Robert Tolksdorf

PageSpace: an architecture to coordinate distributed applications on the Web

Abstract Most applications on the Web require active processing and coordination of services and components. Today, activity within the Web is tied to server machines and there is no integrated mechanism that allows it to coordinate activity located at clients, such as applets. In order to allow for really distributed application in the Web, such coordination platforms have to be built. The PageSpace is a platform to support open distributed application on top of the Web. It utilizes Java to execute distributed agents that coordinate their exchange of services by Linda-like coordination technology. The PageSpace architecture comprises a set of agent classes. The user-interfaces is manifested are Alpha agents displayed in Web browsers. The representation of the user on the net is its homeagent, called Beta, which uses services on behalf of the user. Applications are formed by Delta agents that offer and use services. The coordination amongst agent is performed using a shared space of information and Linda-like primitives that operate on it. With the PageSpace architecture, distributed applications on top of the Web and the Internet are enabled, as the platform decentralizes activity. By combining coordination technology with the Web and Java, the centralized, server-bound structure of todays Web-applications is replaced with a truly open distributed system.

Coordination middleware for XML-centric applications

This paper focuses on coordination middleware for distributed applications based on active documents and XML technologies. It introduces the main concepts underlying active documents and XML Then, the paper goes into details about the problem of defining a suitable middleware architecture to effectively support coordination activities in applications including active documents and mobile agents, by specifically focusing on the role played by XML technologies in that context. According to a simple taxonomy, the characteristics of several middleware systems are analyzed and evaluated. This analysis enables us to identify the advantages and the shortcoming of the different approaches, and to identify the basic requirements of a middleware for XML-centric applications.

Coordinating Web-Based Systems with Documents in XMLSpaces

We describe an extension to the Linda model of coordination for Web-based applications. It allows XML documents to be stored in a coordination space from where they can be retrieved based on multiple matching relations amongst XML documents, including those given by XML query-languages. XMLSpaces is distributed and supports several distribution policies in an extensible manner. We describe the partial replication schema implemented in detail.

Models of Coordination

While future software is becoming decomposed in more and more finegrained entities, issues on interactions amongst those entities grows in importance. While methodologies for building such components are well established, the design and support of their interplay can not build on commonly understood and well defined models. In this paper, we review several coordination models from various disciplines, and describe how a coordination reference model could look like. We use a set of characteristics of coordination models to compare the reviewed ones.

Workspaces: a Web-based workflow management system

Workspaces is a Web-based WFMS developed at the Technical University of Berlin that attempts to deal with various shortcomings of Web-based workflow management systems by employing XML. The proposed architecture combines concepts from workflow management and coordination technology and uses XML for representation and the Extensible Stylesheet Language (XSL) for processing. The author examines the system architecture and describes the prototype implementation developed with several diploma students for an experimental evaluation of the ideas.

Coordinating Services in Open Distributed Systems with LAURA

Open distributed systems are computing systems that can be characterized by a heterogeneity of involved machine- and network architectures as well as of the data processed. They have to be able to integrate existing applications, to cope with the use of multiple programming-languages, and potentially high dynamics by joining and leaving components.

Laura—a service-based coordination language

Abstract Open distributed systems are computing systems that can be characterized by a heterogeneity of involved machine- and network architectures as well as of the data processed. They have to be able to integrate existing applications, to cope with the use of multiple programming-languages, and potentially high dynamics by joining and leaving components. The coordination language Laura is designed to facilitate the use and offer of services in such a system. It is based on a shared collection of forms describing offers, requests, and results of services, called the service-space. Lauras operations permit the exchange of forms via the service-space, guided by a matching-rule based on a subtype-notion on service types. The uncoupled coordination paradigm inherited from Linda allows it to meet the requirements of open systems. An architecture for a distributed Laura-system is described, that is scalable and can be adjusted to organizational borders. The architecture has been implemented on top of the ISIS toolkit. With Laura, we apply Lindas coordination paradigm to open systems. Based on an analysis of the issue of names in open systems, we introduce a new approach to typing of interfaces. The architecture we propose includes new extensions to a partial replication scheme that can cope with dynamically changing set of participating machines.

Coordination Technology for Workflows on the Web: Workspaces

The need for coordination technology in Web applications is evident. It has been shown that Linda-like systems are a suited to facilitate the interaction amongst agents and processes over the Internet. Workspaces is the application of Linda-like coordination technology to the domain of Internet-based workflow management systems. The Workspaces architecture is based on workflows as coordinated transformations of documents. A set of basic steps transform XML documents under the control of an XSL engine. Coordination operations affect the order of execution in the workflow. A meta step compiles a workflow graph from the XML-based Workspaces Coordination Language into a set of XSL rules for single transformation steps. The Workspaces architecture uses a Linda-like data space for coordination by XML documents. This XMLSpace contains documents describing the steps in a workflow and application specific documents to be transformed in the course of work. It involves multiple matching relations on XML documents. The combination of standard Internet technology with coordination technology exhibits various benefits of explicit procedure representation, distributed and uncoupled architecture and ease of access.

Coordinating Patterns of Mobile Information Agents

Mobility is a mechanism to manage accessibility dependencies amongst agents. As such, it is a coordination mechanism in the interaction of agents, users, and information.

Programming distributed systems with the delegation-based object-oriented language dSelf

Common middleware platforms rely on a class-based object model. However, this model introduces several dependencies that are disadvantageous for distributed systems. The family of class-less delegation-based languages offers an alternative object model with lesser such dependencies. dSelf is an extension to the delegation-and prototype-based object-oriented language SELF. It adds distributed objects and transparent remote reference resolution to the language. In consequence, dSelf facilitates distributed inheritance and instantiation mechanisms. We describe the conception and implementation of dSelf and give examples where the flexibility of dSelf can be used with benefit as a middleware concept for programming distributed systems.