In Syskit, data services are used as placeholders for “actual” components. They allow to create models that e.g. refer to “an orientation provider” without actually saying which orientation provider.

Data services represent both an interface (i.e. a set of inputs and outputs) and a functionality (i.e. what happens to the data). This page will describe how to define them, and how to represent that, sometimes, certain data services can used in place of other ones. For instance, that a pose provider can be used when an orientation provider is needed.

The relationship with other component models (task contexts and compositions) is described later on these model’s respective pages: compositions and task contexts.

A data service is defined with

data_service_type "OrientationSrv" do
    output_port "orientation_samples",
        "base/samples/RigidBodyState"
end

This defines a Ruby module whose name is given as a string. It is defined in the local namespace, so one can (and should) organize the data service definitions using modules:

module Base
  data_service_type 'OrientationSrv' do
    ...
  end
end

which is then referred to as Base::OrientationSrv. The syntax for the input and output ports is the same than in oroGen files.

Naming it is good practice to suffix the service names with “Srv”, so that they can be recognized from compositions and task contexts.

Filesystem Data services are defined in models/blueprints/. It is customary to group them in topic groups (e.g. in the rock bundle, models/blueprints/pose.rb regroups all pose-related services). Whenever a data service is used in a syskit file, the file that defines it should be required first. Types must be loaded using import_types_from “project”, where “project” is the oroGen project that defines the type. Use syskit browse to find out what is defined where.

Model Hierarchy

The fact that a data service Y can be used instead of another one X (for instance, a Pose data service that also provides an Orientation), is reflected by telling syskit that “Y provides X”

data_service_type "PoseSrv" do
  output_port "pose_samples", "base/samples/RigidBodyState"
  provides OrientationSrv
end

The resulting data service will have both the ports of Y (here, pose_samples) and the ports of X (orientation_samples) This is clearly not what is needed here: whenever a Pose service is used in place of an Orientation service, one would want the pose_samples port to replace orientation_samples. This information must be given explicitly through port mappings:

data_service_type "PoseSrv" do
    output_port "pose_samples", "base/samples/RigidBodyState"
    provides OrientationSrv, 'orientation_samples' => 'pose_samples'
end

In this definition, the name mapping on the “provides” stanza specifies that the “orientation_samples” port of the provided service (OrientationSrv) should be replaced by the “pose_samples” port of the new service.