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In this paper we present a software toolkit for deploying peer-topeer distributed graphical user interfaces across four dimensions: multiple displays, multiple platforms, multiple operating systems, and multiple users, either independently or concurrently. This toolkit is based on the concept of multi-purpose proxy connected to one or many rendering engines in order to render a graphical user interface in part or whole for any user, any operating system (Linux, Mac OS X and Windows XP or higher), any computing platform (ranging from a pocket PC to a wall screen), and/or any display (ranging from private to public displays). This toolkit is a genuine peer-to-peer solution in that no computing platform is used for a server or for a client: any user interface can be distributed across users, systems, and platforms independently of their location, system constraints, and platform constraints. After defining the toolkit concepts, its implementation is described, motivated, and exemplified on two non-form based user interfaces: a distributed office automation and a distributed interactive game.

Much progress has been made in distributed computing in the areas of distribution structure, open computing, fault tolerance, and security. Yet, writing distributed applications remains difficult because the programmer has to manage models of these areas explicitly. A major challenge is to integrate the four models into a coherent development platform. Such a platform shoUld make it possible to cleanly separate an application's functionality from the other four concerns. Concurrent constraint programming, an evolution of concurrent logic programming, has both the expressiveness and the formal foundation needed to attempt this integration. As a first step, we have designed and built a platform that separates an application's functionality from its distribution structure. We have prototyped several collaborative tools with this platform, including a shared graphic editor whose design is presented in detail. The platform efficiently implements Distributed Oz, which extends the Oz language with constructs to express the distribution structure and with basic primitives for open computing, failure detection and handling, and resource control. Oz appears to the programmer as a concurrent object-oriented language with dataflow synchronization. Oz is based on a higher-order, state-aware, concurrent constraint computation model.

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This paper describes a model-based approach for designing Distributed User Interfaces (DUIs), i.e., graphical user interfaces that are distributed along the following dimensions: end user, display device, computing platform, and physical environment. The three pillars of this model-based approach are: (i) a Concrete User Interface model for DUIs incorporating the distribution dimensions and expressing any DUI element in a XML-compliant format until the granularity of an individual DUI element is reached, (ii) a specification language for DUI distribution primitives that have been defined in a user interface toolkit, and (iii), a step wise method for modeling a DUI based on the concepts of a distribution graph expressing a distribution scenario that can be played, based on the distribution primitives. A distribution graph consists of a state-transition diagram whose states represent significant DUI distribution states and whose transitions are labeled by an Event-Condition-Action (ECA... ABSTRACTThis paper describes a model-based approach for designing distributed user interfaces (DUIs), i.e. graphical user interfaces that are distributed along one or many of the following dimensions: end user, display device, computing platform, and physical environment. The three pillars of this model-based approach are: (i) a Concrete User Interface model for DUIs incorporating the distribution dimensions and able to express in a XML-compliant format any DUI element until the granularity of an individual DUI element is reached, (ii) a specification language for DUI distribution primitives that have been defined in a user interface toolkit, and (iii), a step-wise method for modeling a DUI based on the concepts of distribution graph expressing a distribution scenario that can be played namely based on the distribution primitives. A distribution graph consists of a statetransition diagram whose states represent significant distribution states of a DUI and whose transitions are labeled by an even-condition-action representation. The actions involved in this format may call any distribution primitive of the DUI toolkit. In order to exemplify this model-based approach, two simple DUIs are first designed: a DUI for the Pictionary game and a DUI for the Minesweeper game. They are then incorporated into a larger DUI game of the goose where cells may trigger the two other games.

Some of the most difficult questions to answer when designing a distributed application are related to mobility: what information to transfer between sites and when and how to transfer it. Network-transparent distribution, the property that a program's behavior is independent of how it is partitioned among sites, does not directly address these questions. Therefore we propose to extend all language entities with a network behavior that enables efficient distributed programming by giving the programmer a simple and predictable control over network communication patterns. In particular, we show how to give objects an arbitrary mobility behavior that is independent of the objects definition. In this way, the syntax and semantics of objects are the same regardless of whether they are used as stationary servers, mobile agents, or simply as caches. These ideas have been implemented in Distributed Oz, a concurrent object-oriented language that is state aware and has dataflow synchronization. We prove that the implementation of objects in Distributed Oz is network transparent. To satisfy the predictability condition, the implementation avoids forwarding chains through intermediate sites. The implementation is an extension to the publicly available DFKI Oz 2.0 system.

We present a design for a distributed programming system, Distributed Oz, that abstracts away the network. This means that all network operations are invoked implicitly by the system as an incidental result of using particular language constructs.However, since network operations are expensive, the programmer must retain control over network communication patterns. This control is provided through the language constructs. While retaining their centralized semantics, they me extended with a d~tributed semantics.Distributed Oz is an extension of Oz, a concurrent state-aware language with first-class procedures. Distributed Oz extends Oz with just two concepts: mobility control and asynchronous ordered communicant ion. Mobilit y control provides for truly mobile objects in a simple and clean way. Asynchronous ordered communication aIlows to conveniently build servers.These two concepts give the programmer a control over network communications that is both simple and predictable.We give scenarios to show how common distributed programming tasks can be implemented efficiently. There are two reasons for the simplicity of Distributed Oz. First, Oz has a simple formaf semantics. Second, the distributed extension is built using network protocofs that are naturaf extensions to the centralized language operations. We discuss the operational semantics of Oz and Distributed Oz and the architecture of the distributed implementation.We give an overview of the basic network protocols for communication and mobility.

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