Timed Quorum System for Large-Scale and Dynamic Environments

This paper presents Timed Quorum System (TQS), a new quorum system especially suited for large-scale and dynamic systems. TQS requires that two quorums intersect with high probability if they are used in the same small period of time. It proposed an algorithm that implements TQS and that verifies probabilistic atomicity: a consistency criterion that requires each operation to respect atomicity with high probability. This TQS implementation has quorum of size O(\sqrt{nD}) and expected access time of O(log \sqrt{nD}) message delays, where n measures the size of the system and D is a required parameter to handle dynamism

Implementing a Register in a Dynamic Distributed System

Providing distributed processes with concurrent objects is a fundamental service that has to be offered by any distributed system. The classical shared read/write register is one of the most basic ones. Several protocols have been proposed that build an atomic register on top of an asynchronous messagepassing system prone to process crashes. In the same spirit, this paper addresses the implementation of a regular register (a weakened form of an atomic register) in an asynchronous dynamic message-passing system. The aim is here to cope with the net effect of the adversaries that are asynchrony and dynamicity (the fact that processes can enter and leave the system). The paper focuses on the class of dynamic systems the churn rate c of which is constant. It presents two protocols, one applicable to synchronous dynamic message passing systems, the other one to asynchronous dynamic systems. Both protocols rely on an appropriate broadcast communication service (similar to a reliable brodcast). Each requires a specific constraint on the churn rate c. Both protocols are first presented in an as intuitive as possible way, and are then proved correct

Robust services in dynamic systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (p. 191-202).Our growing reliance on online services accessible on the Internet demands highly- available systems that work correctly without interruption. This thesis extends previous work on Byzantine-fault-tolerant replication to meet the new requirements of current Internet services: scalability and the ability to reconfigure the service automatically in the presence of a changing system membership. Our solution addresses two important problems that appear in dynamic replicated services: First, we present a membership service that provides servers and clients in the system with a sequence of consistent views of the system membership (i.e., the set of currently available servers). The membership service is designed to be scalable, and to handle membership changes mostly automatically. Furthermore, the membership service is itself reconfigurable, and tolerates arbitrary faults of a subset of the servers that are implementing it at any instant. The second part of our solution is a generic methodology for transforming replicated services that assume a fixed membership into services that support a dynamic system membership. The methodology uses the output from the membership service to decide when to reconfigure.(cont.) We built two example services using this methodology: a dynamic Byzantine quorum system that supports read and write operations, and a dynamic Byzantine state machine replication system that supports any deterministic service. The final contribution of this thesis is an analytic study that points out an obstacle to the deployment of replicated services based on a dynamic membership. The basic problem is that maintaining redundancy levels for the service state as servers join and leave the system is costly in terms of network bandwidth. To evaluate how dynamic the system membership can be, we developed a model for the cost of state maintenance in dynamic replicated services, and we use measured values from real-world traces to determine possible values for the parameters of the model. We conclude that certain deployments (like a volunteer-based system) are incompatible with the goals of large- scale reliable services. We implemented the membership service and the two example services. Our performance results show that the membership service is scalable, and our replicated services perform well, even during reconfigurations.by Rodrigo Seromenho Miragaia Rodrigues.Ph.D