Matthias Wiesmann

Database replication techniques: a three parameter classification

Data replication is an increasingly important topic as databases are more and more deployed over clusters of workstations. One of the challenges in database replication is to introduce replication without severely affecting performance. Because of this difficulty, current database products use lazy replication, which is very efficient but can compromise consistency. As an alternative, eager replication guarantees consistency but most existing protocols have a prohibitive cost. In order to clarify the current state of the art and open up new avenues for research, this paper analyses existing eager techniques using three key parameters (server architecture, server interaction and transaction termination). In our analysis, we distinguish eight classes of eager replication protocols and, for each category, discuss its requirements, capabilities and cost. The contribution lies in showing when eager replication is feasible and in spelling out the different aspects a database replication protocol must account for.

Comparison of database replication techniques based on total order broadcast

In this paper, we present a performance comparison of database replication techniques based on total order broadcast. While the performance of total order broadcast-based replication techniques has been studied in previous papers, this paper presents many new contributions. First, it compares with each other techniques that were presented and evaluated separately, usually by comparing them to a classical replication scheme like distributed locking. Second, the evaluation is done using a finer network model than previous studies. Third, the paper compares techniques that offer the same consistency criterion (one-copy serializability) in the same environment using the same settings. The paper shows that, while networking performance has little influence in a LAN setting, the cost of synchronizing replicas is quite high. Because of this, total order broadcast-based techniques are very promising as they minimize synchronization between replicas.

Using optimistic atomic broadcast in transaction processing systems

Atomic broadcast primitives are often proposed as a mechanism to allow fault-tolerant cooperation between sites in a distributed system. Unfortunately, the delay incurred before a message can be delivered makes it difficult to implement high performance, scalable applications on top of atomic broadcast primitives. Recently, a new approach has been proposed for atomic broadcast which, based on optimistic assumptions about the communication system, reduces the average delay for message delivery to the application. We develop this idea further and show how applications can take even more advantage of the optimistic assumption by overlapping the coordination phase of the atomic broadcast algorithm with the processing of delivered messages. In particular, we present a replicated database architecture that employs the new atomic broadcast primitive in such a way that communication and transaction processing are fully overlapped, providing high performance without relaxing transaction correctness.

Beyond 1-Safety and 2-Safety for Replicated Databases: Group-Safety

In this paper, we study the safety guarantees of group communication-based database replication techniques. We show that there is a model mismatch between group communication and database, and because of this, classical group communication systems cannot be used to build 2-safe database replication. We propose a new group communication primitive called end-to-end atomic broadcast that solves the problem, i.e., can be used to implement 2-safe database replication. We also introduce a new safety criterion, called group-safety, that has advantages both over 1-safety and 2-safety. Experimental results show the gain of efficiency of group-safety over lazy replication, which ensures only 1-safety.

Group communication based on standard interfaces

While group communication systems have been proposed for some time, they are still not used much in actual systems. We believe that one reason for this is the lack of standardisation of group communication system interfaces. The paper proposes an architecture, using the standard decomposition into services, where services are based on standard interfaces: both interactions between services and interactions with the application use existing, open standards. A decomposition of the group communication into services is presented, along with a description of applicable standards. As an example, a group membership service based on the LDAP standard is discussed.

Towards JMS compliant group communication - a semantic mapping

Group communication provides communication primitives with various semantics and their use greatly simplifies the development of highly available services. However, despite tremendous advances in research and numerous prototypes, group communication stays confined to small niches and academic prototypes. In contrast, message-oriented middleware such as the Java message service (JMS) is widely used, and has become a de-facto standard. We believe that the lack of a well-defined and easily understandable standard is the reason that hinders the deployment of group communication systems. Since JMS is a well-established technology, an interesting solution is to extend JMS adding group communication primitives to it. Foremost, this requires extending the traditional semantics of group communication in order to take into account various features of JMS, e.g., durable/nondurable subscriptions and persistent/non-persistent messages. The resulting new group communication specification, together with the corresponding API, defines group communication primitives compatible with JMS.

Collision prevention using group communication for asynchronous cooperative mobile robots

The paper presents a fail-safe mobility management and a collision prevention platform for a group of asynchronous cooperative mobile robots. The fail-safe platform consists of a time-free collision prevention protocol, which guarantees that no collision can occur between robots, independently of timeliness properties of the system, and even in the presence of timing errors in the environment. The collision prevention protocol is based on a distributed path reservation system. Each robot in the system knows the composition of the group, and can communicate with all robots of the group. A performance analysis of the protocol provides insights for a proper dimensioning of system parameters in order to maximize the average effective speed of the robots.

Collision Prevention Platform for a Dynamic Group of Asynchronous Cooperative Mobile Robots

This paper presents a fail-safe platform on which cooperative mobile robots rely for their motion. The platform consists of a collision prevention protocol for a dynamic group of cooperative mobile robots with asynchronous communications. The collision prevention protocol is timefree, in the sense that it never relies on physical time, which makes it extremely robust for timing uncertainty common in wireless networks. It guarantees that no two robots ever collide, regardless of the respective activities of the robots. The protocol is based on a fully distributed path reservation system. It assumes a mobile ad hoc network formed by the robots themselves, and takes advantage of the inherent locality of the problem in order to reduce communication. The protocol requires neither initial nor complete knowledge of the composition of the group. A performance analysis of the protocol provides insights for a proper dimensioning of system parameters in order to maximize the average effective speed of the robots.

End-to-end consensus using end-to-end channels

End-to-end consensus ensures delivery of the same value to the application layer running in distributed processes. Deliveries that have not been acknowledged by the application before a failure are delivered again. End-to-end primitives are important for applications that need to enforce persistency. We present an algorithm that solves the end-to-end consensus problem. Our approach is to build end-to-end consensus using a new type of communication channels, end-to-end channels

Anonymous Stabilizing Leader Election using a Network Sequencer

In this paper, we present an anonymous, stable, communication efficient, stabilizing leader election algorithm that works using anonymous communication primitives. The algorithm offers properties similar to that of the Omega failure detector, with the added property of totally ordering the sequence of proposed leaders. The algorithm does not need to know beforehand the identity or the number of processes in the system, and operates using a constant amount of memory. We present the algorithm, discuss performance issues and optimizations and present experimental results of a prototype implementation.