[ros-users] RFC: warning for (lacking) queue_size for python publishers in Indigo?

[Daniel Stonier]

On 6 March 2014 10:58, Dirk Thomas <dthomas at osrfoundation.org> wrote:

> We are looking for feedback from ROS users on a very specific question for
> rospy in Indigo.
>
> In the past rospy publishers sent the messages synchronously to each
> connection.
> That becomes a problem when one of the underlying network buffers runs
> full (which implies that it blocks).
> This commonly happens when e.g. using a connection over wlan and the
> connectivity is lost.
> The client code would then block on the publish() call for an undetermined
> amount of time (potentially forever).
>
> Therefore in Hydro an optional keyword argument was introduced to the
> rospy.Publisher constructor: queue_size=None (see [1])
>
> The default value of `None` maintains the previous behavior (synchronous
> behavior, no queuing).
> When an integer value is passed a queue with the maximum size of N is used
> to queue published messages which are then send to each connection from
> separate threads.
> (A size of zero means an infinite queue, which can be dangerous.)
>
> The implementation follows the existing roscpp implementation - except
> that in roscpp that argument is required.
> It is definitely not possible to make the argument in Python required as
> well since that would imply breaking every existing Python publisher.
>
> Therefore we have two options how to handle this in Indigo:
>
> a) The constructor could output a warning when the queue_size argument is
> missing in order to notify users / developers to update the code and
> actually provide a reasonable queue_size.
>    The warning would be printed using the Python warnings module and can
> therefore be omitted by setting an environment variable (see [2]).
>
> b) We keep the code as it is in Hydro meaning messages are by default
> published synchronously.
>    Users can use the optional keyword but are not given any indication
> from the code at runtime.
>
> The tutorials as well as Indigo migration guide will mention whatever
> option will finally be used.
>
> Which option would you prefer for Indigo?
>
> Thank you for your feedback,
> - Dirk
>
>
> [1] http://wiki.ros.org/rospy/Overview/Publishers%20and%20Subscribers
> [2] https://github.com/ros/ros_comm/pull/372
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I haven't looked into the api here, but I'm guessing from the above
description that the default roscpp implementation accepts a non-zero(?)
queue size and consequently is not synchronous. Is that true?

If it is, what is the primary reason for wanting the python implementation
proposed in b) to be different, i.e. synchronous by default? Is the
threaded non-blocking implementation particularly expensive?

Daniel.
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