<p>Distributed real-time databases can be used to support data sharing</p><p>for applications in wireless ad-hoc networks. In such networks, topology changes frequently and partitions may be unpredictable and last for an unbounded period. In this thesis, the existing database replication protocol PRiDe is extended to handle such long-lasting partitions. The protocol uses optimistic and detached replication to provide predictable response times in unpredictable networks and forward conflict resolution to guarantee progress.</p><p>The extension, pPRiDe, combines update and state transfer strategies. Update transfer for intra-partition communication can reduce bandwidth usage and ease conflict resolution. State transfer for inter partition conflicts removes dependency on a common state between partitions prior to the merge to apply update messages on. This makes the resource usage independent of the life span of partitions. This independence comes at the cost of global data stability guarantees and pPRiDe can thus only provide per partition guarantees. The protocol supports application specific conflict resolution routines for both</p><p>state and update conflicts. A basic simulator for mobile ad-hoc networks has been developed to validate that pPRiDe provides eventual consistency.</p><p>pPRiDe shows that a hybrid approach to change propagation strategy can be beneficial in networks where collaboration by data sharing within long lasting partitions and predictable resource usage is necessary. These types of systems already require the conflict management routines necessary for pPRiDe and can benefit from an existing protocol.</p><p>In addition to pPRiDe and the simulator this thesis provides a flexible object database suitable for future works and an implementation of PRiDe on top of that database.</p>

<p>In distributed database systems, immediate global consistency of replicated data can be achieved by distributed commit protocols that are typically unpredictable. If real-time characteristics are necessary, such unpredictability has to be avoided. In a distributed real-time database, optimistic replication can be used to avoid unpredictable delays by allowing transactions to commit locally. The update of other nodes is performed as soon as possible. If optimistic replication is used, conflicts may occur since data can be changed locally without synchronously informing other nodes. To detect these conflicts, this thesis introduces a conflict detection approach for DeeDS, a distributed, active real-time database that supports a dynamic node set. A comparison of existing conflict detection approaches is performed, and it is found that the dynamic version vector approach is the best fitting approach. The main reason is that it can handle a dynamic node set with a minimum of additional conflict detection data. To show the realization of the approach in DeeDS, dynamic version vectors have been implemented. Additionally, conflict management in DeeDS is redesigned to allow separation of conflict detection and conflict resolution. This makes the software architecture more flexible and is a first step towards application specific conflict resolution.</p>

In distributed database systems, immediate global consistency of replicated data can be achieved by distributed commit protocols that are typically unpredictable. If real-time characteristics are necessary, such unpredictability has to be avoided. In a distributed real-time database, optimistic replication can be used to avoid unpredictable delays by allowing transactions to commit locally. The update of other nodes is performed as soon as possible. If optimistic replication is used, conflicts may occur since data can be changed locally without synchronously informing other nodes. To detect these conflicts, this thesis introduces a conflict detection approach for DeeDS, a distributed, active real-time database that supports a dynamic node set. A comparison of existing conflict detection approaches is performed, and it is found that the dynamic version vector approach is the best fitting approach. The main reason is that it can handle a dynamic node set with a minimum of additional conflict detection data. To show the realization of the approach in DeeDS, dynamic version vectors have been implemented. Additionally, conflict management in DeeDS is redesigned to allow separation of conflict detection and conflict resolution. This makes the software architecture more flexible and is a first step towards application specific conflict resolution.

Distributed real-time databases can be used to support data sharing for applications in wireless ad-hoc networks. In such networks, topology changes frequently and partitions may be unpredictable and last for an unbounded period. In this thesis, the existing database replication protocol PRiDe is extended to handle such long-lasting partitions. The protocol uses optimistic and detached replication to provide predictable response times in unpredictable networks and forward conflict resolution to guarantee progress. The extension, pPRiDe, combines update and state transfer strategies. Update transfer for intra-partition communication can reduce bandwidth usage and ease conflict resolution. State transfer for inter partition conflicts removes dependency on a common state between partitions prior to the merge to apply update messages on. This makes the resource usage independent of the life span of partitions. This independence comes at the cost of global data stability guarantees and pPRiDe can thus only provide per partition guarantees. The protocol supports application specific conflict resolution routines for both state and update conflicts. A basic simulator for mobile ad-hoc networks has been developed to validate that pPRiDe provides eventual consistency. pPRiDe shows that a hybrid approach to change propagation strategy can be beneficial in networks where collaboration by data sharing within long lasting partitions and predictable resource usage is necessary. These types of systems already require the conflict management routines necessary for pPRiDe and can benefit from an existing protocol. In addition to pPRiDe and the simulator this thesis provides a flexible object database suitable for future works and an implementation of PRiDe on top of that database.

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Replikation kann helfen, in einem verteilten System die Fehlertoleranz und Datensicherheit zu verbessern. In Systemen, die über Weitverkehrsnetze kommunizieren oder mobile Endgeräte einschließen, muß das Replikationssystem mit großen Kommunikationslatenzen umgehen können. Deshalb werden in solchen Systemen in der Regel nur asynchrone Replikationsalgorithmen mit schwach-konsistenter Änderungssemantik eingesetzt, da diese die lokale Annahme von Änderungen der Daten und deren Koordinierung mit anderen Replikaten entkoppeln und somit ein schnelles Antwortverhalten bieten können. Diese Dissertation stellt einen Ansatz für die Entwicklung schwach-konsistenter Replikationssysteme mit erweiterten kausalen Konsistenzgarantien vor und weist nach, daß auf seiner Grundlage effiziente Replikationssysteme konstruiert werden können. Dazu werden Mechanismen, Algorithmen und Protokolle vorgestellt, die Änderungen an replizierten Daten aufzeichnen und verteilen und dabei Kausalitätsbeziehungen erhalten. Kern ist ein Änderungsprotokoll, das sowohl als grundlegende Datenstruktur der verteilten Algorithmen agiert, als auch für die Konsistenz der lokalen Daten nach Systemabstürzen sorgt. Die kausalen Garantien werden mit Hilfe von zwei Algorithmen erweitet, die gleichzeitige Änderungen konsistent handhaben. Beide Algorithmen basieren auf der Beobachtung, daß die Divergenz der Replikate durch unkoordinierte, gleichzeitige Änderungen nicht unbedingt als Inkonsistenz gesehen werden muß, sondern auch als das Erzeugen verschiedener Versionen der Daten modelliert werden kann. Distributed Consistent Branching (DCB) erzeugt diese alternativen Versionen der Daten konsistent auf allen Replikaten; Distributed Consistent Cutting (DCC) wählt eine der Versionen konsistent aus. Die vorgestellten Algorithmen und Protokolle wurden in einer Datenbankimplementierung validiert. Mehrere Experimente zeigen ihre Einsetzbarkeit und helfen, ihr Verhalten unter verschiedenen Bedingungen einzuschätzen. / Data replication techniques introduce redundancy into a distributed system architecture that can help solve several of its persistent problems. In wide area or mobile systems, a replication system must be able to deal with the presence of unreliable, high-latency links. Only asynchronous replication algorithms with weak-consistency guarantees can be deployed in these environments, as these algorithms decouple the local acceptance of changes to the replicated data from coordination with remote replicas. This dissertation proposes a framework for building weak-consistency replication systems that provides the application developer with causal consistency guarantees and mechanisms for handling concurrency. By presenting an integrated set of mechanisms, algorithms and protocols for capturing and disseminating changes to the replicated data, we show that causal consistency and concurrency handling can be implemented in an efficient and versatile manner. The framework is founded on log of changes, which both acts the core data structure for its distributed algorithms and protocols and serves as the database log that ensures the consistency of the local data replica. The causal consistency guarantees are complemented with two distributed algorithms that handle concurrent operations. Both algorithms are based on the observation that uncoordinated concurrent operations introduce a divergence of state in a replication system that can be modeled as the creation of version branches. Distributed Consistent Branching (DCB) recreates these branches on all participating processes in a consistent manner. Distributed Consistent Cutting (DCC) selects one of the possible branches in a consistent and application-controllable manner and enforces a total causal order for all its operations. The contributed algorithms and protocols were validated in an database system implementation, and several experiments assess the behavior of these algorithms and protocols under varying conditions.

Pour fournir une disponibilité permanente des données et réduire la latence réseau, les systèmes de partage de données se basent sur la réplication optimiste. Dans ce paradigme, il existe plusieurs copies de l'objet partagé dite répliques stockées sur des sites. Ces répliques peuvent être modifiées librement et à tout moment. Les modifications sont exécutées en local puis propagées aux autres sites pour y être appliquées. Les algorithmes de réplication optimiste sont chargés de gérer les modifications parallèles. L'objectif de cette thèse est de proposer une méthodologie d'évaluation pour les algorithmes de réplication optimiste. Le contexte de notre étude est l'édition collaborative. Nous allons concevoir pour cela un outil d'évaluation qui intègre un mécanisme de génération de corpus et un simulateur d'édition collaborative. À travers cet outil, nous allons dérouler plusieurs expériences sur deux types de corpus: synchrone et asynchrone. Dans le cas d'une édition collaborative synchrone, nous évaluerons les performances des différents algorithmes de réplication sur différents critères tels que le temps d'exécution, l'occupation mémoire, la taille des messages, etc. Nous proposerons ensuite quelques améliorations. En plus, dans le cas d'une édition collaborative asynchrone, lorsque deux répliques se synchronisent, les conflits sont plus nombreux à apparaître. Le système peut bloquer la fusion des modifications jusqu'à ce que l'utilisateur résolut les conflits. Pour réduire le nombre de ces conflits et l'effort des utilisateurs, nous proposerons une métrique d'évaluation et nous évaluerons les différents algorithmes sur cette métrique. Nous analyserons le résultat pour comprendre le comportement des utilisateurs et nous proposerons ensuite des algorithmes pour résoudre les conflits les plus important et réduire ainsi l'effort des développeurs. Enfin, nous proposerons une nouvelle architecture hybride basée sur deux types d'algorithmes de réplication. Contrairement aux architectures actuelles, l'architecture proposéeest simple, limite les ressources sur les dispositifs clients et ne nécessite pas de consensus entre les centres de données / To provide a high availability from any where, at any time, with low latency, data is optimistically replicated. This model allows any replica to apply updates locally, while the operations are later sent to all the others. In this way, all replicas eventually apply all updates, possibly even in different order. Optimistic replication algorithms are responsible for managing the concurrent modifications and ensure the consistency of the shared object. In this thesis, we present an evaluation methodology for optimistic replication algorithms. The context of our study is collaborative editing. We designed a tool that implements our methodology. This tool integrates a mechanism to generate a corpus and a simulator to simulate sessions of collaborative editing. Through this tool, we made several experiments on two different corpus: synchronous and asynchronous. In synchronous collaboration, we evaluate the performance of optimistic replication algorithms following several criteria such as execution time, memory occupation, message's size, etc. After analysis, some improvements were proposed. In addition, in asynchronous collaboration, when replicas synchronize their modifications, more conflicts can appear in the document. In this case, the system cannot merge the modifications until a user resolves them. In order to reduce the conflicts and the user's effort, we propose an evaluation metric and we evaluate the different algorithms on this metric. Afterward, we analyze the quality of the merge to understand the behavior of the users and the collaboration cases that create conflicts. Then, we propose algorithms for resolving the most important conflicts, therefore reducing the user's effort. Finally, we propose a new architecture for supporting cloud-based collaborative editing system. This architecture is based on two optimistic replication algorithms. Unlike current architectures, the proposed one removes the problems of the centralization and consensus between data centers, is simple and accessible for any developers