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[ROS2] best practice for composable node publishing data from blocking call

asked 2021-03-16 09:02:32 -0500

fhwedel-hoe gravatar image

Hi

I am converting a node from ROS1 to ROS2. This node uses a proprietary SDK to connect to a sensor. The function which reads from the sensor performs a blocking call.

This is the code in ROS1:

while (ros::ok()) {
  data =  get_data_from_sensor(); // may block indefinitely or a very short time
  publisher.publish(data);
  ros::spinOnce();
}

For the ROS2 conversion, I want to make use of the composable node architecture. get_data_from_sensor() will likely become a class member function, but how will I call it? When started as a component, ros::spin() is called on the node. As far as I know, no custom entry point exists.

Since the sensor events do not occur on a regular interval, using a timer does not seem feasible. In the node's constructor, I could create and run a thread with a loop like this:

while (rclcpp::ok()) {
  data =  get_data_from_sensor(); // may block indefinitely or a very short time
  publisher.publish(data);
}

However, this feels odd. My gut says, I am not adhering to some ROS2 paradigm. Is there a best practice for composable nodes publishing data from blocking calls at variable intervals?

Kind regards

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answered 2021-03-23 17:56:42 -0500

alsora gravatar image

I don't think that your loop is particularly odd. There are definitely ways to do it differently (i.e. without having to create a new thread), but they will require more code.

What you could do is to pair the get_data_from_sensor with a condition variable that gets notified when new data from the sensor is available. Then you can write a rclcpp::Waitable object that wraps all of this.

The Waitable object is part of a node and it is added to the executor. When the condition variable is notified, the Waitable object will awake a spinning executor, which in turn will invoke the Waitable execute() method, which in your case would result in the publication of the data.

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Thank you for the answer. There is an example at https://answers.ros.org/question/3228... which looks interesting, but rather involved. Maybe I will try both variants to see how they perform.

fhwedel-hoe gravatar image fhwedel-hoe  ( 2021-03-24 06:13:55 -0500 )edit

Ahah I wrote that example some time ago and I forgot about it =) That's exactly what I Indented here.

That approach of using Waitable is currently being used without many changes in rclcpp library.

alsora gravatar image alsora  ( 2021-03-24 07:50:43 -0500 )edit

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Asked: 2021-03-16 09:02:32 -0500

Seen: 182 times

Last updated: Mar 23 '21