Towards Synchronizing Linear Collaborative Objects with Operation Transformation

A collaborative object represents a data type (such as a text document or a filesystem) designed to be shared by multiple geographi- cally separated users. Data replication is a technology to improve perfor- mance and availability of data in distributed systems. Indeed, each user has a local copy of the shared objects, upon which he may perform up- dates. Locally executed updates are then transmitted to the other users. This replication potentially leads, however, to divergent (i.e. different) copies. In this respect, Operational Transformation (OT) algorithms are applied for achieving convergence of all copies, i.e. all users view the same objects. Using these algorithms users can apply the same set of updates but possibly in different orders since the convergence should be ensured in all cases. However, achieving convergence with the OT approach is still a critical and challenging issue. In this paper, we address an open convergence problem when the shared data has a linear structure such as list, text, ordered XML tree, etc. We analyze the source of this problem and we propose a generic solution with its formal correctness

Research report : Collaborative Peer 2 Peer Edition: Avoiding Conflicts is Better than Solving Conflicts

Collaborative edition is achieved by distinct sites that work independently on (a copy of) a shared document. Conflicts may arise during this process and must be solved by the collaborative editor. In pure Peer to Peer collaborative editing, no centralization nor locks nor time-stamps are used which make conflict resolution difficult. We propose an algorithm which relies on the notion or semantics dependence and avoids the need of any integration transformation to solve conflicts. Furthermore, it doesn't use any history file recording operations performed since starting the edition process. We show how to define editing operations for semi-structured documents i.e. XML-like trees, that are enriched with informations derived for free from the editing process. Then we define the semantics dependence relation required by the algorithm and we present preliminary results obtained by a prototype implementation.Comment: 12 page

Verifying Strong Eventual Consistency in Distributed Systems

Data replication is used in distributed systems to maintain up-to-date copies of shared data across multiple computers in a network. However, despite decades of research, algorithms for achieving consistency in replicated systems are still poorly understood. Indeed, many published algorithms have later been shown to be incorrect, even some that were accompanied by supposed mechanised proofs of correctness. In this work, we focus on the correctness of Conflict-free Replicated Data Types (CRDTs), a class of algorithm that provides strong eventual consistency guarantees for replicated data. We develop a modular and reusable framework in the Isabelle/HOL interactive proof assistant for verifying the correctness of CRDT algorithms. We avoid correctness issues that have dogged previous mechanised proofs in this area by including a network model in our formalisation, and proving that our theorems hold in all possible network behaviours. Our axiomatic network model is a standard abstraction that accurately reflects the behaviour of real-world computer networks. Moreover, we identify an abstract convergence theorem, a property of order relations, which provides a formal definition of strong eventual consistency. We then obtain the first machine-checked correctness theorems for three concrete CRDTs: the Replicated Growable Array, the Observed-Remove Set, and an Increment-Decrement Counter. We find that our framework is highly reusable, developing proofs of correctness for the latter two CRDTs in a few hours and with relatively little CRDT-specific code

Mobile Databases: a Selection of Open Issues and Research Directions

International audienceThis paper reports on the main results of a specific action on mobile databases conducted by CNRS in France from October 2001 to December 2002. The objective of this action was to review the state of progress in mobile databases and identify major research directions for the French database community. Rather than provide a survey of all important issues in mobile databases, this paper gives an outline of the directions in which the action participants are now engaged, namely: copy synchronization in disconnected computing, mobile transactions, database embedded in ultra-light devices, data confidentiality, P2P dissemination models and middleware adaptability

Symbolic Model-Checking of Optimistic Replication Algorithms

The original publication is available at www.springerlink.comInternational audienceThe Operational Transformation (OT) approach, used in many collaborative editors, allows a group of users to concurrently update replicas of a shared object and exchange their updates in any order. The basic idea of this approach is to transform any received update operation before its execution on a replica of the object. This transformation aims to ensure the convergence of the different replicas of the object. However, designing transformation algorithms for achieving convergence is a critical and challenging issue. In this paper, we address the verification of OT algorithms with a symbolic model-checking technique. We show how to use the difference bound matrices to explore symbolically infinite state-spaces of such systems and provide symbolic counterexamples for the convergence property

Operation transformation based concurrency control in group editors

Collaborative editing systems (or group editors) allow a geographically dispersed group of human users to view and modify shared multimedia documents, such as research papers, design diagrams, web pages and source code together over a computer network. In addition to being useful tools, group editors are a classic research vehicle and model of interactive groupware applications, based on which a variety of social and technical issues have been investigated. Consistency maintenance as a fundamental problem in group editors has attracted constant research attention. Operational transformation (OT) is an optimistic consistency maintenance method that supports unconstrained collaboration among human users. Although significant progress has been achieved over the past decade, there is still a large space for improvement on the theoretical part of OT. In this dissertation, we are concerned with three problems: (1) How to evaluate the correctness of OT-based consistency maintenance protocols; (2) How to design and prove correct OT-based protocols; (3) What are the consistency correctness conditions for group editing systems in general. This dissertation addresses the above three problems and makes the following contributions: (1) propose a total order based framework including a new consistency model and the associated design methodology. This framework reduces the complexities of the OT design; (2) improve the total order based framework by introducing a natural order based framework. In contrast, this framework removes the requirement of defining a total order that is not necessary to the OT design; (3) establish a generic consistency model and propose the first set of practical design guidelines in OT based on this model

VeriFx: Correct Replicated Data Types for the Masses

Distributed systems adopt weak consistency to ensure high availability and low latency, but state convergence is hard to guarantee due to conflicts. Experts carefully design replicated data types (RDTs) that resemble sequential data types and embed conflict resolution mechanisms that ensure convergence. Designing RDTs is challenging as their correctness depends on subtleties such as the ordering of concurrent operations. Currently, researchers manually verify RDTs, either by paper proofs or using proof assistants. Unfortunately, paper proofs are subject to reasoning flaws and mechanized proofs verify a formalization instead of a real-world implementation. Furthermore, writing mechanized proofs is reserved for verification experts and is extremely time-consuming. To simplify the design, implementation, and verification of RDTs, we propose VeriFx, a specialized programming language for RDTs with automated proof capabilities. VeriFx lets programmers implement RDTs atop functional collections and express correctness properties that are verified automatically. Verified RDTs can be transpiled to mainstream languages (currently Scala and JavaScript). VeriFx provides libraries for implementing and verifying Conflict-free Replicated Data Types (CRDTs) and Operational Transformation (OT) functions. These libraries implement the general execution model of those approaches and define their correctness properties. We use the libraries to implement and verify an extensive portfolio of 51 CRDTs, 16 of which are used in industrial databases, and reproduce a study on the correctness of OT functions