Various protocols and architectures have been proposed to make Byzantine fault tolerance (BFT) increasingly practical. However, the deployment of such systems requires dedicated client-side functionality. This is necessary as clients have to connect to multiple replicas and perform majority voting over the received replies to outvote faulty responses. Deploying custom client-side code is cumbersome, and often not an option, especially in open heterogeneous systems and for well-established protocols (e.g., HTTP and IMAP) where diverse client-side implementations co-exist. We propose Troxy, a system which relocates the BFT-specific client-side functionality to the server side, thereby making BFT transparent to legacy clients. To achieve this, Troxy relies on a trusted subsystem built upon hardware protection enabled by Intel SGX. Additionally, Troxy reduces the replication cost of BFT for read-heavy workloads by offering an actively maintained cache that supports trustworthy read operations while preserving the consistency guarantees offered by the underlying BFT protocol. A prototype of Troxy has been built and evaluated, and results indicate that using Troxy (1) leads to at most 43% performance loss with small ordered messages in a local network environment, while (2) improves throughput by 130% with read-heavy workloads in a simulated wide-area network.

Coordination services are essential building blocks of today's data centers as they provide processes of distributed applications with means to reliably exchange data. Consequently, coordination services must deliver high performance to ensure that they do not become a bottleneck for the applications depending on them. Unfortunately, the design of existing services such as ZooKeeper prevents them from scaling with the number of cores on a machine. In this paper, we address this problem with Agora, a high-performance coordination service that is able to both effectively and efficiently utilize multi-core machines. Agora relies on a primary-backup replication architecture that partitions the workload on each server to achieve parallelism while still providing similar consistency guarantees as ZooKeeper. Our evaluation shows that Agora scales with the number of cores and thus can fully utilize the network resources available.

With the advent of trusted execution environments provided by recent general purpose processors, a class of replication protocols has become more attractive than ever: Protocols based on a hybrid fault model are able to tolerate arbitrary faults yet reduce the costs significantly compared to their traditional Byzantine relatives by employing a small subsystem trusted to only fail by crashing. Unfortunately, existing proposals have their own price: We are not aware of any hybrid protocol that is backed by a comprehensive formal specification, complicating the reasoning about correctness and implications. Moreover, current protocols of that class have to be performed largely sequentially. Hence, they are not well-prepared for just the modern multi-core processors that bring their very own fault model to a broad audience. In this paper, we present Hybster, a new hybrid state-machine replication protocol that is highly parallelizable and specified formally. With over 1 million operations per second using only four cores, the evaluation of our Intel SGX-based prototype implementation shows that Hybster makes hybrid state-machine replication a viable option even for today's very demanding critical services.

Consensus protocols employed in Byzantine fault-tolerant systems are notoriously compute intensive. Unfortunately, the traditional approach to execute instances of such protocols in a pipelined fashion is not well suited for modern multi-core processors and fundamentally restricts the overall performance of systems based on them. To solve this problem, we present the consensus-oriented parallelization (COP) scheme, which disentangles consecutive consensus instances and executes them in parallel by independent pipelines; or to put it in the terminology of our main target, today's processors: COP is the introduction of superscalarity to the field of consensus protocols. In doing so, COP achieves 2.4 million operations per second on commodity server hardware, a factor of 6 compared to a contemporary pipelined approach measured on the same code base and a factor of over 20 compared to the highest throughput numbers published for such systems so far. More important, however, is: COP provides up to 3 times as much throughput on a single core than its competitors and it can make use of additional cores where other approaches are confined by the slowest stage in their pipeline. This enables Byzantine fault tolerance for the emerging market of extremely demanding transactional systems and gives more room for conventional deployments to increase their quality of service.

One of the main reasons why Byzantine fault-tolerant (BFT) systems are not widely used lies in their high resource consumption: 3f+1 replicas are necessary to tolerate only f faults. Recent works have been able to reduce the minimum number of replicas to 2f+1 by relying on a trusted subsystem that prevents a replica from making conflicting statements to other replicas without being detected. Nevertheless, having been designed with the focus on fault handling, these systems still employ a majority of replicas during normal-case operation for seemingly redundant work. Furthermore, the trusted subsystems available trade off performance for security; that is, they either achieve high throughput or they come with a small trusted computing base.