P2P Logging and Timestamping for Reconciliation

In this report, we address data reconciliation in peer-to-peer (P2P) collaborative applications. We propose P2P-LTR (Logging and Timestamping for Reconciliation) which provides P2P logging and timestamping services for P2P reconciliation over a distributed hash table (DHT). While updating at collaborating peers, updates are timestamped and stored in a highly available P2P log. During reconciliation, these updates are retrieved in total order to enforce eventual consistency. In this report, we first give an overview of P2P-LTR with its model and its main procedures. We then present our prototype used to validate P2P-LTR. To demonstrate P2P-LTR, we propose several scenarios that test our solutions and measure performance. In particular, we demonstrate how P2P-LTR handles the dynamic behavior of peers with respect to the DHT

P2P Logging and Timestamping for Reconciliation

ISBN : 978-1-60558-306-8/08/08Dans VLDB 2008International audienceIn this paper, we address data reconciliation in peer-to-peer (P2P) collaborative applications. We propose P2P-LTR (Logging and Timestamping for Reconciliation) which provides P2P logging and timestamping services for P2P reconciliation over a distributed hash table (DHT). While updating at collaborating peers, updates are timestamped and stored in a highly available P2P log. During reconciliation, these updates are retrieved in total order to enforce eventual consistency. In this paper, we first give an overview of P2P-LTR with its model and its main procedures. We then present our prototype used to validate P2P-LTR. To demonstrate P2P-LTR, we propose several scenarios that test our solutions and measure performance. In particular, we demonstrate how P2P-LTR handles the dynamic behavior of peers with respect to the DHT

Um modelo para compartilhamento consistente de dados em ambientes de computação móvel

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Ciência da Computação.Em ambientes de computação móveis, assistentes pessoais digitais podem gravar dados localmente para possibilitar o trabalho do dispositivo móvel mesmo quando ele não está na área de abrangência de uma rede sem fio. Estes mesmos dados gravados localmente podem ser compartilhados entre dois ou mais usuários, sendo necessários mecanismos para prover consistência neste compartilhamento de dados. Assim, é importante ter sempre à mão os dados mais recentes. Para resolver este problema, foi desenvolvido um mecanismo para atualização de dados em dispositivos móveis a partir de um servidor estacionário, usando um protocolo de timestamp. Tal mecanismo foi testado em um ambiente experimental, que possui um servidor em uma rede estruturada e dois clientes móveis buscando e enviando dados entre o cliente móvel e o servidor (quando este estiver na área de abrangência da rede sem fio estruturada) e entre os clientes móveis (quando não há cobertura pela rede sem fio estruturada) usando uma rede ad-hoc. Nesta dissertação, o modelo de compartilhamento de dados entre cliente e servidor, e entre os próprios clientes é descrito em detalhes, bem como o ambiente experimental e o desenvolvimento do protótipo

Causality tracking in dynamic distributed systems

Tese de Doutoramento - Área do Conhecimento Tecnologias da ProgramaçãoA causalidade desempenha um papel central no tratamento de problemas importantes de sistemas distribuídos, tais como na replicação de dados, na análise de execuções, na comunicação em grupo e na determinação de estados globais. Por forma a ser útil, a causalidade precisa de ser concretizada em mecanismos que procedam ao seu registo. Os mecanismos existentes, tais como os vectores versão e os relógios vectoriais, assumem a existência de um mapeamento entre identificadores globalmente únicos e contadores inteiros. Num sistema em que é conhecido o número de entidades, é possível pré-configurar estes identificadores por forma a ocuparem posições distintas num vector ou serem-lhe atribuídos nomes distintos. A gestão destas identidades é bem mais problemática em ambientes dinâmicos, com grande número de entidades e onde estas são permanentemente criadas e destruídas. Esta situação é agravada na presença de partições de rede. As soluções actuais para o registo de causalidade não se revelam apropriadas a estes cenários, cada vez mais relevantes. Esta tese apresenta novos mecanismos de registo de causalidade que têm a propriedade de poder ser usados em cenários com um número dinâmico de entidades. Estes mecanismos permitem a criação descentralizada de entidades (processos ou réplicas) sem requerer identificadores globais ou coordenação global para a sua geração. Estes mecanismos apresentam codificações com tamanho variável, o que permite uma adaptação automática ao número de entidades em jogo, crescendo e colapsando de acordo com as necessidades.Causality plays a central role as a building block in solving important problems in distributed systems, such as replication, debugging, group communication and global snapshots. To be useful, causality must be realised by actual mechanisms that can track it and encode it. Existing causality tracking mechanisms, such as vector clocks and version vectors, rely on mappings from globally unique identifiers to integer counters. In a system with a well known set of entities these identifiers can be pre-configured and given distinct positions in a vector or distinct names in a mapping. Identity management is more problematic in dynamic systems, with a large and highly variable number of entities, being worsened when network partitions occur. Present solutions for causality tracking are not appropriate to these increasingly common scenarios. This thesis introduces novel causality tracking mechanisms that can be used in scenarios with a dynamic number of entities. These allow completely decentralised creation of entities (processes or replicas) with no need for global identifiers or global coordination. These mechanisms have a variable size representation that adapts automatically to the number of entities, growing or shrinking appropriately

Convergence et sécurité d'accès dans les systèmes d'édition collaborative massivement répartis

RÉSUMÉ Parmi les défis des systèmes d'édition collaborative figure la cohérence des objets partagés. Dans la perspective d'une édition cohérente, le système doit garantir la convergence. Pour assurer la cohérence des objets partagés, la littérature propose plusieurs solutions. Les différentes approches majeures proposées sont : l'approche des types de données commutatives répliquées (CRDT) et l'approche de la transformée opérationnelle (OT). L'approche CRDT considère des opérations commutatives qui peuvent être exécutées dans un ordre différent. L'une des difficultés auxquelles CRDT se bute réside en la commutativité des opérations. Toutes les opérations d'édition doivent être commutatives afin d'être exécutées dans un ordre quelconque. L'approche de la transformée opérationnelle quant à elle propose une transformation des opérations distantes reçues par rapport aux opérations qui lui sont concurrentes ; même si elles sont déjà exécutées. Pour effectuer les transformations, l'approche OT utilise un algorithme de transformation inclusive (IT). Dans la littérature, plusieurs travaux ont prouvé que les principaux algorithmes de transformation inclusive proposés ne satisfont pas le critère de convergence. Outre la cohérence, la sécurisation des interactions est un autre défi des systèmes d'édition collaborative. Le contrôle d'accès est l'un des modèles de politiques de sécurité applicable dans ce cadre. Il s'agit d'autoriser ou d'interdire l'édition à certains utilisateurs. Le contrôle d'accès doit être uniformément déployé pour éviter de compromettre la cohérence des opérations d'édition. Une opération d'édition valide sur un site doit l'être partout. Une opération refusée sur un site doit être refusée partout. Dans le contexte étudié, le protocole de sécurité est fiable s'il préserve la cohérence du système. Fournir cette preuve de fiabilité est une tâche ardue. Le nombre de cas à examiner est infini. De plus, pour une vérification automatique, le défaut de ressources survient si des techniques appropriées ne sont pas exploitées. Dans cette thèse, nous nous intéressons aux défis que constituent la convergence et le contrôle d'accès dans les systèmes d'édition collaborative répartis. Nous considérons un objet textuel à structure linéaire qui est massivement édité dans une architecture répartie. L'approche de gestion de cohérence utilisée est la transformée opérationnelle. Ainsi, chaque utilisateur a sa copie locale du document partagé. Les opérations générées sur un site sont aussitôt diffusées aux autres utilisateurs. Elles peuvent être exécutées dans un ordre quelconque. Les types d'opérations d'édition sont : l'insertion d'un caractère et la suppression de caractère. Nous intégrons également un protocole de contrôle d'accès à l'édition collaborative. Notre thèse se présente sous la forme de trois articles scientifiques, chacun traitant d'une problématique bien spécifique. Dans le premier article, nous abordons la problématique de la convergence. Nous avons adopté une démarche en plusieurs étapes. Une exploration a été initialement faite afin de vérifier s'il est possible d'avoir une fonction IT convergente. En utilisant la méthode de la synthèse de contrôleur et les automates de jeu, nos investigations ont révélé qu'aucune IT basée uniquement sur le caractère et la position ne peut garantir une convergence. L'identification des causes de divergence a permis d'amorcer la résolution du « problème de synthèse de contrôleur ». Ainsi, un troisième paramètre a été ajouté aux opérations d'insertion. Il permet de manipuler le nombre de caractères supprimés avant la position d'insertion indiquée. Une fonction de détermination de la valeur de ce paramètre a été proposée. Une fonction IT a été par la suite proposée, en tenant compte des propositions précédentes. En utilisant la vérification sur modèle (model-checking), la preuve a été apportée que notre IT garantit bien la convergence. Le deuxième article propose l'intégration d'un protocole de sécurité optimiste. L'article aborde la problématique de la fiabilité du protocole dans un espace d'états infini. Il est déployé au dessus de protocole de synchronisation du système d'édition collaborative. Nous faisons l'hypothèse que le système vérifie la propriété de cohérence en l'absence du contrôle d'accès. Pour affronter les difficultés relatives à la preuve de fiabilité, l'approche du model-checking symbolique a été préférée. Le model-checking borné a été utilisé avec l'outil Alloy. L'exploration faite pour des instances dont la taille maximale est de treize « signatures », a permis de conclure la préservation de la cohérence par le protocole de contrôle d'accès. Notons que ces instances ne sont pas massives mais la combinatoire résultante n'est pas négligeable. Le troisième article aborde la problématique de réduction de système. Des investigations ont été menées afin d'avoir un modèle fini équivalent au système d'édition collaborative, au regard de la propriété de cohérence. Le modèle abstrait proposé comporte trois sites coopératifs, dont l'un est administrateur. Ce modèle à espace d'états fini étant prouvé équivalent par rapport à la propriété de cohérence, au système à espace d'états infini, il a servi de cadre pour la vérification automatique. En utilisant l'outil Uppaal et le formalisme d'automate, nous avons prouvé par model-checking que le modèle abstrait préserve la cohérence. Par conséquent, le protocole de contrôle d'accès préserve la cohérence de système d'édition collaborative. Nos travaux comportent quelques limitations liées à leur portée. Nous avons manipulé des objets textuels à structure linéaire sur lesquels ne sont appliquées que des opérations d'insertion et de suppression de caractères. De plus, la gestion des droits d'accès est basée sur un modèle mono-administrateur. La performance du protocole de contrôle d'accès n'a pas non plus été prise en compte. Les travaux auraient sans doute plus d'envergure s'ils couvraient plusieurs types d'objets, plusieurs types d'opérations d'édition, plusieurs administrateurs et une étude de performance. Nos futures travaux pourraient être consacrés à l'élargissement de la portée de la présente thèse.----------ABSTRACT The consistency of the shared documents is one of the most important challenges in collaborative editing systems. To achieve consistency, a solution must ensure the convergence criteria. Several solutions are proposed in litterature to achieve consistency of the shared documents. The major approaches are: commutative replicated data type (CRDT) and operational transformation (OT). CRDT considers some commutative operations which could be executed in different order. The main difficulty of CRDT is to compute commutative operations. OT approach proposes to transform remote operations against their concurrent operations, even if they are already executed. An inclusive transformation function is used to compute the transformations. In the litterature, several works show that the main inclusive transformation (IT) functions proposed do not ensure convergence. Besides consistency, security of the edition is another challenge in distributed collaborative systems. Access control is a model of security policy that could be used. It consists of granting or revoking editing authorizations for users. Access control must be uniformly deployed to not compromise the consistency of the system. A valid editing operation at one site must be valid at all other sites. As the same time, an invalid operation at one site, must be invalid everywhere. In the current context, the security protocol is reliable if it preserves the consistency of the system. Produce the proof of reliability is difficult. It requires examining infinite number of cases. In addition, with automatic verification, ressources become insufficient if appropriate techniques are not used. This thesis is interested in consistency and access control challenges in distributed collaborative editing systems. It considers a textual object with a linear structure that is massively edited in a distributed architecture. OT is used to manage consistency. Each user has a local copy of the shared document. Locally-generated operations are immediately broadcast to other users. Operations could be executed in any order. Their types are inserting and deleting characters. To ensure security, collaborative edition is combined with an access control protocol. The thesis consists of three scientific articles. Each of them deals with a specific problem. In the first article, we adress the problem of consistency and proceed in several steps. Initially, we explore the existence of convergent IT functions of OT, which ensure data consistency. Using the controller synthesis approach and game automata, we conclude that there is no IT function, based only on character and position as parameters of insert and delete operations, which ensure data consistency. The investigation of the causes of divergence led to solve the controller synthesis problem. Thus, a new parameter is added to the insert operation signature. It handles the number of characters deleted before the inserting position. The function needed to compute the value of this parameter is provided. Finally, based on these contributions, we propose an IT function and show that it ensures convergence. The proof is achieved by a symbolic model-checking emulated using the tool Uppaal. The second article adresses the reliability of security protocol in an infinite state space. An optimist access control protocol is considered to be deployed over any correct synchronization protocol. The symbolic model-checking approach is choosen to deal with the proof of reliabi--lity. For this purpose, bounded model-checking is used with the tool Alloy. Exploration made with instances whose maximum size is thirteen allow to conclude the preservation of consistency by the access control protocol. These instances are not massive but the resulting combinatorial is important. The third article adresses the problem of system reduction. In this article, we investigate a finite model equivalent to a distributed collaborative editing system, with regard to consistency. The abstract model proposed consists of three cooperative sites including the administrator. This finite state model is proved by model-checking to preserve consistency. Consequently, the access control protocol preserves consistency of any correct distributed collaborative editing system. The model-checking techniques exploits Uppaal tool and automata. Our work has several limitations. We consider textual objects with linear structure. These objects are edited by applying some operations which are inserting and deleting characters. In addition, the management of access rights is based on one-administrator model. The performance study of the access control protocol is not done. The work would probably be more extensive if it covered several types of objects, several types of editing operations, many administrators and the performance study. Our future work could be devoted to the widening of the scope of this thesis

Measuring knowledge sharing processes through social network analysis within construction organisations

The construction industry is a knowledge intensive and information dependent industry. Organisations risk losing valuable knowledge, when the employees leave them. Therefore, construction organisations need to nurture opportunities to disseminate knowledge through strengthening knowledge-sharing networks. This study aimed at evaluating the formal and informal knowledge sharing methods in social networks within Australian construction organisations and identifying how knowledge sharing could be improved. Data were collected from two estimating teams in two case studies. The collected data through semi-structured interviews were analysed using UCINET, a Social Network Analysis (SNA) tool, and SNA measures. The findings revealed that one case study consisted of influencers, while the other demonstrated an optimal knowledge sharing structure in both formal and informal knowledge sharing methods. Social networks could vary based on the organisation as well as the individuals’ behaviour. Identifying networks with specific issues and taking steps to strengthen networks will enable to achieve optimum knowledge sharing processes. This research offers knowledge sharing good practices for construction organisations to optimise their knowledge sharing processes