Total order

Most practical work on AI planning systems during the last fifteen years has been based on hierarchical task network (HTN) d ecomposition, but until now, there has been very little analytical work on the properties of HTN planners. This paper describes how the complexity of HTN planning varies with various conditions on the task networks.

... group communication, fault-tolerance, agreement problems, message passing, total ordering, global ordering, atomic multicast, atomic broadcast, classification, taxonomy ... Part of this research was conducted for the program “Fostering Talent in Emergent Research Fields ...

We consider a sorting (classification) problem in the presence of multiple attributes and criteria, called the MA&C sorting problem. It consists in assignment of some actions to some pre-defined and preference-ordered decision classes. The actions are described by a finite set of attributes and criteria. Both attributes and criteria take values from their domains; however, the domains of attributes are not preference-ordered, while the domains of criteria (scales) are totally ordered by preference relations. Among the attributes we distinguish between qualitative attributes and quantitative attributes. In order to construct a comprehensive preference model that could be used to support the sorting task, we consider preferential information of the decision maker DM) in the form of assignment examples, i.e. exemplary assignments of some reference actions to the decision classes. The preference model inferred from these examples is a set of ''if . . . , then . . . '' decision rules. The rules are derived from rough approximations of decision classes made up of reference actions. They satisfy conditions of completeness and dominance, and manage with possible ambiguity (inconsistencies) in the set of examples. Our idea of rough approximations involves three relations together: indiscernibility, similarity and dominance defined on qualitative and quantitative attributes, and on criteria, respectively. The usefulness of this approach is illustrated by an example. Ó 2002 Published by Elsevier Science B.V.

Fault-tolerant distributed systems are becoming more important but, in existing systems, maintaining the consistency of replicated data is quite expensive. The Totem single-ring protocol supports consistent concurrent operations by placing a total order on broadcast messages. This total order is derived from the sequence number in a token that circulates around a logical ring imposed on a set of processors in a broadcast domain. The protocol handles recon guration of the system when processors fail and restart or the network partitions and remerges. Extended virtual synchrony ensures that processors deliver messages and con guration changes to the application in a consistent total order system-wide. An e ective ow control mechanism enables the Totem single-ring protocol to achieve message ordering rates signi cantly higher than the best prior total ordering protocols.

The Stable Marriage Problem and its many variants have been widely studied in the literature (Gusÿeld and Irving, The Stable Marriage Problem: Structure and Algorithms, MIT Press, Cambridge, MA, 1989; Roth and Sotomayor, Two-sided matching: a study in game-theoretic modeling and analysis, because of the inherent appeal of the problem, partly because of the elegance of the associated structures and algorithms, and partly because of important practical applications, such as the National Resident Matching Program (Roth, J. Political Economy 92(6) (1984) 991) and similar large-scale matching schemes. Here, we present the ÿrst comprehensive study of variants of the problem in which the preference lists of the participants are not necessarily complete and not necessarily totally ordered. We show that, under surprisingly restrictive assumptions, a number of these variants are hard, and hard to approximate. The key observation is that, in contrast to the case where preference lists are complete or strictly ordered (or both), a given problem instance may admit stable matchings of di erent sizes. In this setting, examples of problems that are hard are: ÿnding a stable matching of maximum or minimum size, determining whether a given pair is stable-even if the indi erence takes the form of ties on one side only, the ties are at the tails of lists, there is at most one tie per list, and each tie is of length 2; and ÿnding, or approximating, both an 'egalitarian' and a 'minimum regret' stable matching. However, we give a 2-approximation algorithm for the problems of ÿnding a stable matching of maximum or minimum size. We also discuss the signiÿcant implications of our results for practical matching schemes.

In this paper we describe and compare two frameworks for constraint solving where classical CSPs, fuzzy CSPs, weighted CSPs, partial constraint satisfaction, and others can be easily cast. One is based on a semiring, and the other one on a totally ordered commutative monoid. While comparing the two approaches, we show how to pass from one to the other one, and we discuss when this is possible. The two frameworks have been independently introduced in ijcai95,jacm and schiex-ijcai95.

We view the incidence relation of a graph G=(I: E) as an order relation on ~ts vertices and edges, i.e. a <6 b if and only ifa is a vertex and b is an edge incident on a. This leads to the definition of the order-dimension of G as the minimum number of total orders on I'~ E whose intersection is <G. Our main result ~s the characterization of planar graphs as the graphs whose order-dimension does not exceed three. Strong versions of several known properties of planar graphs are ~mphed by this characterizahon These properhes include: each planar graph has arboricn) at most three and each planar graph has a plane embedding ~hose edges are straight line segments. A nice feature of this embedding ~s that the coordinates of the vertices have a purely combmatoriaI meaning.

A general purpose group communication protocol suite called Newtop is described. It is assumed that processes can simultaneously belong to many groups, group size could be large, and processes could be communicating over the Internet. Asynchronous communication environment is therefore assumed where message transmission times cannot be accurately estimated, and the underlying network may well get partitioned, preventing functioning processes from communicating with each other. Newtop can provide causality preserving total order delivery to members of a group, ensuring that total order delivery is preserved for multi-group processes. Both symmetric and asymmetric order protocols are supported, permitting a process to use say symmetric version in one group and asymmetric version in other.

The ability to quickly deploy new protocols, and provide flexibility to both applications and network administrators has been an oft-cited reason for active networks to be seriously examined. This paper describes an architecture and implementation that provides dynamic installation of network services and flexibility to active code. An example of weighted fair queuing being installed dynamically into a running network is examined.

We examine the computational complexity of testing and nding small plans in probabilistic planning domains with both at and propositional representations. The complexity of plan evaluation and existence varies with the plan type sought; we examine totally ordered plans, acyclic plans, and looping plans, and partially ordered plans under three natural de nitions of plan value. We show that problems of interest are complete for a variety of complexity classes: PL, P, NP, co-NP, PP, NP PP , co-NP PP , and PSPACE. In the process of proving that certain planning problems are complete for NP PP , we introduce a new basic NP PP -complete problem, E-Majsat, which generalizes the standard Boolean satis ability problem to computations involving probabilistic quantities; our results suggest that the development of good heuristics for E-Majsat could be important for the creation of e cient algorithms for a wide variety of problems.

This paper presents a simple, flexible, and efficient algorithm for constructing a possibly non-convex, simple polygon that characterizes the shape of a set of input points in the plane, termed a characteristic shape. The algorithm is based on the Delaunay triangulation of the points. The shape produced by the algorithm is controlled by a single normalized parameter, which can be used to generate a finite, totally ordered family of related characteristic shapes, varying between the convex hull at one extreme and a uniquely defined shape with minimum area. An optimal O(n log n) algorithm for computing the shapes is presented. Characteristic shapes possess a number of desirable properties, and the paper includes an empirical investigation of the shapes produced by the algorithm. This investigation provides experimental evidence that with appropriate parameterization the algorithm is able to accurately characterize the shape of a wide range of different point distributions and densities. The experiments detail the effects of changing parameter values and provide an indication of some "good" parameter values to use in certain circumstances.

Orca is a portable, object-based distributed shared memory system. This paper studies and evaluates the design choices made in the Orca system and compares Orca with other DSMs. The paper gives a quantitative analysis of Orca's coherence protocol (based on write-updates with function shipping), the totally-ordered group communication protocol, the strategy for object placement, and the all-software, user-space architecture. Performance measurements for ten parallel applications illustrate the tradeoffs made in the design of Orca, and also show that essentially the right design decisions have been made. A write-update protocol with function shipping is effective for Orca, especially since it is used in combination with techniques that avoid replicating objects that have a low read/write ratio. The overhead of totally-ordered group communication on application performance is low. The Orca system is able to make near-optimal decisions for object placement and replication.

Many protocols exist to support the maintenance of consistency of data in fault-tolerant distributed systems; these protocols are quite expensive and thus have not been widely adopted. The Totem protocol supports consistent concurrent operations by placing a total order on broadcast messages. This total order is achieved by including a sequence number in a token circulated around a logical ring that is imposed on a set of processors in a broadcast domain. A membership algorithm handles recon guration, including restarting of a failed processor and remerging of a partitioned network. E ective ow-control allows the protocol to achieve message ordering rates two to three times higher than the best prior protocols.

As citation practices strongly depend on fields, field normalisation is recognised as necessary for fair comparison of figures in bibliometrics and evaluation studies. However fields may be defined at various levels, from small research areas to broad academic disciplines, and thus normalisation values are expected to vary. The aim of this project was to test the stability of citation ratings of articles as the level of observation - hence the basis of normalisation - changes. A conventional classification of science based on ISI subject categories and their aggregates at various scales was used, namely at five levels: all science, large academic discipline, sub-discipline, speciality and journal. Among various normalisation methods, we selected a simple ranking method (quantiles), based on the citation score of the article in each particular aggregate (journal, speciality, etc.) it belonged to at each level. The study was conducted on articles in the full SCI range, for publication year 1998 with a four-year citation window. Stability is measured in three ways: overall comparison of article rankings; individual trajectory of articles; survival of the top-cited class across levels. Overall rank correlations on the observed empirical structure are benchmarked against two fictitious sets that keep the same embedded structure of articles but reassign citation scores either in a totally ordered or in a totally random distribution. These sets act respectively as a &apos;worst case&apos; and &apos;best case&apos; for the stability of citation ratings. The results show that: (a) the average citation rankings of articles substantially change with the level of observation (b) observation at the journal level is very particular, and the results differ greatly in all test circumstances from all the other levels of observation (c) the lack of cross-scale stability is confirmed when looking at the distribution of individual trajectories of articles across the levels; (d) when considering the top-cited fractions, a standard measure of excellence, it is found that the contents of the &apos;top-cited&apos; set is completely dependent on the level of observation. The instability of impact measures should not be interpreted in terms of lack of robustness but rather as the co-existence of various perspectives each having their own form of legitimacy. A follow-up study will focus on the micro levels of observation and will be based on a structure built around bibliometric groupings rather than conventional groupings based on ISI subject categories.

The Low Latency Fault Tolerance (LLFT) middleware provides fault tolerance for distributed applications deployed within a cloud computing or data center environment, using the leader/follower replication approach. The LLFT middleware consists of a Low Latency Messaging Protocol, a Leader-Determined Membership Protocol, and a Virtual Determinizer Framework. The Messaging Protocol provides a reliable, totally ordered message delivery service by employing a direct group-to-group multicast where the ordering is determined by the primary replica in the group. The Membership Protocol provides a fast reconfiguration and recovery service when a replica becomes faulty and when a replica joins or leaves a group. The Virtual Determinizer Framework captures ordering information at the primary replica and enforces the same ordering at the backup replicas for major sources of nondeterminism. The LLFT middleware maintains strong replica consistency, offers application transparency, and achieves low end-to-end latency.

The Horus system supports a communication architecture that treats protocols as instances of an abstract data type. This approach encourages developers to partition complex protocols into simple microprotocols, each of which is implemented by a protocol layer. Protocol layers can be stacked on top of each other in a variety of ways, at run-time. First, we describe the classes of protocols that can be supported this way. Next, we present the Horus object model that we designed for this technology, and the interface between the layers that makes it all work. We then present an example layer that implements a group membership protocol. Next, we show how, given a set of required properties, an appropriate stack can be constructed. We look at an example stack of protocols, which provides fault-tolerant, totally ordered communication between a group of processes. The work contributes a standard framework for protocol development and experimentation, provides a high performance implementation of the virtual synchrony model, and introduces a methodology for increasing the robustness of the protocol development process.

-based publish-subscribe system. The model is used to derive an efficient and scalable protocol for exactly-once delivery to large numbers (tens of thousands per broker) of content-based subscribers in either publisher order or uniform total order. Our protocol allows intermediate content filtering at each hop, but requires persistent storage only at the publishing site. It is tolerant of message drops, message reorderings, node failures, and link failures, and maintains only "soft" state at intermediate nodes. We evaluate the performance of our implementation both under failure-free conditions and with fault injection. 2. A protocol based on this knowledge model that tolerates broker crashes and dropped and re-ordered messages, that does not require hop-by-hop reliability of messages, and that requires stable storage only at the publishing broker and only soft-state [10] everywhere else.

In this paper, we present a performance comparison of database replication techniques based on total order broadcast. While the performance of total order broadcast-based replication techniques has been studied in previous papers, this paper presents many new contributions. First, it compares with each other techniques that were presented and evaluated separately, usually by comparing them to a classical replication scheme like distributed locking. Second, the evaluation is done using a finer network model than previous studies. Third, the paper compares techniques that offer the same consistency criterion (one-copy serializability) in the same environment using the same settings. The paper shows that, while networking performance has little influence in a LAN setting, the cost of synchronizing replicas is quite high. Because of this, total order broadcast-based techniques are very promising as they minimize synchronization between replicas.

This paper presents a new methodology for solving multiobjective integer linear programs (MOILP) using tools from algebraic geometry. We introduce the concept of partial Gröbner basis for a family of multiobjective programs where the right-hand side varies. This new structure extends the notion of Gröbner basis for the single objective case to the case of multiple objectives, i.e., when there is a partial ordering instead of a total ordering over the feasible vectors. The main property of these bases is that the partial reduction of the integer elements in the kernel of the constraint matrix by the different blocks of the basis is zero. This property allows us to prove that this new construction is a test family for a family of multiobjective programs. An algorithm “á la Buchberger” is developed to compute partial Gröbner bases, and two different approaches are derived, using this methodology, for computing the entire set of Pareto-optimal solutions of any MOILP problem. Some examples illustrate the application of the algorithm, and computational experiments are reported on several families of problems.

Forward-chammg rule systems must test each newly asserted fact agamst a collection of predlcates to find those rules that match the fact Expert system rule engines use a simple combmatlon of hashmg and sequential search for this matching We introduce an algorithm for findmg the matching predicates that IS more efficient than the standard algorithm when the number of predlcates 1s large We focus on equahty and mequahty predicates on totally ordered domams This algorithm 1s well-suited for database rule systems, where predicate-testing speed 1s crltlcal A key component of the algorithm 1s the rnterval bznary search tree (IBS-tree)

The extension of mathematical morphology operators to multi-valued functions, and in particular to colour images, is neither direct nor general. In this paper, a generalisation of distance-based and lexicographical-based approaches is proposed, allowing the extension of morphological operators to colour images for any colour representation (e.g., RGB, LSH and L * a * b * ) and for any metric distance to a reference colour. The performance of the introduced operators is illustrated by means of different applications: colour feature extraction using openings (closings) by reconstruction, colour gradients for segmenting, colour denoising by the centre operator and colour enhancement by the contrast mapping. Examples from natural colour images and biomedical microscopic colour images are given.

In this paper we describe two frameworks for constraint solving where classical CSPs, fuzzy CSPs, weighted CSPs, partial constraint satisfaction, and others can be easily cast. One is based on a semiring, and the other one on a totally ordered commutative monoid. We then compare the two approaches and we discuss the relationship between them. The two frameworks have been independently introduced in 2] and 28].

In this paper, we explore data replication protocols that provide both fault tolerance and good performance without compromising consistency. We do this by combining transactional concurrency control with group communication primitives. In our approach, transactions are executed at only one site so that not all nodes incur in the overhead of producing results. To further reduce latency, we use an optimistic multicast technique that overlaps transaction execution with total order message delivery. The protocols we present in the paper provide correct executions while minimizing overhead and providing higher scalability.

The all nearest smaller values problem is defined as follows. Let A = (a 1 , a 2 ,..., a n ) be n elements drawn from a totally ordered domain. For each a i , 1 ≤ i ≤ n, find the two nearest elements in A that are smaller than a i (if such exist): the left nearest smaller element a j (with j &lt; i) and ...

A dictionary data structure supports e cient search, insert, and delete operations on n keys from a totally ordered universe. Red-black trees, 2-3 trees, AVL trees, skip lists and other classic data structures facilitate O(log n) time search, insert and deletes, matching the information theoretic lower bound when access probabilities are uniform i.i.d. If access probabilities are non-uniform but still i.i.d., there are other weighted data structures such as D-trees, biased search trees, splay trees and treaps which can achieve optimality.

This paper presents in detail an efficient and provably correct algorithm for database replication over partitionable networks. Our algorithm avoids the need for end-to-end acknowledgments for each action while supporting network partitions and merges and allowing dynamic instantiation of new replicas. One round of end-to-end acknowledgments is required only upon a membership change event such as a network partition. New actions may be introduced to the system at any point, not only while in a primary component. We show how performance can be further improved for applications that allow relaxation of consistency requirements. We provide experimental results that demonstrate the superiority of this approach.

Many distributed services are hosted at large, shared, geographically diverse data centers, and they use replication to achieve high availability despite the unreachability of an entire data center. Recent events show that non-crash faults occur in these services and may lead to long outages. While Byzantine-Fault Tolerance (BFT) could be used to withstand these faults, current BFT protocols can become unavailable if a small fraction of their replicas are unreachable. This is because existing BFT protocols favor strong safety guarantees (consistency) over liveness (availability).

The authors present the construction of a multicast service, called agreed multicast, that guarantees that messages arrive reliably and in the same total-order to all their destinations. ToTo, a novel protocol, implements the agreed multicast service of Transis, a communication subsystem for the High Availability project, currently developed at the Hebrew University of Jerusalem. This service is desired in distributed systems, and supports high level coordination among groups of processes in distributed applications. The ToTo protocol is genuinely symmetric and fairly simple for implementing. It provides early delivery latency, and requires as little as n/2 messages for forming agreement on the order of delivery. Experimental results show up to O(log (n)) speedup over previous protocols, which matches the authors' prediction of the expected speedup. Using the Transis membership service, ToTo can operate in a dynamic environment, and continue to form an agreed total order among the connected machines despite failures and recoveries.

For many years, the intuitions underlying partial-order planning were largely taken for granted. Only in the past few years has there been renewed interest in the fundamental principles underlying this paradigm. In this paper, we present a rigorous comparative analysis of partial-order and total-order planning by focusing on two speci c planners that can be directly compared. We show that there are some subtle assumptions that underly the wide-spread intuitions regarding the supposed e ciency of partial-order planning. For instance, the superiority of partial-order planning can depend critically upon the search strategy and the structure of the search space. Understanding the underlying assumptions is crucial for constructing e cient planners.

This paper examines algorithms for detecting when a property @ holds during the execution of a distributed system. The properties we consider axe expressed over the state of the system and are not assumed to have properties that facilitate detection, such as stability. ...

Large databases with uncertain information are becoming more common in many applications including data integration, location tracking, and Web search. In these applications, ranking records with uncertain attributes needs to handle new problems that are fundamentally different from conventional ranking. Specifically, uncertainty in records' scores induces a partial order over records, as opposed to the total order that is assumed in the conventional ranking settings.

By "fusion" this work means integration of disparate types of data including (intervals of) real numbers as 2 well as possibility/probability distributions defined over the totally-ordered lattice (R, ≤) of real numbers. Such 3 data may stem from different sources including (multiple/multimodal) electronic sensors and/or human judgement. 4

A general algebraic method for decoding all types of binary cyclic codes is presented. It is shown that such a method can correct t=[(d-1)/2] errors, where d is the true minimum distance of the given cyclic code. The key idea behind this decoding technique is a systematic application of the algorithmic procedures of Grobner bases to obtain the error-locator polynomial L(z). The discussion begins from a set of syndrome polynomials F and the ideal T(F) generated by F. It is proved here that the process of transforming F to the normalized reduced Grobner basis of I(F) with respect to the “purely lexicographical” ordering automatically converges to L(z). Furthermore, it is shown that L(z) can be derived from any normalized Grobner basis of I(F) with respect to any admissible total ordering. To illustrate this new approach, the procedures for decoding certain BCH codes and quadratic residue codes are demonstrated

In this paper we compare the utility of different commitment strategies in planning. Under a "least commitment strategy", plans are represented as partial orders and operators are ordered only when interactions are detected. We investigate claims of the inherent advantages of planning with partial orders, as compared to planning with total orders. By focusing our analysis on the issue of operator ordering commitment, we are able to carry out a rigorous comparative analysis of two planners. We show that partial-order planning can be more efficient than total-order planning, but we also show that this is not necessarily so.

Totally ordered multicast protocols have proved to be extremely useful in supporting fault-tolerant distributed applications. This paper compares the performance of the two main classes of protocols providing total order in large-scale systems: token-site and symmetric protocols. The paper shows that both classes of protocols can exhibit a latency close to 2D, where D is the message transit delay between two processes.

This paper presents the results of morphotectonic and morphometric research carried out in order to determine the neotectonic development of the volcanic mountains and a drainage network in SW Cappadocia. The study area extends among the Aksaray, Niğde, and Nevşehir Provinces. The study area comprises Hasandağ, Melendiz, Keçiboyduran, Göllüdağ Mountains and the adjacent parts of these volcanic mountains. Data collected exclusively from 1:25,000 digitised topographic maps and 10 m-resolution DEMs were used to define parameters related to the longitudinal profile of streams. The study area was divided into 10 volcanic units. Longitudinal profiles of 20 streams and stream orders were analysed to determine a regional tectonic differentiation pattern in these units. The streams in the study area drain into four different tectonic depressions. These depressions are Aksaray plain controlled by the Tuz Gölü fault (TGF), Çiftlik plain controlled by the Keçiboyduran-Melendiz fault (KMF), Misli plain controlled by the Derinkuyu fault (DF), and Bor plain controlled by the Niğde Fault Zone (NFZ). An analysis of morphometric parameters shows that the development of a drainage network is associated with faults and rock resistance. Occurrence of morphometric parameters with different values in units reveals that the volcanic mountains were not uplifted in the same period and were subjected to different morphologic processes. High total order number in the south of Hasandağ (unit 3) and Melendiz Mountains (unit 7) indicate that the uplift ratio of the southern part is much greater than that of the northern part. Moreover, development of the drainage network in the south is in a more advanced phase than in the north. Indeed, the drainage network in the north is in the youngest erosional phase of all parts of the study area. The increased stream length-gradient indices (SL), and stream gradients and an analysis of headward erosion show that the streams displaying the longest and highest reach of the erosional phase are all in the southern part of Keçiboyduran and Melendiz Mountains. The longitudinal profile (Lp) of the present thalweg of the streams is irregular. The irregular Lp are associated with four different causes. These are geological variations in resistance, tectonics, and volcanic topography and downcutting in response to stream incision. The beginning of the fluvial incision in the northern part is younger than in the south.

Priority-Based Total and Semi-Total Ordering Broadcast Protocols Akihito Nakamura and Makoto Takizawa Dept. of Information and Systems Engineering Tokyo Denki University Ishizaka, Hatoyama, Hiki, Saitama 350-03, Japan E-mail {naka, t aki} Q t akilab.k.dendai.ac. jp ...

The Totem multiple-ring protocol provides reliable totally ordered delivery of messages across multiple local-area networks interconnected by gateways. This consistent message order is maintained in the presence of network partitioning and remerging, and of processor failure and recovery. The protocol provides accurate topology change information as part of the global total order of messages. It addresses the issue of scalability and achieves a latency that increases logarithmically with system size by exploiting process group locality and selective forwarding of messages through the gateways. Pseudocode for the protocol and an evaluation of its performance are given.

Middleware platforms are becoming very popular among system developers. Due to its popularity, there is an increasing demand for reliable middleware support. In the past few years several research efforts have concentrated in augmenting the reliability of middleware infrastructures that have led to the definition of the FT-Corba standard.

A class of finite discrete dynamical systems, called Sequential Dynamical Systems (SDSs), was introduced in [BR99] as a formal model for analyzing simulation systems. Here, we address the complexity of two basic problems and their generalizations for SDSs.

The Totem system supports fault-tolerant applications in which distributed processes cooperate to perform a commontask and in which replicated data must be updated consistently in the presence of asynchrony and faults. Reliable totally ordered delivery of messages to processes within process groups is provided on a single local-area network or over multiple local-area networks interconnected by gateways. Message ordering is consistent across the entire network, despite processor and communication faults, without requiring all processes to deliver all messages. The Totem system handles processor failure and recovery, as well as network partitioning and remerging, and provides membership and topology maintenance services.

Total order multicast greatly simplifies the implementation of fault-tolerant services using the replicated state machine approach. The additional latency of total ordering can be masked by taking advantage of spontaneous ordering observed in LANs: A tentative delivery allows the application to proceed in parallel with the ordering protocol. The effectiveness of the technique rests on the optimistic assumption that a large share of correctly ordered tentative deliveries offsets the cost of undoing the effect of mistakes. This paper proposes a simple technique which enables the usage of optimistic delivery also in WANs with much larger transmission delays where the optimistic assumption does not normally hold. Our proposal exploits local clocks and the stability of network delays to reduce the mistakes in the ordering of tentative deliveries. An experimental evaluation of a modified sequencer-based protocol is presented, illustrating the usefulness of the approach in fault-tolerant database management.