We present an algorithm for distributed mutual exclusion in a computer network of N nodes that communicate by messages rather than shared memory. The algorithm uses a spanning tree of the computer network, and the number of messages exchanged per critical section depends on the topology of this tree. However, typically the number of messages exchanged is O (log N ) under light demand, and reduces to approximately four messages under saturated demand. Each node holds information only about its immediate neighbors in the spanning tree rather than information about all nodes, and failed nodes can recover necessary information from their neighbors. The algorithm does not require sequence numbers as it operates correctly despite message overtaking.

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Abstract. In this paper we explore the issue of transforming models to models, an essential part of the OMG's Model Driven Architecture (MDA) vision. Drawing from the literature and our experiences implementing a number of transformations using different technologies, we explore the strengths and weaknesses of the different technologies and identify requirements for a transformation language for performing the kind of model-to-model transformations required to realise the MDA vision.

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While developments in distributed object computing environments, such as the Common Object Request Broker Architecture (CORBA) [17] and the Telecommunication Intelligent Network Architecture (TINA) [16], have enabled interoperability between domains in large open distributed systems, managing the resources within such systems has become an increasingly complex task. This challenge has been considered for several years within the distributed systems management research community and policy-based management has recently emerged as a promising solution.Large evolving enterprises present a significant challenge for policy-based management partly due to the requirement to support both mutual transparency and individual autonomy between domains [2], but also because the fluidity and complexity of interactions occurring within such environments requires an ability to cope with the coexistence of multiple, potentially inconsistent policies. This paper discusses the need of providing both dynamic (run-time) and static (compile-time) conflict detection and resolution for policies in such systems and builds on our earlier conflict detection work [7,8] to introduce the methods for conflict resolution in large open distributed systems.

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The Reference Model of Open Distributed Processing (RM-ODP) was a joint effort by the international standards bodies ISO and ITU-T to develop a coordinating framework for the standardisation of open distributed processing (ODP). The model describes an architecture within which support of distribution, interworking, interoperability and portability can be integrated. The RM-ODP framework defines ODP concerns using five "viewpoints" (abstractions), namely enterprise, information, computational, engineering, and technology. This tutorial introduces the reference model, describing the viewpoints and some of the ODP functions and transparencies.Keyword Codes: C.2.4 Keywords: Computer-Communication Networks, Distributed Systems WHAT IS RM-ODP?Advances in computer networking have allowed computer systems across the world to be interconnected. Despite this, heterogeneity in interaction models prevents interworking between systems. Open distributed processing (ODP) describes systems that support heterogeneous distributed processing both within and between organisations through the use of a common interaction model. ISO and ITU-T (formerly CCITT) have developed a Reference Model of Open DistributedProcessing (RM-ODP) to provide a coordinating framework for the standardisation of ODP by creating an architecture which supports distribution, interworking, interoperability and portability. The Goals and Deliverables of RM-ODPRM-ODP aims to achieve:• portability of applications across heterogeneous platforms • interworking between ODP systems, i.e. meaningful exchange of information and convenient use of functionality throughout the distributed system • distribution transparency, i.e. hide the consequences of distribution from both the applications programmer and user

While advances in open distributed systems have undoubtedly provided a uniquely diverse environment for users, managing the resources within such an environment has become an increasingly complex task. This challenge has been considered for several years within the distributed systems management research community and we have recently seen policy-based management emerge as one such promising exemplification.The focus of our work has been predominantly on supporting the requirements of large evolving enterprises. Such environments present a significant challenge for policybased management as the fluidity and complexity of interactions occurring in such environments mean that prevailing static-based specification and analysis of policies and roles, would be inadequate in many instances. We are therefore interested in providing support for a dynamic policy-based management environment. This paper discusses the critical nature of providing both dynamic and static conflict detection and resolution and introduces a scalable computationally-efficient dynamic conflict detection mechanism.