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All ros2 topics have a status topic for monitoring them. To have a look launch an action server, then list the topics with hidden topics shown:

$ ros2 run examples_rclcpp_minimal_action_server action_server_member_functions

...

$ ros2 topic list --include-hidden-topics
/fibonacci/_action/feedback
/fibonacci/_action/status
/parameter_events
/rosout

Echo the _action/status topic while sending a goal

$ ros2 topic echo /fibonacci/_action/status
status_list:
- goal_info:
    goal_id:
      uuid:
      - 82
      - 166
      - 223
      - 231
      - 14
      - 16
      - 94
      - 134
      - 45
      - 224
      - 247
      - 104
      - 135
      - 179
      - 196
      - 14
    stamp:
      sec: 1606145080
      nanosec: 966343610
  status: 2
---
status_list:
- goal_info:
    goal_id:
      uuid:
      - 82
      - 166
      - 223
      - 231
      - 14
      - 16
      - 94
      - 134
      - 45
      - 224
      - 247
      - 104
      - 135
      - 179
      - 196
      - 14
    stamp:
      sec: 1606145080
      nanosec: 966343610
  status: 4
---

...

$ ros2 run examples_rclcpp_minimal_action_client action_client_member_functions

Notice the first message status 2 is EXECUTING, and the second is status 4 SUCCEEDED. This server chose to execute the goal right away, but other servers may accept the goal, hold on to it for a bit, then execute it later.

See the ROS 2 action design doc for more info.