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rospy Subscriber callback threading test

asked 2020-07-14 05:26:26 -0500

Cry-A-Lot gravatar image

I try to use rospy Subscriber callback handler to manipulate some object but it doesn't work like i would like to.

In,subscriber handler is look like this :

import rospy from std_msgs.msg import String import time

verboser = True

def counter() :
    counter = 0
    while(verboser and counter < 20 ):
        counter += 1
        time.sleep(1)
        print(counter)

def callback(data):
    print("data INcoming : ",data.data)
    if int(data.data) == 1:
        counter()
    else :
        global verboser
        verboser = False

def main():
    rospy.init_node('listener')
    sub = rospy.Subscriber('chatter', String,callback)

if __name__ == '__main__':
    main()
    rospy.spin()

if i try to command counter() function with code like this :

import rospy
from std_msgs.msg import String
import time

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    hello_str = "1"
    pub.publish(hello_str)
    time.sleep(5)
    hello_str = "0"
    pub.publish(hello_str)

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

it produce this output :

data INcoming :  1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
data INcoming :  0

but if i change talker to publish only one and re-run it with change variable 'hello_str' it seem counter() function does stop after run the process

import rospy
from std_msgs.msg import String
import time

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    hello_str = "1"
    pub.publish(hello_str)

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

ouput :

data INcoming :  1
1
2
3
4
5
6
data INcoming :  0
7

and i try to check if publisher publish after sleep 5 second via 'rostopic echo /chatter' and it seem it does send. so i think the rospy.Subscriber() is refuse to new threading callback that have been publish from the same process until it finish. Is there anyway to config rospy to accept this ?

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Comments

1

I think the main problem is that you are not giving the callback enough time to perform its task. The callback, counter funtion will take at least 20 sec to be executed, but if you are publishing again at max rate the callback will be executed again while the other one is still producing.

So in your set up, 1 is published and callback start the loop until a 0 is again published. Then the flag verboser is set to False forcing the other thread loop to stop right after receiveng the 0.

As a comment: Callbacks in rospy are not threadsafe. You should use locks in any rospy node where multiple callbacks of any kind might interact.

Weasfas gravatar image Weasfas  ( 2020-07-14 13:44:16 -0500 )edit

the point in this test is i try to start then stop it by using the same subscriber,but it doesn't work out when i using the same node to publishing to a topic and it work when i run it separately which run one node to publish 1 to start a counter() function and another node to publish 0 the set the verboser to stop it.

Cry-A-Lot gravatar image Cry-A-Lot  ( 2020-07-16 06:32:29 -0500 )edit

1 Answer

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answered 2020-07-16 08:51:01 -0500

Weasfas gravatar image

Hi @Cry-A-Lot,

The main problem here is that you want to use rospy beyond its limitations. I will explain myself:

In rospy, for a particular topic, there is only one thread that all the subscribers share. Hence, when your node publish 1 and after 5 sec publish 0, it is the same connection/subscriber and the thread is blocked, thus not setting the verbose flag to False.

Your other method works exactly for this reason, when you publish 1 and then exits the program and relaunch again publishing 0, the subscriber connection is new and the callback is called in another thread changing the value of the flag and stopping the other loop thread.

For this reason why you want to do cannot be done if not using tricks to be able to not block your processing, and one of those tricks is unregistering the subscriber for each callback called. It will look something like:

#!/usr/bin/env python
import rospy
from std_msgs.msg import String
import time

verboser = True
sub = None

def counter() :
    counter = 0
    while(verboser and counter < 20 ):
        counter += 1
        time.sleep(1)
        print(str(counter) + ". Verbose is: " + str(verboser))

def callback(data):
    global sub
    sub.unregister()
    sub = rospy.Subscriber('chatter', String, callback)
    print("data INcoming : ",data.data)
    if int(data.data) == 1:
        counter()
    else:
        global verboser
        verboser = False

def main():
    global sub
    rospy.init_node('listener')
    sub = rospy.Subscriber('chatter', String, callback)

if __name__ == '__main__':
    main()
    rospy.spin()

If someone came up with a more elegant solution rather than this I would glad to discuss it here, however, and that is my opinion, if you want to have more control about threats and synchronization I will try to use roscpp. But as I said that is my personal preference to deal with this kind of things.

Hope that helps you.

Regards!

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so, even though ros publisher is asynchronous, pyros still has it own limitation that there is only one thread that all the subscribers share. then, if i want to call my callback function again i need to use unregister() function to unsubscribe/disconnect to the topic and do registering to new connection by re-implement() Subscriber() function again that will wait for a new publishing. Your answer helps a lot. THANK!

Cry-A-Lot gravatar image Cry-A-Lot  ( 2020-07-17 05:45:47 -0500 )edit

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Asked: 2020-07-14 05:25:40 -0500

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Last updated: Jul 16