Snapshot Algorithm Research Articles

Distributed snapshots for ad hoc network systems

Snapshot algorithms are fundamental algorithms in recording consistent global states of a distributed system. They find a large class of important applications such as monitoring and debugging of distributed systems and detection of stable properties (e.g. deadlock, termination and loss of tokens). Unfortunately, most existing snapshot algorithms were designed for static network systems and cellular systems only. They cannot be applied directly in recent emerging ad hoc network systems, which usually have no fixed infrastructure and may experience dynamic topology changes in the mid of execution. This paper presents a snapshot algorithm that can work in an ad hoc network system. Compared to the traditional snapshot algorithms, the proposed algorithm can guarantee both the causal consistency and the topological consistency. Simple methods are also discussed to reduce logged system states and exchanged message timestamps when applying the algorithm.

Time-Based Software Transactional Memory

Software transactional memory (STM) is a concurrency control mechanism that is widely considered to be easier to use by programmers than other mechanisms such as locking. The first generations of STMs have either relied on visible read designs, which simplify conflict detection while pessimistically ensuring a consistent view of shared data to the application, or optimistic invisible read designs that are significantly more efficient but require incremental validation to preserve consistency, at a cost that increases quadratically with the number of objects read in a transaction. Most of the recent designs now use a “time-based” (or “time stamp-based”) approach to still benefit from the performance advantage of invisible reads without incurring the quadratic overhead of incremental validation. In this paper, we give an overview of the time-based STM approach and discuss its benefits and limitations. We formally introduce the first time-based STM algorithm, the Lazy Snapshot Algorithm (LSA). We study its semantics and the impact of its design parameters, notably multiversioning and dynamic snapshot extension. We compare it against other classical designs and we demonstrate that its performance is highly competitive, both for obstruction-free and lock-based STM designs.

Empire of colonies: Self-stabilizing and self-organizing distributed algorithm

Self-stabilization ensures automatic recovery from an arbitrary state; we define self-organization as a property of algorithms which display local attributes. More precisely, we say that an algorithm is self-organizing if (1) it converges in sublinear time and (2) reacts “fast” to topology changes. If s ( n ) is an upper bound on the convergence time and d ( n ) is an upper bound on the convergence time following a topology change, then s ( n ) ∈ o ( n ) and d ( n ) ∈ o ( s ( n ) ) . The self-organization property can then be used for gaining, in sub-linear time, global properties and reaction to changes. We present self-stabilizing and self-organizing algorithms for many distributed algorithms, including distributed snapshot and leader election. We present a new randomized self-stabilizing distributed algorithm for cluster definition in communication graphs of bounded degree processors. These graphs reflect sensor networks deployment. The algorithm converges in O ( log n ) expected number of rounds, handles dynamic changes locally and is, therefore, self-organizing. Applying the clustering algorithm to specific classes of communication graphs, in O ( log n ) levels, using an overlay network abstraction, results in a self-stabilizing and self-organizing distributed algorithm for hierarchy definition. Given the obtained hierarchy definition, we present an algorithm for hierarchical distributed snapshots. The algorithms are based on a new basic snap-stabilizing snapshot algorithm, designed for message passing systems in which a distributed spanning tree is defined and in which processors communicate using bounded links capacity. The algorithm is on-demand self-stabilizing when no such distributed spanning tree is defined. Namely, it stabilizes regardless of the number of snapshot invocations. The combination of the self-stabilizing and self-organizing distributed hierarchy construction and the snapshot algorithm forms an efficient self-stabilizer transformer. Given a distributed algorithm for a specific task, we are able to convert the algorithm into a self-stabilizing algorithm for the same task with an expected convergence time of O ( log 2 n ) rounds.

Visual homing in the absence of feature-based landmark information

Despite that fact that landmarks play a prominent role in human navigation, experimental evidence on how landmarks are selected and defined by human navigators remains elusive. Indeed, the concept of a ‘landmark’ is itself not entirely clear. In everyday language, the term landmark refers to salient, distinguishable, and usually nameable objects, rendering the problem of landmark recognition a special case of the general object recognition problem. In contrast, in the insect and robot literature, this notion of landmarks is often replaced by the “local position information” [e.g., Trullier, O., Wiener, S., Berthoz, A., & Meyer, J.-A. (1997). Biologically based artificial navigation systems: Review and prospects. Progress in Neurobiology, 51, 483–544], referring to the entire set of sensor readings obtained at one location. This set may then serve as a characteristic of the particular location. Honey bees have been shown to base place recognition and homing on a snapshot-like memory of the place’s visual environment, not on the distances to recognized objects [Cartwright, B., & Collett, T. (1983). Landmark learning in bees. Experiments and models. Journal of Comparative Physiology A, 151, 521–543]. A number of theoretical models of snapshot-based homing [e.g., Franz, M., Schölkopf, B., Mallot, H. A., Bülthoff, H. H. (1998). Where did I take that snapshot? Scene-based homing by image matching. Biological Cybernetics, 79, 191–202; Vardy, A., & Möller, R. (2005). Biologically plausible visual homing methods based on optical flow techniques. Connection Science, 17, 47–89] predict that the accuracy of snapshot-based homing should depend on image contrast. For rodent hippocampal place fields, models have been developed using additional image information such as three-dimensional depth and allocentric orientations (e.g., room axes) and are thus less sensitive to image contrast and noise [e.g. Barry, C., Lever, C., Hayman, R., Hartley, T., Burton, S., O’Keefe, J., et al. (2006). The boundary vector cell model of place cell firing and spatial memory. Reviews in the Neurosciences, 17, 71–79]. Here, we study human visual homing in a virtual environment void of objects and readily detected image features. The environment was a circular room with a homogenous colour gradient covering the wall and uniform floor and ceiling. Subjects were able to approach remembered places. Accuracy decreased with colour gradient modulation and room size, in qualitative agreement with the snapshot model but not with other models of place recognition. We conclude that human memory for places can make use of a snapshot algorithm.

Dynamic snapshot algorithm and partial rollback algorithm for internet agents

AbstractThis paper considers an Internet agent system in which a tremendous number of agents operate, frequently appearing and disappearing, and discusses the fault‐tolerant algorithm. Application of the snapshot algorithm to the agent system is considered. The snapshot algorithm is used to view the whole situation (snapshot) of the distributed system. The snapshot algorithm of Chandy and Lamport [2] is considered as a representative snapshot algorithm, in terms of the high efficiency and the simplicity of the procedure. It is not practical, however, to apply their snapshot algorithm to the distributed agent system in which a tremendous number of agents operate. From such a viewpoint, this paper extends the idea of Chandy and Lamport's algorithm and proposes a subsnapshot algorithm, in which the snapshot is taken among the agents who are in the causal relation, through message exchange and agent creation. Then, an efficient rollback algorithm is proposed, which is based on the snapshots taken by the subsnapshot algorithm. In the general rollback algorithm utilizing the snapshot, all agents must roll back. In contrast, in the rollback algorithm proposed in this paper, it suffices that only some agents should roll back. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 88(12): 43–57, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20208

Towards a practical snapshot algorithm

An atomic snapshot memory is an implementation of a multiple-location shared memory that can be atomically read in its entirety without preventing concurrent writing. The design of wait-free implementations of atomic snapshot memories has been the subject of extensive theoretical research in recent years. This paper introduces the coordinated-collect algorithm, a novel wait-free atomic snapshot construction which we believe is a first step in taking snapshots from theory to practice. Unlike previous algorithms, it uses currently available multiprocessor synchronization operations to provide an algorithm that has only O(1) update complexity and O(n) scan complexity, with very small constants. We evaluated the performance of known snapshot algorithms for a collection of benchmarks on a simulated distributed shared-memory multiprocessor. Our empirical evidence suggests that coordinated-collect will outperform all known wait-free, lock-free, and locking snapshot algorithms in terms of overall throughput and latency.

Optimization issues in multimedia systems

Multimedia systems must meet stringent real-time performance criteria in order to satisfy customer requirements. Because of these criteria, and because of the rich structure inherent in video-on-demand (VOD) applications, there is both need and opportunity to employ sophisticated mathematical optimization techniques. In this paper we present an overview of several recent optimization algorithms for multimedia systems, concentrating on the techniques themselves. In particular, we will describe a VOD batching algorithm known as the maximum factored queue length policy, based in part on solving a simple instance of a so-called mathematical resource allocation problem. Next we will describe an optimization problem arising when employing a VOD technique known as adaptive piggybacking. This problem can be solved as a dynamic program, and the scheme which results is known as the snapshot algorithm. Finally we will describe a so-called DASD dancing algorithm for VOD disk load balancing, which depends on the solution to a set of three somewhat more advanced resource allocation problems. © 1998 John Wiley & Sons, Inc.

The partitioned synchronization rule for planar extendible partial orders

The partitioned synchronization rule is a technique for proving the correctness of concurrency control algorithms. Prior work has shown the applicability of the partitioned synchronization rule to hierarchically decomposed databases whose structure is restricted to semitrees. The principal contribution of the paper is a demonstration that the partitioned synchronization rule also applies to more general structures than semitrees, specifically, to any planar extendible partial order, a partial order which when extended with a least and a greatest element still remains planar. To demonstrate utility, the paper presents two applications of the partitioned synchronization rule. The first application shows correctness of a component based timestamp generation algorithm suitable for implementing a timestamp ordering concurrency control algorithm. The second application shows correctness of a snapshot algorithm for concurrency control in a replicated multilevel secure database; we choose this application to highlight that hierarchically decomposed databases and multilevel secure databases are structurally similar. In both cases, the correctness proofs via the partitioned synchronization rule are substantially simpler than corresponding direct proofs. >

Efficient Algorithms for Distributed Snapshots and Global Virtual Time Approximation

This paper presents snapshot algorithms for determining a consistent global state of a distributed system without significantly affecting the underlying computation. These algorithms do not require channels to be FIFO or messages to be acknowledged. Only a small amount of storage is needed. An important application of a snapshot algorithm is Global Virtual Time determination for distributed simulations. The paper proposes new and efficient Global Virtual Time approximation schemes based on snapshot algorithms and distributed termination detection principles.

On distributed snapshots

We develop an efficient snapshot algorithm that needs no control messages and does not require channels to be first-in-first-out. We also show that several stable properties (e.g., termination, deadlock) can be detected with uncoordinated distributed snapshots. For such properties, our algorithm can be further simplified.

Discarding Obsolete Information in a Replicated Database System

A replicated database architecture is described in which updates processed at a site must be saved to allow reconcilliation of newly arriving updates in a way that preserves mutual consistency. The storage space occupied by the saved updates increases indefinitely, and periodic discarding of old updates is needed to avoid running out of storage. A protocol is described which allows sites in the system to agree that updates older than a given timestamp are no longer needed and can be discarded. This protocol uses a distributed snapshot algorithm of Chandy and Lamport and represents a practical application of that algorithm. A protocol for permanent removal of sites is also described, which will allow the discarding of updates to continue when one or more sites crash and are expected not to recover.

Repeated snapshots in distributed systems with synchronous communications and their implementation in CSP

A new version of the Snapshot Algorithm of Chandy and Lamport (1985) is presented. It considers synchronous communications and partially ordered semantics and allows for repeated snapshots. Its implementation in the language CSP is described: it is symmetric, generic and bounded in storage. It yields a symmetric and generic solution to the ‘Distributed Termination Detection’ problem of Francez.

Global and logical time in distributed algorithms

The use of global time can simplify the design and description of distributed algorithms. It is, however, an expensive mechanism to implement. In this paper we show that in some cases global time can be assumed while designing an algorithm, but need not be implemented—in these cases it can be replaced with Lamport's logical time in a routine manner, which clearly preserves the correctness of the algorithm. The technique is illustrated by three examples. Two existing algorithms are described as if they had been designed while assuming global time; they are Chandy and Lamport's distributed snapshot algorithm, and Banatre and Lapalme's fair agreement algorithm for the distributed auction system Enchere. The third example is a new distributed algorithm for restarting from a saved checkpoint state. It was designed while assuming global time, but can be implemented without it.