[ros-users] actionlib multiple goal types

[Eitan Marder-Eppstein]

Ben,

I'm not sure that there's a recommended way to handle multiple goal types as
this is something that we haven't done with great regularity, but I can try
to give insight on the options and their trade-offs so that you can make the
decision that's best for you.

Option 1 - Having extra fields in the .action file: We've used this approach
for the grasping pipeline that we've built here at Willow and it seems to
work pretty well. A clear disadvantage is that you have an .action file that
contains more information than it strictly needs to and you need to check
somehow which interface the user wants rather than having a separate type.
Some might argue that this is also not as clean as providing two interfaces
on separate ROS channels (in this case over the action protocol), but it
also helps avoid race conditions that can occur from having two separate
channels.

Option 2 - Two .action files and two action servers: Actionlib doesn't have
support for multiple goal types by default, though you could build tools for
handling this on top of the current protocol. The benefit here is that you
have two clearly separated ROS interfaces, but the disadvantage is that you
have to deal with a whole bunch of race conditions in how you want to
process goals. You also have to handle the interaction between the two
action servers.... like preempting a goal in one when a new one comes in
over the wire for the other. Given the programming pain that comes along
with this, we haven't actually used this option in practice... though it
should be doable.

Option 3 - Conversion on the client side: Another option is to expose only
one interface from the server, but to provide conversion utilities on the
client side that make it easy to convert from a second type to the one the
server requires. This assumes that there is a clear conversion procedure
between the two desired goal types, but may work well in many cases.

Hope this helps,

Eitan

On Thu, Sep 16, 2010 at 10:22 AM, Axelrod, Benjamin <baxelrod at irobot.com>wrote:

>  What is the recommended way to handle multiple goal types with
> actionlib?
>
> 1)      Put flags in the .action file such that it can handle multiple
> goal types?  This means always have a single action file for each
> server/client pair.
>
> 2)      Make two different .action files:
>
> a.      Can a single server/client pair handle multiple .action files?
>
> b.      Should you have 2 actionlib servers and 2 actionlib clients?  And
> if so, how can they interact?
>
>
>
> For example, suppose you have a node which drives a simple differential
> drive robot base.  You want this server to take either a drive straight
> distance, or the number of degrees to rotate in place.  Both actions can’t
> be going at the same time.  A new goal of one type should preempt the
> other.  And the goal specification, feedback, and result are different
> enough that it might make a single .action file rather awkward.
>
>
>
> I could imagine a single .action file such as:
>
>
>
> float32 driveStraightDistance
>
> float32 rotateDegrees
>
> bool driveStraight  #true to drive straight, false to rotate
>
> ---
>
> float32 metersTraveled
>
> float32 degreesRotated
>
> bool driveStraight  #true if this goal was for a drive straight command
>
> ---
>
> float32 metersTraveled
>
> float32 degreesRotated
>
> bool driveStraight  #true if this goal was for a drive straight command
>
>
>
> Of course there are some redundant fields there.  This isn’t my exact use
> case, but you get the idea…
>
>
>
> Thanks,
>
> -Ben
>
>
>
>
>
>
>
> --
>
> Ben Axelrod
>
> Research Scientist
>
> iRobot Corporation
>
> 8 Crosby Drive, Mail Stop 8-1
>
> Bedford, MA 01730
>
> (781) 430-3315 (Tel)
>
> (781) 960-2628 (Fax)
>
> baxelrod at irobot.com <ofitch at irobot.com>
>
>
>
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