Hybrid fault models are known to be an effective means for enhancing the robustness of consensus-based replicated systems. However, existing hybridization approaches suffer from limited flexibility with regard to the composition of crash-tolerant and Byzantine fault-tolerant system parts and/or are associated with a significant diversification overhead. In this paper we address these issues with ShellFT, a framework that leverages the concept of micro replication to allow system designers to freely choose the parts of the replication logic that need to be resilient against Byzantine faults. As a key benefit, such a selective hybridization makes it possible to develop hybrid solutions that are tailored to the specific characteristics and requirements of individual use cases. To illustrate this flexibility, we present three custom ShellFT protocols and analyze the complexity of their implementations. Our evaluation shows that compared with traditional hybridization approaches, ShellFT is able to decrease diversification costs by more than 70 %.

When put under stress, traditional state-machine replication protocols typically exhibit response times that by far exceed the average level of normal-case operation. The common way to mitigate such overload-induced tail latency is to overprovision computing and network resources. However, this method often leads to large amounts of resources left unused over extended periods of time, especially in application scenarios in which high loads are mostly limited to short phases. In this paper, we circumvent the need for overprovisioning with Idem, a replication protocol specifically designed to process client requests with low latency even during load spikes. Most notably, Idem replicas avoid overload by proactively rejecting requests in a collaborative manner. In contrast to centralized overload-prevention strategies, the collaboration among replicas allows Idem to always timely notify clients about rejections of their requests, not only under favorable conditions but also in the presence of replica crashes.

State-machine replication protocols represent the foundation of many fault-tolerant services. Unfortunately, their inherent complexity makes existing implementations notoriously difficult to debug and test. To address this problem, we propose a novel design approach, micro replication, whose main goal is to reduce bugs and enable replication protocols with improved debuggability properties. At its core, our concept consists of a set of principles that, if followed during protocol design, later significantly facilitate crucial tasks such as bug-source isolation, state-information retrieval, as well as root-cause identification. To achieve this, micro replication organizes a protocol as a composition of specialized modules (“micro replicas”) that each encapsulate a particular protocol phase or mechanism, and therefore are easier to test and monitor than traditional monolithic replicas. Besides discussing the underlying ideas of our approach, to show its feasibility we also present and evaluate Mirador, the first micro-replicated Byzantine fault-tolerant protocol.