Can't retrieve data from /odom topic

Hello. I am currently doing the quiz 5 and I’m not able to get the odom message all the time.
I mean, when I echo the /odom topic by terminal, I can see the messages updating all the time.
When I try to subscribe by python script, I only receive the message each ~10 seconds.

import rclpy
from rclpy.action import ActionServer
from rclpy.node import Node

from actions_quiz_msg.action import Distance
from std_msgs.msg import Float64
from nav_msgs.msg import Odometry
from rclpy.qos import ReliabilityPolicy, QoSProfile
import math
import time

class MyActionServer(Node):
    def __init__(self):
        self._action_server = ActionServer(self, Distance, 'distance_as',self.execute_callback) 
        self.cmd = Float64()
        self.x_past = 0.0
        self.y_past = 0.0
        self.x_act = 0.0
        self.y_act = 0.0
        self.calculated_distance = 0.0
        self.start = True
        self.publisher_ = self.create_publisher(Float64, 'total_distance', 10)
        self.subscriber = self.create_subscription(Odometry, '/odom', self.odom_callback, QoSProfile(depth=10, reliability=ReliabilityPolicy.RELIABLE))
    def odom_callback(self, msg):
        self.x_act = msg.pose.pose.position.x
        self.y_act = msg.pose.pose.position.y
        print ("Message received: ", self.x_act)

    def execute_callback(self, goal_handle):
        self.get_logger().info('Executing goal...')

        feedback_msg = Distance.Feedback()

        for i in range(0, goal_handle.request.seconds):
            self.get_logger().info('X Actual: {0}, Y Actual: {1}'.format(str(self.x_act), str(self.y_act)))
            #self.get_logger().info('X Pasada: {0}, Y Pasada: {1}'.format(str(self.x_past), str(self.y_past)))
            self.get_logger().info('Executing goal...')

            if (self.start):
                self.calculated_distance = 0.0
                self.x_past = self.x_act
                self.y_past = self.y_act
                self.start = False

            self.calculated_distance = self.calculated_distance +  math.sqrt(math.pow(self.x_act - self.x_past,2) + math.pow(self.y_act - self.y_past,2))

   = self.calculated_distance

            feedback_msg.current_dist =  self.calculated_distance
            self.get_logger().info('Feedback: Calculated distance: {0} '.format(str(self.calculated_distance)))

            self.x_past = self.x_act
            self.y_past = self.y_act


        result = Distance.Result()
        result.status = True
        result.total_dist = self.calculated_distance

        self.get_logger().info('Result: Status:{0}, Total distance: {1}'.format(str(result.status), str(result.total_dist)))
        return result

def main(args=None):

    my_action_server = MyActionServer()


if __name__ == '__main__':

output of ros2 topic info -v /odom

What am I doing wrong?

Thank you

Hi @ShikurM56 ,

You forgot to add rclpy.spin_once(self) inside the for loop of the execute_callback. That is why your subscriber callback was in “blocking” mode to allow the for loop inside execute_callback to complete and return. This is something very common in the ROS [1 and 2] architecture.

Tip / Hint : The other way to solve this would be to use Executors and Callback groups. [Use Reentrant callback group with MultiThreadedExecutor], only in the case of ROS2.

for i in range(0, goal_handle.request.seconds):

    rclpy.spin_once(self)   # add this line here <<<------------

    self.get_logger().info('X Actual: {0}, Y Actual: {1}'.format(str(self.x_act), str(self.y_act)))
    # self.get_logger().info('X Pasada: {0}, Y Pasada: {1}'.format(str(self.x_past), str(self.y_past)))
    self.get_logger().info('Executing goal...')

This should fix your problem. Let me know if this does not fix.


1 Like

Hello @girishkumar.kannan
Thank you for your answer. With that line, I’m able to compute correctly the distance traveled, but now I can’t call the goal more than one time. It keeps in … Waiting for an action server to become available…

Hi @ShikurM56 ,

Firstly, glad to know that your problem is fixed and now it is working!

Yes, even I observed that happening. I do not know why. I fixed that problem when I learned the course in C++.

But, a way to solve that problem in Python (that I have learned) is when you make the action server run with MultiThreadedExecutor by adding server callback as Reentrant Callback Group type.
That way you can call the action server several times after it finishes.

To learn more about Executors and Callback Groups watch this video:
The explanation starts at 15:00 minute mark onwards.

Use this guide by ROS2 documentation: Using Callback Groups — ROS 2 Documentation: Foxy documentation

Let me know if you still need help.


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