Here's my python launch script to have node1 started only after nodes 2, 3, and 4 have been started. In this example "started" means that launch has processed that action, and the ProcessStarted event has been emitted.
from rclpy.node import Node
from launch import LaunchContext, LaunchDescription, LaunchService
from launch.actions import RegisterEventHandler
from launch.events.process import ProcessStarted
from launch.event_handlers.on_process_start import OnProcessStart
from launch_ros.actions import Node
def generate_launch_description():
node1 = Node(package='demo_nodes_cpp',
executable='talker',
name='node1',
exec_name='node1')
node2 = Node(package='demo_nodes_cpp',
executable='listener',
name='node2',
exec_name='node2')
node3 = Node(package='demo_nodes_cpp',
executable='listener',
name='node3',
exec_name='node3')
node4 = Node(package='demo_nodes_cpp',
executable='listener',
name='node4',
exec_name='node4')
already_started_nodes = set()
def start_next_node(event: ProcessStarted, context: LaunchContext):
print(f'node {event.process_name} started.')
already_started_nodes.update([event.process_name])
if len(already_started_nodes) == 3:
print(f'all required nodes are up, time to start node1')
return node1
return LaunchDescription([
RegisterEventHandler(event_handler=OnProcessStart(target_action=node2,
on_start=start_next_node)),
RegisterEventHandler(event_handler=OnProcessStart(target_action=node3,
on_start=start_next_node)),
RegisterEventHandler(event_handler=OnProcessStart(target_action=node4,
on_start=start_next_node)),
node2,
node3,
node4,
])
if __name__ == '__main__':
ls = LaunchService()
ls.include_launch_description(generate_launch_description())
ls.run()
The three RegisterEventHandler actions will call start_next_node after the specific node has been started. Once three nodes have already been started, then we'll start node1. We could do something to look at which node actually was started, So if we needed node3 to wait until node2 to start, and node 4 to wait for node3 to start, we could do that kind of logic there.
Example output:
[INFO] [launch]: Default logging verbosity is set to INFO
[INFO] [node2-1]: process started with pid [5499]
[INFO] [node3-2]: process started with pid [5500]
[INFO] [node4-3]: process started with pid [5501]
node node2-1 started.
node node3-2 started.
node node4-3 started.
all required nodes are up, time to start node1
[INFO] [node1-4]: process started with pid [5502]
[node1-4] [INFO] [1679095729.989045466] [node1]: Publishing: 'Hello World: 1'
[node4-3] [INFO] [1679095729.990325048] [node4]: I heard: [Hello World: 1]
[node3-2] [INFO] [1679095729.990340845] [node3]: I heard: [Hello World: 1]
[node2-1] [INFO] [1679095729.990431971] [node2]: I heard: [Hello World: 1]
[node1-4] [INFO] [1679095730.988866943] [node1]: Publishing: 'Hello World: 2'
[node3-2] [INFO] [1679095730.989625772] [node3]: I heard: [Hello World: 2]
[node4-3] [INFO] [1679095730.989626316] [node4]: I heard: [Hello World: 2]
[node2-1] [INFO] [1679095730.989626457] [node2]: I heard: [Hello World: 2]
Thanks a lot @ChuiV this is exactly what I was looking for!!!
according to my experience , i will use a shell script to do this, an untested example needs your own completion maybe as follows:
#!/bin/bash
AMENT_PREFIX_PATH=/opt/ros/foxy # your distro
source $AMENT_PREFIX_PATH/setup.bash
source /path/to/your/pkg/setup.bash
ros2 run pkg node1 node2> /dev/null 2>&1 &
# in case screen flushing and set the node run background
# and pls choose your prefered launch method
i=0
while(( $i<=3 )) # try 3 times
do
sleep 1
echo check for the $i time
let i++
node1=`ros2 node list | grep -i node1 | grep -v grep`
node2=`ros2 node list | grep -i node2 | grep -v grep`
# or other critics your choose, e.g. PID
if [[ -n $node1 && -n $node2 ]]
then
ros2 run pkg node3 &
echo nodes launched
break
fi
done
if [[ -z $node1 ]]
then
echo node1 failed
fi
if [[ -z $node2 ]]
then
echo node2 failed
fi
btw, you may need killall -i -r node to stop the background nodes...
@fury.nerd Thanks for the idea. I am specifically looking into realizing it using launch file.
hi, @TinyTurtle, how about using ros2 launch /path/to/your/launch.py instead of ros2 run pkg node in this shell script? as commented choose your prefered launch method
@fury.nerd, your solution is fine. But I have close to 25 nodes, checking these using shell script would be a problem, as the solution should run on linux/windows.
Asked: 2023-03-14 05:01:19 -0500
Seen: 929 times
Last updated: Mar 17 '23
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