Expose matched subscriber count for publishers

[paulbovbel]

Feature request

Feature description

Please expose a mechanism for getting matched pub/sub counts (rcl_publisher_get_subscription_count).

Implementation considerations

I was catching up on https://answers.ros.org/question/286069/implementing-subscriberstatuscallback-within-ros2/ and noticed that Expose matched publisher / subscriber count (rather than only based on the topic name) is no longer in the roadmap! It looks like rcl_publisher_get_subscription_count was imlemented in rcl/rmw but never plumbed out to the client libraries. Is there any (technical) reason for that?

Also would it be feasible to get that data in a callback/async fashion, rather than by polling?

[mjcarroll]

Yes, it didn't quite get all the way up into the rclcpp library, but it hasn't been completely dropped. It has been partially rolled in to: ros2/ros2#649

As far as callback/async, it is possible, just currently not implemented. It would work similar to how incoming messages work in rcl, with a corresponding mapping in rclcpp.

[paulbovbel]

Thanks, I'll use the rcl handle for now!

Providing an async mechanism would go a long way for ROS1-style lazy pub/sub pipelines (see pcl_ros). An application-side pattern using a timer or something is the alternative that comes to mind.

[mjcarroll]

Agreed.

The approach would be to use another guard condition (or perhaps piggy-back off an existing one) to trigger when publisher and subscriber counts change. This could then be used at the rclcpp-layer to provide the behavior you describe.

[ivanpauno]

This was solve in #628, I forgot referencing this

[paulbovbel]

Thanks @ivanpauno!

Is adding an async mechanism for notifying the application regarding new subscribers still on the roadmap for rclcpp, to enable lazy pub/sub pipelines?

[dirk-thomas]

This was solve in #628

Closing...

Is adding an async mechanism for notifying the application regarding new subscribers still on the roadmap for rclcpp

It is mentioned as a desired feature on the roadmap but it is not planned to be worked on by Dashing.