the Robot Construction Kit

Dynamic Services

This section hits one of the functionalities that Syskit is the only one to offer in robotic model-based systems: the ability to deal with dynamic interfaces.

It is sometimes convenient or event necessary to add the ability, in components, to create ports at runtime. For instance, certain device drivers would know how many devices they are attached to only when they access the hardware. Or, motion task merging paradigms such as itasc or whole body control would create ports as required by the number of motion tasks they should handle.

In Syskit, this aspect is handled in two parts: - on the modelling side, by declaring that a given task context (not a composition) has the ability to create new data services. - at runtime, by auto-configuring the underlying RTT task context according to the data services that have been created.

Modelling API

The dynamic part of a task context interface is represented by declaring that the task context can create new data services. This is done through by calling dynamic_service on the task context model:

class Examples::TaskWithDynamicInterface
  dynamic_service AServiceSrv, :as => 'dispatch' do
    # The 'name' local variable contains the name provided
    # when instanciating this dynamic service
    provides AnotherServiceSrv, :as => name

The service model given to the dynamic_service call declares what kind of service can be created. The :as option gives a dynamic service name, i.e. a way to refer to this particular dynamic service when you will want to create one.

Finally, the block is what needs to be performed to instanciate the given service. It must call #provides with the expected service model, or one of its submodels. Unlike “normal” #provides calls, ports that this #provides call expects will be added to the interface. Syskit will check that a corresponding dynamic_input_port / dynamic_output_port stanza exists on the oroGen model.

New services can then be created with

model = Examples::TaskWithDynamicInterface.specialize
model.require_dynamic_service('dispatch', :as => 'service_name')

The name given to require_dynamic_service is accessible in the dynamic_service block as the ‘name’ local variable. It is customary to use this name in the corresponding #provides, either plain, or prefixed/suffixed as needed.

The object returned by #with_dynamic_service is a proper task context model. As such, it can be used everywhere a task context model is used, as for instance subsystem definitions. It can e.g. be selected in place of the base task context:

AComposition.use(Examples::TaskWithDynamicInterface => model)

If you plan to have a more permanent use for this model, assign it to a Ruby constant:

model = Examples::TaskWithDynamicInterface.specialize
model.require_dynamic_service('dispatch', :as => 'service_name')
ConfiguredTask = model

Since dynamic services are a bit of a ‘raw’ API, it is common to create an API on top of it that represents the semantic of the created services. For instance, assuming that the services we are creating in our examples are used for monitoring, one could do

class Examples::TaskWithDynamicInterface
  def self.with_monitor(name)
    model = specialize
    model.require_dynamic_service('dispath', :as => 'name')

which then allows to use this interface with

ConfiguredTask = Examples::TaskWithDynamicInterface.

Dynamic Services and RTT Task Contexts

What we have seen so far is how to model dynamic services. However, since there is no common protocol on the oroGen task context side (and I don’t believe there could be one), you will need to ensure that the task context gets configured according to the required dynamic services.

This is done by adding code to the task’s #configure method

class Examples::TaskWithDynamicInterface
  def configure
    super # this applies the necessary
          # configuration files. We will
          # override using the dyn srv
          # definitions
    each_data_service do |srv|
      # There are currently no way to be sure that 'srv' is dynamic.
      # Use the names to check it out
      # Configuration needs to be stored in task arguments. Note that
      # you can extend the task with new arguments by calling
      # component_model.argument in the block given to dynamic_service

      # Do what is necessary on #orocos_task
      # srv is an instance of Syskit::BoundDataService
      # srv.model is an instance of Syskit::Models::BoundDataService
      if srv.fullfills?(Examples::InputMonitorSrv)
      elsif srv.fullfills?(Examples::OutputMonitorSrv)

Complete Example

In this example, we will model a task context that can have a set of inputs and merges them based on a given priority. The data type is base/samples/Joints

# Define the services related to our I/O
import_types_from 'base'
module Base
  data_service_type 'JointsProviderSrv' do
    output_port 'out', 'base/samples/Joints'
  data_service_type 'JointsConsumerSrv' do
    input_port 'in', 'base/samples/Joints'

module Merger
  class Task
    # Declare the merged inputs as a dynamic data serivce
    # The corresponding orogen description must have
    # a dynamic port declaration that matches, e.g.
    # dynamic_input_port /^in_\w+$/, 'base/samples/Joints'
    # If the name does not have a fixed pattern, 'nil' can
    # be used
    # dynamic_input_port nil, 'base/samples/Joints'
    dynamic_service Base::JointsConsumerSrv, :as => 'merged_inputs' do
      # Create an argument on the final task to give the priority for this
      # service. It will be retrieved later in the configure method using the
      # service name, i.e.
      #   priority = arguments["#{}_priority"]
      component_model.argument "#{name}_priority",
        :default => options[:priority]
      # This service's port will be mapped to a
      # dynamically created in_#{name} port
      # Note that it is possible to also map to static
      # ports !
      provides Base::JointsConsumerSrv, 'in' => "in_#{name}"

    # This method is overloaded so that we can autoconfigure the task
    # based on the dynamic services that got instanciated. Part of the
    # configuration is stored in task arguments, such as the _priority arguments
    # that are added in the dynamic_service block above
    def configure
      # On this task, the list of inputs is given
      # as a property, so generate the property value
      # and save it
      merging_conf = []
      each_data_service do |srv|
        if srv.fullfills?(Base::JointsConsumerSrv)
          merging_conf << [, arguments["#{}_priority"]]
      orocos_task.merging = merging_conf

  # Define ourselves a composition that is going to
  # be the resulting merged joints
  class MergedJoints < Syskit::Composition
    # The merging task
    add Merger::Task, :as => 'merger'
    # The output of the merge allows us to make
    # the composition a joint provider itself
    export merger_child.merged_joints_port
    provides Base::JointsProviderSrv, :as => 'joints'

  # Create a composition that merges the given providers
  # @param [{name => [Component, Integer]}] a name to subsystem
  #   definition mapping that describes the joints providers
  # @return a submodel of MergedJoints
  def self.merge(providers =
    # Allocate the services
    task = Merger::Task.specialize
    # Define one dynamic service per required provider. The
    # providers are given as a mapping of a name to a
    # provider service (the data service that will
    # give us the data) and a priority.
    providers.each do |name, (provider, priority)|
      task.require_dynamic_service 'merged_inputs',
        :as => name,
        :priority => (priority || 0)
    # Create a submodel of the composition, that will
    # use our specialized task context
    MergedJoints.new_submodel do
      merger = overload 'merger', task
      # Add the children and create the connections
      providers.each do |name, (provider, priority)|
        child = add provider, :as => name
        # We should be able to refer to the data
        # service here, but it is currently buggy
        # Name the port directly
        child.connect_to merger.find_input_port("in_#{name}")

You can test the modelling part of the example above by copy/pasting it to a file called for instance example.rb and modifying it in the following way:

  1. modify the “class Task” first lines to match

    class Task < Syskit::TaskContext
      orogen_model.output_port 'merged_joints',
      orogen_model.dynamic_input_port /in_\w+/,
  2. add the following lines at the end

        'first' => [Base::JointsProviderSrv, 0],
        'second' => [Base::JointsProviderSrv, 1]))

Then, run

syskit instanciate ./example.rb

to see the result