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Chapter 1

Urbi SDK is a fully-featured environment to orchestrate complex organizations of components. It is an open source framework for complex systems. It relies on a middleware architecture that coordinates components named UObjects. It also features urbiscript, a scripting language tailored to write orchestration programs.

 1.1 Urbi and UObjects
 1.2 The Big Picture
 1.3 Urbi and urbiscript
 1.4 Genesis
 1.5 Outline
 1.6 Documentation

1.1 Urbi and UObjects

Urbi makes the orchestration of independent and concurrent components easier. It was first designed for robotics: it provides all the needed features to coordinate the execution of various components (actuators, sensors, software devices that provide features such as text-to-speech, face recognition and so forth). Languages such as C++ are well suited to program the local, low-level, handling of these hardware or software devices; indeed one needs efficiency, small memory footprint, and access to low-level hardware details. Yet, when it comes to component orchestration and coordination, in a word, when it comes to addressing concurrency, it can be tedious to use such languages.

Middleware infrastructures make possible to use remote components as if they were local, to allow concurrent execution, to make synchronous or asynchronous requests and so forth. The UObject C++ architecture provides exactly this: a common API that allows conforming components to be used seamlessly in highly concurrent settings. Components need not be designed with UObjects in mind, rather, UObjects are typically “shells” around “regular” components.

Components with an UObject interface are naturally supported by the urbiscript programming language. This provides a tremendous help: one can interact with these components (making queries, changing them, observing their state, monitoring various kinds of events and so forth), which provides a huge speed-up during development.

Although made with robots in mind, the UObject architecture is well suited to tame any heavily concurrent environment, such as video games or complex systems in general.

1.2 The Big Picture

The Section 1.1 shows the architecture of Urbi. Let’s browse it bottom up.


Figure 1.1: A Bird-View of the Urbi Architecture

At the lowest level, Urbi requires a (possibly very limited) embedded computer. This is the case for most robots today, but on occasion, some device cannot even run reasonably small pieces of code. In that case, Urbi can still be used, but then the robot is actually remote-controlled from a computer running Urbi.

Right on top of the hardware, is running the Operating System. Urbi supports the major OSes; it was also ported on top of real-time OSes such as Xenomai, and on specific OSes such as Aperios, Sony’s proprietary system running its Aibo robotic dog.

The Urbi Runtime, which is the actual core of the system, also known as the engine or the kernel, is interfacing the OS with the rest of the Urbi world, urbiscript and UObjects.

UObjects are used to bind hardware or software components, such as actuators and sensors on the one hand, and voice synthesis or face recognition on the other hand. They can be run locally on the robot, or on a remote, more powerful, computer.

To orchestrate all the components, urbiscript is a programming language of choice (see below).

1.3 Urbi and urbiscript

urbiscript is a programming language primarily designed to handle concurrent programming. It’s a dynamic, prototype-based, object-oriented scripting language. It supports and emphasizes parallel and event-based programming, which are very popular paradigms in robotics, by providing core primitives and language constructs.

Its main features are:

1.4 Genesis

Urbi what first designed and implemented by Jean-Christophe Baillie, together with Matthieu Nottale. Because its users wildly acclaimed it, Jean-Christophe founded Gostai, a France-based Company that develops software for robotics with a strong emphasis on personal robotics. Gostai has been acquired by Aldebaran Robotics in 2012. Urbi is now a community-driven open source project available at

Authors Urbi SDK 1 was further developed by Akim Demaille, Guillaume Deslandes, Quentin Hocquet, and Benoît Sigoure.

The Urbi SDK 2 project was started and developed by Akim Demaille, Quentin Hocquet, Matthieu Nottale, and Benoît Sigoure. Samuel Tardieu provided an immense help during the year 2008, in particular for the concurrency and event support.

The maintenance in Gostai was carried out by Akim Demaille, Quentin Hocquet, and Matthieu Nottale. Jean-Christophe Baillie is still deeply involved in the development of urbiscript, he regularly submits ideas, and occasionally even code!

Contributors Many people contributed significantly to Urbi, including Alexandre Morgand, Romain Bezut, Thomas Moulard, Clément Moussu, Nicolas Pierron.

1.5 Outline

This multi-part document provides a complete guide to Urbi. See Listing 28 for the various notations that are used in the document.

Listing IV Urbi and UObjects User Manual
This part covers the Urbi architecture: its core components (client/server architecture), how its middleware works, how to include extensions as UObjects (C++ components) and so forth.

No knowledge of the urbiscript language is needed. As a matter of fact, Urbi can be used as a standalone middleware architecture to orchestrate the execution of existing components.

Yet urbiscript is a feature that “comes for free”: it is easy using it to experiment, prototype, and even program fully-featured applications that orchestrate native components. The interested reader should read either the urbiscript user manual (Listing I), or the reference manual (Section 20).

  Listing 24 Quick Start
This chapter, self-contained, shows the potential of Urbi used as a middleware.
  Section 25 The UObject API
This section shows the various steps of writing an Urbi C++ component using the UObject API.
  Listing 26 The UObject Java API
UObjects can also be written in Java. This section demonstrates it all.
  Section 27 Use Cases
Interfacing a servomotor device as an example of how to use the UObject architecture as a middleware.
Listing I urbiscript User Manual  
This part, also known as the “urbiscript tutorial”, teaches the reader how to program in urbiscript. It goes from the basis to concurrent and event-based programming. No specific knowledge is expected. There is no need for a C++ compiler, as UObject will not be covered here (see Listing IV). The reference manual contains a terse and complete definition of the Urbi environment (Section III).
  Listing 3 First Steps
First contacts with urbiscript.
  Section 4 Basic Objects, Value Model
A quick introduction to objects and values.
  Section 5 Flow Control Constructs
Basic control flow: if, for and the like.
  Listing 6 Advanced Functions and Scoping
Details about functions, scopes, and lexical closures.
  Listing 7 Objective Programming, urbiscript Object Model
A more in-depth introduction to object-oriented programming in urbiscript.
  Listing 9 Functional Programming
Functions are first-class citizens.
  Listing 10 Parallelism, Concurrent Flow Control
The urbiscript operators for concurrency, tags.
  Listing 11 Event-based Programming
Support for event-driven concurrency in urbiscript.
  Listing 12 Urbi for ROS Users
How to use ROS from Urbi, and vice-versa.
paragraph II Guidelines and Cook Books  
This part contains guides to some specific aspects of Urbi SDK.
  paragraph 13 Installation
Complete instructions on how to install Urbi SDK binary packages.
  paragraph 14 Frequently Asked Questions
Some answers to common questions.
  Section 15 Urbi Guideline
Based on our own experience, and code that users have submitted to us, we suggest a programming guideline for Urbi SDK.
  Listing 16 Migration from urbiscript 1 to urbiscript 2
This chapter is intended to people who want to migrate programs in urbiscript 1 to urbiscript 2.
  Listing 18 Building Urbi SDK
Building Urbi SDK from the sources. How to install it, how to check it and so forth. This chapter is meant only for people who want to build Urbi SDK, which is orders of magnitude more delicate than simply installing the binary packages from Gostai.
Section III Urbi SDK Reference Manual  
This part defines the specifications of the urbiscript language. It defines the expected behavior from the urbiscript interpreter, the standard library, and the SDK. It can be used to check whether some code is valid, or browse urbiscript or C++ API for a desired feature. Random reading can also provide you with advanced knowledge or subtleties about some urbiscript aspects.

This part is not an urbiscript tutorial; it is not structured in a progressive manner and is too detailed. Think of it as a dictionary: one does not learn a foreign language by reading a dictionary. For an urbiscript Tutorial, see Listing I.

This part does not aim at giving advanced programming techniques. Its only goal is to define the language and its libraries.

  Section 19 Programs
Presentation and usage of the different tools available with the Urbi framework related to urbiscript, such as the Urbi server, the command line client, umake, …
  Section 20 urbiscript Language Reference Manual
Core constructs of the language and their semantics.
  Listing 21 urbiscript Standard Library
The classes and methods provided in the standard library.
  Section 22 Communication with ROS
Urbi provides a set of tools to communicate with ROS (Robot Operating System). For more information about ROS, see
  Section 23 Urbi Standard Robotics API
Also known as “The Urbi Naming Standard”: naming conventions in for standard hardware/software devices and components implemented as UObject and the corresponding slots/events to access them.
Listing V Tables and Indexes  
This part contains material about the document itself.
  Listing 28 Notations
Conventions used in the type-setting of this document.
  Listing 29 Grammar
Grammar of the urbiscript language.
  Section 31 Licenses
Licenses of components used in Urbi SDK.
  Listing 30 Release Notes
Also known as the Urbi ChangeLog, or Urbi NEWS, this chapter lists the user-visible changes in Urbi SDK releases.
  Section 32 ł::bel sec:bibliographyBibliography
References to other documents such as documentation, scientific papers, etc.
  Section 33 Glossary
Definition of the terms used in this document.
  Section 34 List of Tables
Index of all the tables: list of keywords, operators, etc.
  Section 35 List of Figures
Index of all the figures: snapshots, schema, etc.

1.6 Documentation