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We define a practical algorithm for distributed rational tree unification and prove its correctness in both the off-line and on-line cases. We derive the distributed algorithm from a centralized one, showing clearly the trade-offs between local and distributed execution. The algorithm is used to realize logic variables in the Mozart Programming System, which implements the Oz language (see

In this State of the art Report (SotA), we will give an introduction to work presented in the area of debugging large software systems with modern hardware architectures. We will discuss techniques used for single- multi- and distributed systems. In addition we will provide pointers to work by large players in the field, and major conferences of importance.

In a match-making system, sources (producers) advertise generated data without any particular destination in mind. Destinations (consumers) are determined based on their interests (via subscriptions) in receiving the produced data. Advertisements and subscriptions are matched by the underlying network routing system. We propose to facilitate match-making capability in ad hoc and sensor networks by adapting the idea of quorum system. A quorum system is formed by organizing nodes into subsets called quorums, where every two quorums intersect and no quorum includes another quorum. To accommodate node mobility and network scale, we propose that producers and consumers systematically forward their advertisement and subscription messages to form `pseudo' quorums, where they are matched at intersecting nodes. Simulation results shows that pseudo quorum based match-making system achieves a very high matching rate with much less messaging overhead as compared to that of event and query flooding.

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Last years have witnessed a dramatic growth in the number as well as in the variety of Distributed Virtual Environment systems. These systems allow multiple users, working on different client computers that are interconnected through different networks, to interact in a shared virtual world. One of the key issues in the design of scalable and cost-effective DVE systems is the partitioning problem. This problem consists of efficiently assigning the existing clients to the servers in the system, and some techniques have been already proposed for solving it. This paper experimentally analyzes the correlation of the quality function proposed in the literature for solving the partitioning problem with the performance of DVE systems. Since the results show an absence of correlation, we also propose the experimental characterization of DVE systems. The results show that the reason for that absence of correlation is the non-linear behavior of DVE systems with regard to the number of clients in the system. DVE systems reach saturation when any of the servers reaches 100 % of CPU utilization. The system performance greatly decreases if this limit is exceeded in any server. Also, as a direct application of these results we present a partitioning method that is targeted to keep all the servers in the system below a certain threshold value of CPU utilization, regardless of the amount of network traffic. Evaluation results show that the proposed partitioning method can improve DVE system performance, regardless of both the movement pattern of clients and the initial distribution of clients in the virtual world.

In numerous practical applications, it is necessary to find the smallest possible tree with a bounded diameter. A diameter-constrained minimum spanning tree (DCMST) of a given undirected, edge-weighted graph, G, is the smallest-weight spanning tree of all spanning trees of G which contain no path with more than k edges, where k is a given positive integer. The problem of finding a DCMST is NP-complete for all values of k; 4 k (n -- 2), except when all edge-weights are identical. A DCMST is essential for the efficiency of various distributed mutual exclusion algorithms, where it can minimize the number of messages communicated among processors per critical section. It is also useful in linear lightwave networks, where it can minimize interference in the network by limiting the traffic in the network lines. Another practical application requiring a DCMST arises in data compression, where some algorithms compress a file utilizing a tree data-structure, and decompress a path in the tree to access a record. A DCMST helps such algorithms to be fast without sacrificing a lot of storage space. We present a survey of the literature on the DCMST problem, study the expected diameter of a random labeled tree, and present five new polynomial-time algorithms for an approximate DCMST. One of our new algorithms constructs an approximate DCMST in a modified greedy fashion, employing a heuristic for selecting an edge to be added to iii the tree in each stage of the construction. Three other new algorithms start with an unconstrained minimum spanning tree, and iteratively refine it into an approximate DCMST. We also present an algorithm designed for the special case when the diameter is required to be no more than 4. Such a diameter-4 tree is also used for evaluating the quality of o...

Planet is a mobile object system designed to be used as a platform for distributed collaborative applications in a network environment that amalgams local networks and world-wide networks such as the Internet. Planet adopts an approach that treats distribution and persistency in a unified way. This paper describes remote memory-mapping architecture suited to unification and a scheme that incorporates an object passing mechanism into the architecture. Furthermore the paper describes an object interaction mechanism that orthogonalizes the object encapsulation concept during programmers and the protection domain concept at runtime.

In a previous paper, Garcia-Molina speci es the leader election problem for syn-chronous and asynchronous distributed systems with crash and link failures and gives an elegant algorithm for each type of system. This paper points out a aw in Garcia-Molina's speci cation of leader election in asynchronous systems and proposes a new speci cation. Index terms: leader election, group membership, asynchronous distributed systems, crash failures In a previous paper, Garcia-Molina speci es the leader election problem for synchronous and asynchronous distributed systems with crash and link failures and gives an elegant algorithm for each type of system � the algorithm for asynchronous systems is called the Invitation Algorithm [3]. The group communication system in Amoeba [5] uses the Invitation Algorithm to recon gure a group after a node crashes. In a recent textbook [1], Chow and Johnson write: \We will examine the classic election algorithms of Garcia-Molina. Several variations of election have been proposed, but the Garcia-Molina algorithm best de nes and handles the possible failures."

In this paper we present a self-stabilizing quorum-based distributed mutual exclusion algorithm. Our algorithm is designed for an asynchronous message-passing model. The algorithm scales well since it has constant synchronization delay and its message complexity is proportional to the square root of the number of processes in the system. The algorithm tolerates message loss. The algorithm places few assumptions on timeouts needed for its implementation. All this allows for a ready implementation of the algorithm on practical distributed architectures.