Optimistic Replication Research Articles

Version Vector Weighted Voting protocol: efficient and fault‐tolerant commitment for weakly connected replicas

AbstractMobile and other loosely coupled environments call for decentralized optimistic replication protocols that provide highly available access to shared objects, while ensuring eventual consistency. We propose a protocol based on epidemic weighted voting for achieving such a goal with better availability than traditional primary commit approaches. We improve previous epidemic weighted voting solutions by allowing commitment of multiple, happened‐before related updates at a single distributed election round. We demonstrate that our protocol, in contrast to basic weighted voting solutions, achieves similar update commitment ratios to the primary commit alternative. The improvement over basic weighted voting is especially amplified with weaker replica connectivity, as in mobile and other loosely coupled environments. We support such claims by presenting comparison performance results obtained from side‐by‐side execution of reference protocols in a simulated environment. Copyright © 2007 John Wiley & Sons, Ltd.

P2P replica synchronization with vector sets

This paper describes an enhanced replica synchronization mechanism built in Microsoft's WinFS replica management system. The system reconciles autonomously-operating replicas in a completely peer-to-peer manner, without employing a central master or locking. The main challenge is for two replicas to exchange meta-information efficiently about (potentially numerous) data objects in order to discover what updates they are missing, and detect conflicts. The paper introduces a novel bundling mechanisms called VS, that groups together multiple objects and represents their state in a single version-vector. VS provides improved storage and communication overheads over previously known optimistic replication schemes, in the following sense. Under normal, low-fault situations, it maintains and communicates as little as a single version vector in order to represent precedence ordering of the entire set of data objects. Moreover, under settings of severe communication disruptions, VS degenerates to no worse than a single vector per object. This dramatically improves the complexities described in a preliminary write-up of the WinFS replication scheme. The VS mechanism has potentially wide applicability as a mechanism for compactly handling synchronization of arbitrarily overlapping groups of objects.

Design and analysis of a fault tolerant hybrid mobile scheme

Mobile computing systems provide users with access to information regardless of their geographical location. In these systems, Mobile Support Stations (MSSs) play the role of providing reliable and uninterrupted communication and computing facilities to mobile hosts. The failure of a MSS can cause interruption of services provided by the mobile system. Two basic schemes for tolerating the failure of MSSs exist in the literature. The first scheme is based on the principle of checkpointing used in distributed systems. The second scheme is based on state information replication of mobile hosts in a number of secondary support stations. Depending on the replication scheme used, the second approach is further classified as a pessimistic or an optimistic technique. In this paper, we propose a hybrid scheme which combines the pessimistic and the optimistic replication schemes. In the proposed scheme, an attempt is made to strike a balance between the long delay caused by the pessimistic and the high memory requirements of the optimistic schemes. In order to find the best ratio between the number of pessimistic to the number of optimistic secondary stations in the proposed scheme, we used fuzzy logic. We also used simulation to compare the performance of the proposed scheme with those of the optimistic and the pessimistic schemes. Simulation results showed that the proposed scheme performs better than either schemes in terms of delay and memory requirements.

Introducing permuted states for analyzing conflict rates in optimistic replication

No abstract available.

Optimistic replication

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.Optimistic replication deploys algorithms not seen in traditional “pessimistic” systems. Instead of synchronous replica coordination, an optimistic algorithm propagates changes in the background, discovers conflicts after they happen, and reaches agreement on the final contents incrementally.We explore the solution space for optimistic replication algorithms. This article identifies key challenges facing optimistic replication systems---ordering operations, detecting and resolving conflicts, propagating changes efficiently, and bounding replica divergence---and provides a comprehensive survey of techniques developed for addressing these challenges.

XML 반복부 데이터의 변경 협상 방법

모바일 환경에서 여러 사용자가 XML 트리를 공유하는 응용이 점차 많아지고 있다. XML 트리를 낙관적인 중복 방식으로 모바일 환경에서 공유하려면 동시에 수정된 데이터를 협상하여 하나의 버전을 만들 수 있어야 한다. 특히 XML 트리의 부분 트리를 삽입/삭제한 구조 변경 행위들을 협상하기 위해서는 트리의 노드 매핑과 비교 과정이 필요한데, 이것은 O(n<TEX>$^2$</TEX>)의 복잡도를 가지는 문제로 알려져 있다. 또한 응용에 따라 달라지는 의미 기반의 충돌 해결 방식이 도입되어야 할 필요성이 제기되고 있다. 본 연구에서는 모바일 환경에서 XML 트리의 편집 기록(edit script)을 협상하기 위한 효율적인 협상 방법을 제시하였다 본 논문에서는 이러한 요구를 만족시킬 수 있는 XML 데이터 공유 모델로서 문서타입에서 반복부에 해당하는 부분만 삽입과 삭제를 허용하고 반복부의 부분 트리들이 형제들과 구분되는 키 값을 가지는 리스트 공유 모델을 이용한다. 이러한 리스트 공유 모델은 구조 변경 행위가 항상 문서의 유효성을 보장할 수 있다는 장점을 가지며, 키 기반의 리스트 협상을 하게 되므로 효율적인 협상 알고리즘을 얻는 것이 가능하다. 본 논문에서는 리스트 공유 모델에서 서로 키의 충돌이 없는 편집 기록에 대해 편집기록의 길이를 m이라 할 때 O(m)복잡도를 가지는 협상 알고리즘을 제안하였다. 기존 트리 협상 방법이 키의 충돌이 없는 경우 트리 크기에 선형 비례하는 시간 복잡도를 가지므로 제안된 방법은 모바일 환경에서 효과적인 협상 방법으로 사용될 수 있을 것이다. Sharing XML trees on mobile devices has become more and more popular. Optimistic replication of XML trees for mobile devices raises the need for reconciliation of concurrently modified data. Especially for reconciling the modified tree structures, we have to compare trees by node mapping which takes O(<TEX>$n^2$</TEX>) time. Also, using semantic based conflict resolving policy is often discussed in the literature. In this research, we focused on an efficient reconciliation method for mobile environments, using edit scripts of XML data sent from each device. To get a simple model for mobile devices, we use the XML list data sharing model, which allows inserting/deleting subtrees only for the repetitive parts of the tree, based on the document type. Also, we use keys for repetitive part subtrees, keys are unique between nodes with a same parent. This model not only guarantees that the edit action always results a valid tree but also allows a linear time reconciliation algorithm due to key based list reconciliation. The algorithm proposed in this paper takes linear time to the length of edit scripts, if we can assume that there is no insertion key conflict. Since the previous methods take a linear time to the size of the tree, the proposed method is expected to provide a more efficient reconciliation model in the mobile environment.

Transaction-Centric Reconciliation in Disconnected Client–Server Databases

As mobile computing devices become more and more common, mobile databases are becoming popular. An important feature of these database systems is their ability to allow optimistic replication of data by providing disconnected mobile devices the ability to perform local updates. The key problem to this approach is the reconciliation problem, i.e. the problem of serializing potentially conflicting updates from disconnected clients on all replicas of the database. Reconciliation of conflicting updates is especially critical for disconnected databases where disconnected updates are allowed. We examine some choices for providing solutions to the reconciliation problem. We then present an algorithm that combines multiversion concurrency control schemes on a server with reconciliation of updates from disconnected clients. We also describe a simple but illustrative sample application. Finally we present our conclusions.

Optimistic replication in Pharos, a collaborative application on the web

Pharos is a collaborative application which enables users to share document annotations. Annotations of a same subject are stored altogether in channels, and channels are replicated to improve performance. What characterises data replication in a collaborative application like Pharos is the way users see data. If, for instance, mutual consistency is required, then collaboration should be synchronous. On the other hand, if remote users can work in disconnected mode, collaboration can be asynchronous with weaker constraints on data freshness. In this paper, we focus on asynchronous replication which is typically required on the Web. We propose an optimistic replication model based on lazy group replication and a protocol to detect and resolve potential conflicts to refresh the replicas. This protocol is based on the ordering of write operations at each site using its timestamps values. Careful log management is the key to its implementation. We describe the implementation of our model in the Pharos application.

The Bengal Database Replication System

Mobile computers often suffer from limited connectivity, or even complete lack of network access. Moreover, in wireless networks some machines are often more accessible to mobile computers than others are. Ideally, nomadic users want the same quality and speed of database access as when they are well connected to the network. Replicated databases meet this need by allowing each mobile machine to carry its own copy of key data. But existing replicated systems are not well suited for all nomadic scenarios. Certain important mobile computing situations require optimistic peer-oriented database replication. The Bengal Database Replication System has these characteristics and was designed to operate in difficult nomadic conditions. This paper presents the design assumptions of the system, describes its architecture, presents performance data on its operation, and discusses future enhancements for the system. The paper also contrasts the system to other replicated databases, concentrating on their suitability for nomadic computing. The Bengal technology can play an important role in the development of highly scalable, highly available, fault-tolerant database systems.