There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Many practical computing problems concern large graphs. Standard examples include the Web graph and various social networks. The scale of these graphs - in some cases billions of vertices, trillions of edges - poses challenges to their efficient processing. In this paper we present a computational model suitable for this task. Programs are expressed as a sequence of iterations, in each of which a vertex can receive messages sent in the previous iteration, send messages to other vertices, and modify its own state and that of its outgoing edges or mutate graph topology. This vertex-centric approach is flexible enough to express a broad set of algorithms. The model has been designed for efficient, scalable and fault-tolerant implementation on clusters of thousands of commodity computers, and its implied synchronicity makes reasoning about programs easier. Distribution-related details are hidden behind an abstract API. The result is a framework for processing large graphs that is expressive and easy to program.

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Pregel: a system for large-scale graph processing

Recent archaeological discoveries on the island of Paxos reveal that the parliament functioned despite the peripatetic propensity of its part-time legislators. The legislators maintained consistent copies of the parliamentary record, despite their frequent forays from the chamber and the forgetfulness of their messengers. The Paxon parliament's protocol provides a new way of implementing the state machine approach to the design of distributed systems.

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The part-time parliament

This ebook is the first authorized digital version of Kernighan and Ritchie's 1988 classic, The C Programming Language (2nd Ed.). One of the best-selling programming books published in the last fifty years, "K&R" has been called everything from the "bible" to "a landmark in computer science" and it has influenced generations of programmers. Available now for all leading ebook platforms, this concise and beautifully written text is a "must-have" reference for every serious programmers digital library.

As modestly described by the authors in the Preface to the First Edition, this "is not an introductory programming manual; it assumes some familiarity with basic programming concepts like variables, assignment statements, loops, and functions. Nonetheless, a novice programmer should be able to read along and pick up the language, although access to a more knowledgeable colleague will help."

The C programming language

ing WS1S Systems to Verify Parameterized Networks . . . . . . . . . . . . 188 Kai Baukus, Saddek Bensalem, Yassine Lakhnech and Karsten Stahl FMona: A Tool for Expressing Validation Techniques over Infinite State Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 J.-P. Bodeveix and M. Filali Transitive Closures of Regular Relations for Verifying Infinite-State Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Bengt Jonsson and Marcus Nilsson Diagnostic and Test Generation Using Static Analysis to Improve Automatic Test Generation . . . . . . . . . . . . . 235 Marius Bozga, Jean-Claude Fernandez and Lucian Ghirvu Efficient Diagnostic Generation for Boolean Equation Systems . . . . . . . . . . . . 251 Radu Mateescu Efficient Model-Checking Compositional State Space Generation with Partial Order Reductions for Asynchronous Communicating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Jean-Pierre Krimm and Laurent Mounier Checking for CFFD-Preorder with Tester Processes . . . . . . . . . . . . . . . . . . . . . . . 283 Juhana Helovuo and Antti Valmari Fair Bisimulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Thomas A. Henzinger and Sriram K. Rajamani Integrating Low Level Symmetries into Reachability Analysis . . . . . . . . . . . . . 315 Karsten Schmidt Model-Checking Tools Model Checking Support for the ASM High-Level Language . . . . . . . . . . . . . . 331 Giuseppe Del Castillo and Kirsten Winter Table of

Tools and Algorithms for the Construction and Analysis of Systems. Proc. TACAS 2009

This paper presents an algorithm that emulates atomic read/write shared objects in a dynamic network setting. To ensure availability and faulttolerance, the objects are replicated. To ensure atomicity, reads and writes are performed using quorum configurations, each of which consists of a set of members plus sets of read-quorums and write-quorums. The algorithm is reconfigurable: the quorum configurations may change during computation, and such changes do not cause violations of atomicity. Any quorum configuration may be installed at any time. The algorithm tolerates processor stopping failure and message loss. The algorithm performs three major tasks, all concurrently: reading and writing objects, introducing new configurations, and "garbage-collecting" obsolete configurations. The algorithm guarantees atomicity for arbitrary patterns of asynchrony and failure. The algorithm satisfies a variety of conditional performance properties, based on timing and failure assumptions. In the "normal case", the latency of read and write operations is at most 8d, where d is the maximum message delay.

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RAMBO: A Reconfigurable Atomic Memory Service for Dynamic Networks

The paper presents a robust emulation of multi-writer/multireader registers in message-passing systems using dynamic quorum configurations. In addition to processor and link failures, this emulation tolerates changes in quorum configurations, i.e., on-line replacements of one quorum system consisting of read and write quorums with another such system. The new emulation is specified using a modular two-layer architecture. The lower layer uses unreliable broadcast to disseminate a client request to a set of processors, and then to collect responses from a subset of the processors. The higher layer emulates robust multi-writer/multi-reader registers where quorum configurations are used to ensure register atomicity. A unique feature of the read/write service is that it implements dynamically changing quorum configurations. The processor designated as the reconfigured executes requests that replace the current configuration with a new configuration. The combination of the higher and lower layers allows essentially unlimited concurrency and does not involve locks. Waiting can occur only due to processor or link failures that disconnect at least one processor in each read quorum or at least one processor in each write quorum of the specified configurations. Additional computation and communication overhead can be incurred by the read and write operations when they encounter frequent reconfigurations. The algorithms are specified here in terms of I/O automata and their correctness is proved using invariants and partial-order methods.

Robust emulation of shared memory using dynamic quorum-acknowledged broadcasts

Group communication services are becoming accepted as effective building blocks for the construction of fault-tolerant distributed applications. Many specifications for group communication services have been proposed. However, there is still no agreement about what these specifications should say, especially in cases where the services are partitionable, i.e., where communication failures may lead to simultaneous creation of groups with disjoint memberships, such that each group is unware of the existence of any other group. In this paper, we present a new, succinct specification for a view-oriented partitionable group communication service. The service associates each message with a particular view of the group membership. All send and receive events for a message occur within the associated view. The service provides a total order on the messages within each view, and each processor receives a prefix of this order. Our specification separates safety requirements from performance and fault-tolerance requirements. The safety requirements are expressed by an abstract, global state machine. To present the performance and fault-tolerance requirements, we include failure-status input actions in the specification; we then give properties saying that consensus on the view and timely message delivery are guaranteed in an execution provided that the execution stabilizes to a situation in which the failure-status stops changing and corresponds to consistently partioned system. Because consensus is not required in every execution, the specification is not subject to the existing impossibility results for partionable systems. Our specification has a simple implementation, based on the membership algorithm of Christian and Schmuck. We show the utility of the specification by constructing an ordered-broadcast application, using an algorithm (based on algorithms of Amir, Dolev, Keidar, and others) that reconciles information derived from different instantiations of the group. The application manages the view-change activity to build a shared sequence of messages, i.e., the per-view total orders of the group service are combined to give a universal total order. We prove the correctness and analyze the performance and fault-tolerance of the resulting application.

Specifying and using a partitionable group communication service

List of Figures. List of Tables. List of Examples and Remarks. List of Symbols. Foreword Franco P. Preparata. 1. Introduction. 2. Models for Robust Computation. 3. The Write-All Problem: Algorithms. 4. Lower Bounds, Snapshots and Approximation. 5. Fault-Tolerant Simulations. 6. Shared Memory Randomized Algorithms and Distributed Models and Algorithms. Bibliography and References. Author Index. Subject Index.

Fault-Tolerant Parallel Computation

One of the most significant challenges introduced by mobile networks is coping with the unpredictable motion and the unreliable behavior of mobile nodes. In this paper, we define the Virtual Mobile Node Abstraction, which consists of robust virtual nodes that are both predictable and reliable. We present the Mobile Point Emulator, a new algorithm that implements the Virtual Mobile Node Abstraction. This algorithm replicates each virtual node at a constantly changing set of real nodes, modifying the set of replicas as the real nodes move in and out of the path of the virtual node. We show that the Mobile Point Emulator correctly implements a virtual mobile node, and that it is robust as long as the virtual node travels through well-populated areas of the network. The Virtual Mobile Node Abstraction significantly simplifies the design of efficient algorithms for highly dynamic mobile ad hoc networks.

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Alexander A. Shvartsman

Papers

RAMBO: A Reconfigurable Atomic Memory Service for Dynamic Networks

Robust emulation of shared memory using dynamic quorum-acknowledged broadcasts

Specifying and using a partitionable group communication service

Fault-Tolerant Parallel Computation

Virtual Mobile Nodes for Mobile Ad Hoc Networks