Recovering Shared Objects Without Stable Storage

This paper considers the problem of building fault-tolerant shared objects when processes can crash and recover but lose their persistent state on recovery. This Diskless Crash-Recovery (DCR) model matches the way many long-lived systems are built. We show that it presents new challenges, as operations that are recorded at a quorum may not persist after some of the processes in that quorum crash and then recover. To address this problem, we introduce the notion of crash-consistent quorums, where no recoveries happen during the quorum responses. We show that relying on crash-consistent quorums enables a recovery procedure that can recover all operations that successfully finished. Crash-consistent quorums can be easily identified using a mechanism we term the crash vector, which tracks the causal relationship between crashes, recoveries, and other operations. We apply crash-consistent quorums and crash vectors to build two storage primitives. We give a new algorithm for multi-writer, multi-reader atomic registers in the DCR model that guarantees safety under all conditions and termination under a natural condition. It improves on the best prior protocol for this problem by requiring fewer rounds, fewer nodes to participate in the quorum, and a less restrictive liveness condition. We also present a more efficient single-writer, single-reader atomic set - a virtual stable storage abstraction. It can be used to lift any existing algorithm from the traditional Crash-Recovery model to the DCR model. We examine a specific application, state machine replication, and show that existing diskless protocols can violate their correctness guarantees, while ours offers a general and correct solution

A software architecture for consensus based replication

Orientador: Luiz Eduardo BuzatoTese (doutorado) - Universidade Estadual de Campinas, Instituto de ComputaçãoResumo: Esta tese explora uma das ferramentas fundamentais para construção de sistemas distribuídos: a replicação de componentes de software. Especificamente, procuramos resolver o problema de como simplificar a construção de aplicações replicadas que combinem alto grau de disponibilidade e desempenho. Como ferramenta principal para alcançar o objetivo deste trabalho de pesquisa desenvolvemos Treplica, uma biblioteca de replicação voltada para construção de aplicações distribuídas, porém com semântica de aplicações centralizadas. Treplica apresenta ao programador uma interface simples baseada em uma especificação orientada a objetos de replicação ativa. A conclusão que defendemos nesta tese é que é possível desenvolver um suporte modular e de uso simples para replicação que exibe alto desempenho, baixa latência e que permite recuperação eficiente em caso de falhas. Acreditamos que a arquitetura de software proposta tem aplicabilidade em qualquer sistema distribuído, mas é de especial interesse para sistemas que não são distribuídos pela ausência de uma forma simples, eficiente e confiável de replicá-losAbstract: This thesis explores one of the fundamental tools for the construction of distributed systems: the replication of software components. Specifically, we attempted to solve the problem of simplifying the construction of high-performance and high-availability replicated applications. We have developed Treplica, a replication library, as the main tool to reach this research objective. Treplica allows the construction of distributed applications that behave as centralized applications, presenting the programmer a simple interface based on an object-oriented specification for active replication. The conclusion we reach in this thesis is that it is possible to create a modular and simple to use support for replication, providing high performance, low latency and fast recovery in the presence of failures. We believe our proposed software architecture is applicable to any distributed system, but it is particularly interesting to systems that remain centralized due to the lack of a simple, efficient and reliable replication mechanismDoutoradoSistemas de ComputaçãoDoutor em Ciência da Computaçã

Exploiting replication in distributed systems

Techniques are examined for replicating data and execution in directly distributed systems: systems in which multiple processes interact directly with one another while continuously respecting constraints on their joint behavior. Directly distributed systems are often required to solve difficult problems, ranging from management of replicated data to dynamic reconfiguration in response to failures. It is shown that these problems reduce to more primitive, order-based consistency problems, which can be solved using primitives such as the reliable broadcast protocols. Moreover, given a system that implements reliable broadcast primitives, a flexible set of high-level tools can be provided for building a wide variety of directly distributed application programs

FastPay: High-Performance Byzantine Fault Tolerant Settlement

FastPay allows a set of distributed authorities, some of which are Byzantine, to maintain a high-integrity and availability settlement system for pre-funded payments. It can be used to settle payments in a native unit of value (crypto-currency), or as a financial side-infrastructure to support retail payments in fiat currencies. FastPay is based on Byzantine Consistent Broadcast as its core primitive, foregoing the expenses of full atomic commit channels (consensus). The resulting system has low-latency for both confirmation and payment finality. Remarkably, each authority can be sharded across many machines to allow unbounded horizontal scalability. Our experiments demonstrate intra-continental confirmation latency of less than 100ms, making FastPay applicable to point of sale payments. In laboratory environments, we achieve over 80,000 transactions per second with 20 authorities---surpassing the requirements of current retail card payment networks, while significantly increasing their robustness