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ROS Action Server

❗This is for ROS 1, not ROS 2. We are keeping this for reference to users who want to use ROS 1❗

What is an Action Server?

An action server is a node that provides an action. An action is a long-running task that can be awaited and provides feedback on progress. Actions are a powerful way for nodes to communicate with each other and await long-running actions while receiving feedback on progress. Actions can be used to perform motor control or locomotion, or any other task that benefits from being awaited (which means that it can be pre-empted) and has definite progress feedback to provide, as opposed to a service which is a one-shot request-response mechanism.

Overview

In this exercise you will create a ROS action server and client. The action server will be a simple counter that counts from 0 to 9. The action client will request the action server to count digits. Each progress is relayed as feedback back to the client who also prints the result upon completion.

The steps below assume that you have a working understanding of ROS workspace and packages.

It's also assumed that you have already created a ROS workspace called ros_ws in your home directory. If you have not, please follow the instructions at Creating a ROS Workspace.

Instructions

  1. Create a new ROS package called action_server:

    cd ~/ros_ws/src
catkin new_pkg action_server

  2. Create a new action definition file called Count.action in the action_server package:

    cd ~/ros_ws/src/action_server
mkdir action
touch action/Count.action

  3. Edit the Count.action file and add the following:

    int32 count
---

int32 result

  4. Edit the package.xml file and add the following:

    <build_depend>actionlib_msgs</build_depend>
<exec_depend>actionlib_msgs</exec_depend>

  5. Edit the CMakeLists.txt file and add the following:

    find_package(actionlib_msgs REQUIRED)
add_action_files(DIRECTORY action FILES Count.action)
generate_messages(DEPENDENCIES actionlib_msgs)

  6. Build the action_server package:

    cd ~/ros_ws
catkin build action_server

  7. Create a new ROS package called action_client:

    cd ~/ros_ws/src
catkin new_pkg action_client

  8. Edit the package.xml file and add the following:

    <build_depend>actionlib_msgs</build_depend>
<exec_depend>actionlib_msgs</exec_depend>
<build_depend>action_server</build_depend>
<exec_depend>action_server</exec_depend>

  9. Edit the CMakeLists.txt file and add the following:

    find_package(actionlib_msgs REQUIRED)
find_package(action_server REQUIRED)
add_action_files(DIRECTORY action FILES Count.action)
generate_messages(DEPENDENCIES actionlib_msgs)

  10. Build the action_client package:

    cd ~/ros_ws
catkin build action_client

  11. Create a new file called count_server.py in the action_server package:

    cd ~/ros_ws/src/action_server
touch scripts/count_server.py

  12. Edit the count_server.py file and add the following:

    #!/usr/bin/env python

import rospy
import actionlib

from action_server.msg import CountAction, CountGoal, CountResult

class CountServer(object):
    def __init__(self):
        self._as = actionlib.SimpleActionServer('count', CountAction, execute_cb=self.execute_cb, auto_start=False)
        self._as.start()

    def execute_cb(self, goal):
        r = rospy.Rate(1)
        success = True

        for i in range(0, goal.count):
            if self._as.is_preempt_requested():
                rospy.loginfo('Preempted')
                self._as.set_preempted()
                success = False
                break

            rospy.loginfo(i)
            r.sleep()

        if success:
            self._as.set_succeeded(CountResult(i))

if __name__ == '__main__':

    rospy.init_node('count_server')
    server = CountServer()
    rospy.spin()

  13. Make the count_server.py file executable:

    chmod +x scripts/count_server.py

  14. Create a new file called count_client.py in the action_client package:

    cd ~/ros_ws/src/action_client
touch scripts/count_client.py

  15. Edit the count_client.py file and add the following:

    #!/usr/bin/env python

import rospy
import actionlib

from action_client.msg import CountAction, CountGoal, CountResult

def feedback_cb(feedback):
    rospy.loginfo(feedback)

if __name__ == '__main__':
    rospy.init_node('count_client')
    client = actionlib.SimpleActionClient('count', CountAction)
    client.wait_for_server()

    goal = CountGoal(10)
    client.send_goal(goal, feedback_cb=feedback_cb)

    client.wait_for_result()
    rospy.loginfo(client.get_result())

  16. Make the count_client.py file executable:

    chmod +x scripts/count_client.py

  17. Run the count_server.py file:

    rosrun action_server count_server.py

  18. In a new terminal, run the count_client.py file:

    rosrun action_client count_client.py

  19. You should see the following output:

    [INFO] [1631635475.000000]: 0
[INFO] [1631635476.000000]: 1
[INFO] [1631635477.000000]: 2
[INFO] [1631635478.000000]: 3
[INFO] [1631635479.000000]: 4
[INFO] [1631635480.000000]: 5
[INFO] [1631635481.000000]: 6
[INFO] [1631635482.000000]: 7
[INFO] [1631635483.000000]: 8
[INFO] [1631635484.000000]: 9
[INFO] [1631635484.000000]: 9

Next Steps

Summary

Further Reading