With a variety of services rapidly evolving at all architectural levels of cloud computing, there is an increasing demand for a standardized way to coordinate their interactions. Business process management, that is, more general, the management of Web-service-based workflows, could satisfy this demand and, indeed, first corresponding o↵erings have gained instant popularity. While from a functional perspective, these Platform-as-a-Service (PaaS) solutions are already quite mature, their support for fault tolerance is still very limited, making them inapplicable for critical tasks.Concerning the deficiencies of currently existing systems, this paper presents a practical solution for executing critical Web-service-based workflows, particularly within clouds, in a fault-tolerant, highly available and highly configurable manner. We achieve this by actively replicating workflows as well as Web services in a combined architecture, reusing existing standard systems and coordination services. By providing an automated transformation tool, replication is realized transparently to existing systems and workflows. Measurements show that our proposed architecture achieves lower response times than existing systems and that the integration of a coordination service imposes only moderate costs, while simplifying the implementation and leading to a dynamically adaptable solution.

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 normalcase 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. This paper presents CheapBFT, a BFT system that, for the first time, tolerates that all but one of the replicas active in normal-case operation become faulty. CheapBFT runs a composite agreement protocol and exploits passive replication to save resources; in the absence of faults, it requires that only f + 1 replicas actively agree on client requests and execute them. In case of suspected faulty behavior, CheapBFT triggers a transition protocol that activates f extra passive replicas and brings all non-faulty replicas into a consistent state again. This approach, for example, allows the system to safely switch to another, more resilient agreement protocol. CheapBFT relies on an FPGA-based trusted subsystem for the authentication of protocol messages that provides high performance and comprises a small trusted computing base.

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Abstract. Platform-as-a-Service (PaaS) clouds free companies of building infrastructures dimensioned for peak service demand and allow them to only pay for the resources they actually use. Being a PaaS cloud customer, on the one hand, o↵ers a company the opportunity to provide applications in a dynamically scalable way. On the other hand, this scalability may lead to financial loss due to costly use of vast amounts of resources caused by program errors, attacks, or careless use. To limit the e↵ects of involuntary resource usage, we present DQMP, a decentralized, fault-tolerant, and scalable quota-enforcement protocol. It allows customers to buy a fixed amount of resources (e. g., CPU cycles) that can be used flexibly within the cloud. DQMP utilizes the concept of di↵usion to equally balance unused resource quotas over all processes running applications of the same customer. This enables the enforcement of upper bounds while being highly adaptive to all kinds of resourcedemand changes. Our evaluation shows that our protocol outperforms a lease-based centralized implementation in a setting with 1,000 processes.