Most geo-replicated storage systems use weak consistency to avoid the performance penalty of coordinating replicas in different data centers. This departure from strong semantics poses problems to application programmers, who need to address the anomalies enabled by weak consistency. In this paper we use a recently proposed isolation level, called Non-Monotonic Snapshot Isolation, to achieve ACID transactions with low latency. To this end, we present Blotter, a geo-replicated system that leverages these semantics in the design of a new concurrency control protocol that leaves a small amount of local state during reads to make commits more efficient, which is combined with a configuration of Paxos that is tailored for good performance in wide area settings. Read operations always run on the local data center, and update transactions complete in a small number of message steps to a subset of the replicas. We implemented Blotter as an extension to Cassandra. Our experimental evaluation shows that Blotter has a small overhead at the data center scale, and performs better across data centers when compared with our implementations of the core Spanner protocol and of Snapshot Isolation on the same codebase.

Wireless ad hoc networks, due to their inherent unreliability, pose significant challenges to the task of achieving tight coordination among nodes. The failure of some nodes and momentary breakdown of communications, either of accidental or malicious nature, should not result in the failure of the entire system. This paper presents an asynchronous Byzantine consensus protocol-called Turquois-specifically designed for resource-constrained wireless ad hoc networks. The key to its efficiency is the fact that it tolerates dynamic message omissions, which allows an efficient utilization of the wireless broadcasting medium. The protocol also refrains from computationally expensive public-key cryptographic during its normal operation. The protocol is safe despite the arbitrary failure of f <; n/3 nodes from a total of n nodes, and unrestricted message omissions. Progress is ensured in rounds where the number of omissions is σ ≤ [n-t/2] (n - k - t) + k - 2, where k is the number of nodes required to terminate and t ≤ f is the number of nodes that are actually faulty. These characteristics make Turquois the first consensus protocol that simultaneously circumvents the FLP and the Santoro-Widmayer impossibility results, which is achieved through randomization. Finally, the protocol was prototyped and subject to a comparative performance evaluation against two well-known Byzantine fault-tolerant consensus protocols. The results show that, due to its design, Turquois outperforms the other protocols by more than an order of magnitude as the number of nodes in the system increases.

Randomized agreement protocols have been around for more than two decades. Often assumed to be inefficient due to their high expected communication and computation complexities, they have remained overlooked by the community-at-large as a valid solution for the deployment of fault-tolerant distributed systems. This paper aims to demonstrate that randomization can be a very competitive approach even in hostile environments where arbitrary faults can occur. A stack of randomized intrusion-tolerant protocols is described and its performance evaluated under several settings in both local-area-network (LAN) and wide-area-network environments. The stack provides a set of relevant services ranging from basic communication primitives up to atomic broadcast. The experimental evaluation shows that the protocols are efficient, especially in LAN environments where no performance reduction is observed under certain Byzantine faults.

The operation of wireless ad hoc networks is intrinsically tied to the ability of nodes to coordinate their actions in a dependable and efficient manner. The failure of some nodes and momentary breakdown of communications, either of accidental or malicious nature, should not result in the failure of the entire system. This paper presents Turquois - an intrusion-tolerant consensus protocol specifically designed for resource-constrained wireless ad hoc networks. Turquois allows an efficient utilization of the broadcasting medium, avoids synchrony assumptions, and refrains from public-key cryptography during its normal operation. The protocol is safe despite the arbitrary failure of f <; n/3 processes from a total of n processes, and unrestricted message omissions. The protocol was prototyped and subject to a comparative performance evaluation against two well-known intrusion-tolerant consensus protocols. The results show that, as the system scales, Turquois outperforms the other protocols by more than an order of magnitude.

This paper presents a study on the performance of intrusion-tolerant protocols in wireless LANs. The protocols are evaluated in several different environmental settings, and also within the context of a car platooning application for distributed cruise control. The experimental evaluation reveals how performance is affected by the various environmental parameters such as the wireless standard, group size, and network topology. The distributed cruise control application demonstrates the practicability of such protocols, even when subjected to malicious faults.

Randomized agreement protocols, often assumed to be inefficient due to their high expected communication and time complexities, they have remained largely overlooked by the community-at-large as a valid solution for the deployment of fault-tolerant distributed systems. This paper aims to demonstrate that randomization can be a very competitive approach even in hostile environments where arbitrary faults can occur. A stack of randomized intrusion-tolerant protocols is described and its performance evaluated under different faultloads. The stack provides a set of relevant services ranging from basic communication primitives up to atomic broadcast. The experimental evaluation shows that the protocols are efficient and no performance reduction is observed under certain Byzantine faults

The paper presents a comparative performance study of the two main classes of randomized binary consensus protocols: a local coin protocol, with an expected high communication complexity and cheap symmetric cryptography, and a shared coin protocol, with an expected low communication complexity and expensive asymmetric cryptography. The experimental evaluation was conducted on a LAN environment, by varying several system parameters, such as the fault types and number of processes. The analysis shows that there is a significant gap between the theoretical and the practical performance results of these protocols, and provides an important insight into what actually happens during their execution