Decentralised commitment for optimistic semantic replication (original) (raw)
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How to design optimistic operations for peer-to-peer replication
Proceedings of the 9th Joint Conference on Information Sciences (JCIS), 2006
As collaboration over the Internet becomes an everyday affair, it is increasingly important to provide high quality of interactivity. Distributed applications can replicate collaborative objects at every site for the purpose of achieving high interactivity. Replication, however, has a fatal weakness that it is difficult to maintain consistency among replicas. This paper introduces operation commutativity as a key principle in designing operations in order to manage distributed replicas consistent. In addition, we suggest effective schemes that make operations commutative using the relations of objects and operations. Finally, we apply our approaches to some simple replicated abstract data types, and achieve their consistency without serialization and locking.
An Efficient and Fault-Tolerant Update Commitment Protocol for Weakly Connected Replicas
Lecture Notes in Computer Science, 2005
Mobile and other loosely-coupled environments call for decentralized optimistic replication protocols that provide highly available access to shared objects, while ensuring eventual convergence towards a strongly consistent state. In this paper we propose a novel epidemic weighted voting protocol for achieving such goal. Epidemic weighted voting approaches eliminate the single point of failure limitation of primary commit approaches. Our protocol introduces a significant improvement over other epidemic weighted voting solutions by allowing multiple, happened-before related updates to be committed at a single distributed election round. We demonstrate that our protocol is especially advantageous with the weak connectivity levels that characterize mobile and other loosely-coupled networks. We support such claims by presenting comparison results obtained from side-by-side execution of reference protocols in a simulated environment.
Agreeing to agree: Conflict resolution for optimistically replicated data
2006
Current techniques for reconciling disconnected changes to optimistically replicated data often use version vectors or related mechanisms to track causal histories. This allows the system to tell whether the value at one replica dominates another or whether the two replicas are in conflict. However, current algorithms do not provide entirely satisfactory ways of repairing conflicts. The usual approach is to introduce fresh events into the causal history, even in situations where the causally independent values at the two replicas are actually equal. In some scenarios these events may later conflict with each other or with further updates, slowing or even preventing convergence of the whole system. To address this issue, we enrich the set of possible actions at a replica to include a notion of explicit conflict resolution between existing events, where the user at a replica declares that one set of events dominates another, or that a set of events are equivalent. We precisely specify the behavior of this refined replication framework from a user's point of view and show that, if communication is assumed to be "reciprocal" (with pairs of replicas exchanging information about their current states), then this specification can be implemented by an algorithm with the property that the information stored at any replica and the sizes of the messages sent between replicas are bounded by a polynomial function of the number of replicas in the system.
Semi-passive replication and Lazy Consensus
2004
This paper presents two main contributions: semi-passive replication and Lazy Consensus. The former is a replication technique with parsimonious processing. It is based on the latter; a variant of Consensus allowing the lazy evaluation of proposed values. Semi-passive replication is a replication technique with parsimonious processing. This means that, in the normal case, each request is processed by only one single process. The most significant aspect of semi-passive replication is that it requires a weaker system model than existing techniques of the same family. For semi-passive replication, we give an algorithm based on the Lazy Consensus. Lazy Consensus is a variant of the Consensus problem that allows the lazy evaluation of proposed values, hence the name. The main difference with Consensus is the introduction of an additional property of laziness. This property requires that proposed values are computed only when they are actually needed. We present an algorithm based on Chandra and Toueg's Consensus algorithm for asynchronous distributed systems with a ♦S failure detector.
Deco: A Decentralized, Cooperative Atomic Commit Protocol
Journal of Computer Networks and Communications, 2012
An atomic commit protocol can cause long-term locking of databases if the coordinator crashes or becomes disconnected from the network. In this paper we describe how to eliminate the coordinator. This decentralized, cooperative atomic commit protocol piggybacks transaction statuses of all transaction participants onto tokens which are passed among the participants. Each participant uses the information in the tokens to make a decision of when to go to the next state of a three-phase commit protocol. Transactions can progress to ensure a uniform agreement on success or failure, even if the network is partitioned or nodes temporarily crash.
2005
Abstract Data replication is a key technology in distributed systems that enables higher availability and performance. This article surveys optimistic replication algorithms. They allow replica contents to diverge in the short term to support concurrent work practices and tolerate failures in low-quality communication links. The importance of such techniques is increasing as collaboration through wide-area and mobile networks becomes popular.
On Model-Checking Optimistic Replication Algorithms
2009
Collaborative editors consist of a group of users editing a shared document. The Operational Transformation (OT) approach is used for supporting optimistic replication in these editors. It allows the users to concurrently update the shared data and exchange their updates in any order since the convergence of all replicas, i.e. the fact that all users view the same data, is ensured in all cases. However, designing algorithms for achieving convergence with the OT approach is a critical and challenging issue. In this paper, we address the verification of OT algorithms with a model-checking technique. We formally define, using tool UPPAAL, the behavior and the convergence requirement of the collaborative editors, as well as the abstract behavior of the environment where these systems are supposed to operate. So, we show how to exploit some features of such systems and the tool UPPAAL to attenuate the severe state explosion problem. We have been able to show that if the number of users exceeds 2 then the convergence property is not satisfied for five OT algorithms. A counterexample is provided for every algorithm.
Decentralized managing of replication objects in massively distributed systems
2008
Data replication is a central technique to increase availability and performance of distributed systems. While offering many advantages it also requires more effort for ensuring data consistency in case of updates. In the research literature various approaches for replication management in distributed databases have been presented, but they are mostly limited either in scalability or in the consistency guarantees they provide. On the other hand, P2P systems usually provide replication support but ignore the update problem.
2018
The problem of ensuring consistency in applications that manage replicated data is one of the main challenges of distributed computing. Among the several invariants that may be enforced, ensuring that updates are applied and made visible respecting causality has emerged as a key ingredient among the many consistency criteria and client session guarantees that have been proposed and implemented in the last decade. Techniques to keep track of causal dependencies, and to subsequently ensure that messages are delivered in causal order, have been widely studied. It is today well known that, in order to accurately capture causality one may need to keep a large amounts of metadata, for instance, one vector clock for each data object. This metadata needs to be updated and piggybacked on update messages, such that updates that are received from remote datacenters can be applied locally without violating causality. This metadata can be compressed; ultimately, it is possible to preserve causal...
Asynchronous active replication in three-tier distributed systems
2002 Pacific Rim International Symposium on Dependable Computing, 2002. Proceedings., 2002
The deployment of server replicas of a given service across an asynchronous distributed system (e.g. Internet) is a real practical challenge. This target cannot be indeed achieved by classical software replication techniques (e.g. passive and active replication) as these techniques usually rely on group communication toolkits that require server replicas to run over a partially synchronous distributed system. This paper proposes a threetier architecture for software replication that encapsulates the need of partial synchrony in a specific software component of a mid-tier to free replicas (end-tier) and clients (client-tier) from the need of underlying partial synchrony assumptions. Then we propose how to specialize the mid-tier in order to manage active replication of server replicas.