[Rock-dev] Device/Deployment Matching in Roby

[Chris Mueller]

Hi Roby-Users,

i've currently met another magic behaviour in roby that is probably not 
known for everyone and i think personally not very obvious, especially
for new roby users. The documentation mentioned this feature indeed, but 
i would recommend to add more concret details for it.

I've already mentioned in the past, that it was not clear for myself how 
roby is choosing the correct physical deployment
if there exists more than one deployment providing a specific Orocos 
Task. This issue is documented in:
http://rock-robotics.org/master/documentation/system/subsystem_design.html 
(Topic: Orocos Deployments)

It says this is usually a policy issue and especially for roby devices 
there is clear rule how it works.

"If the task context is a device driver, i.e. needs to access hardware, 
ambiguities are not acceptable (one deployment might have access to the 
required hardware while the other has not). In this case, the deployment 
that has the same name than the device will be used (if there is one), 
or an error will be generated."

If have currently some trouble to use this description for the concret 
implemention.

Here is a short example. Assume you have the following deployments:

simple_deployment("pwm_led", "pwm_gumstix::Task")
simple_deployment("pwm_servo", "pwm_gumstix::Task")

In your config/ directory you have a ruby file that defines all robot 
devices. e.G:

Robot.devices do
   device(Dev::PwmControl, :as => "led").with_conf("led")
   device(Dev::PwmControl, :as => "servo").with_conf("servo")
end

With driver_mapping in models/orogen/pwm_gumstix.rb:

class PwmGumstix::Task
   driver_for "Dev::PwmControl" do
       provides Srv::AnyService
   end
end

The concret problem is now: How can Roby match the correct deployment 
process to the right device and if you try to instanciate the current model
for this example you get an error of the form:
    multiple deployments for Dev::PwmControl:   pwm_led, pwm_servo

The solution is very simple: the ":as" property is not only an internal 
identifier for the roby devices.  The name of this property must also 
match the
name of a existing deployment. This is some explicit mapping mechanism 
that was completely unknown for myself and its very helpful to know,
if your system has several devices using the same TaskContext type.

Here is the simple fix for this problem:
   device(Dev::PwmControl, :as => "pwm_led").with_conf("led")  # for 
deployment pwm_led
   device(Dev::PwmControl, :as => "pwm_servo").with_conf("servo") # for 
deployment pwm_servo

Hope you are not struggling about that like I did.

Chris
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