View-oriented group communication is an important and widely used building block for many distributed applications. Much current research has been dedicated to specifying the semantics and services of view-oriented group communication systems (GCSs). However, the guarantees of different GCSs are formulated using varying terminologies and modeling techniques, and the specifications vary in their rigor. This makes it difficult to analyze and compare the different systems. This survey provides a comprehensive set of clear and rigorous specifications, which may be combined to represent the guarantees of most existing GCSs. In the light of these specifications, over 30 published GCS specifications are surveyed. Thus, the specifications serve as a unifying framework for the classification, analysis, and comparison of group communication systems. The survey also discusses over a dozen different applications of group communication systems, shedding light on the usefulness of the presented specifications. This survey is aimed at both system builders and theoretical researchers. The specification framework presented in this article will help builders of group communication systems understand and specify their service semantics; the extensive

We present Byzantine Disk Paxos, an asynchronous sharedmemory consensus protocol that uses a collection of n > 3t disks, t of which may fail by becoming non-responsive or arbitrarily corrupted. We give two constructions of this protocol; that is, we construct two different building blocks, each of which can be used, along with a leader oracle, to solve consensus. One building block is a shared wait-free safe register. The second building block is a regular register that satisfies a weaker termination (liveness) condition than wait freedom: its write operations are wait-free, whereas its read operations are guaranteed to return only in executions with a finite number of writes. We call this termination condition finite writes (FW), and show that consensus is solvable with FW-terminating registers and a leader oracle. We construct each of these reliable registers from n > 3t base registers, t of which can be non-responsive or Byzantine. All the previous wait-free constructions in this model used at least 4t + 1 fault-prone registers, and we are not familiar with any prior FW-terminating constructions in this model.

We present Brahms, an algorithm for sampling random nodes in a large dynamic system prone to malicious behavior. Brahms stores small membership views at each node, and yet overcomes Byzantine attacks by a linear portion of the system. Brahms is composed of two components. The first one is a resilient gossip-based membership protocol. The second one uses a novel memory-efficient approach for uniform sampling from a possibly biased stream of ids that traverse the node. We evaluate Brahms using rigorous analysis, backed by simulations, which show that our theoretical model captures the protocol's essentials. We study two representative attacks, and show that with high probability, an attacker cannot create a partition between correct nodes. We further prove that each node's sample converges to a uniform one over time. To our knowledge, no such properties were proven for gossip protocols in the past.

This paper deals with the emulation of atomic read/write (R/W) storage in dynamic asynchronous message passing systems. In static settings, it is well known that atomic R/W storage can be implemented in a fault-tolerant manner even if the system is completely asynchronous, whereas consensus is not solvable. In contrast, all existing emulations of atomic storage in dynamic systems rely on consensus or stronger primitives, leading to a popular belief that dynamic R/W storage is unattainable without consensus.In this paper, we specify the problem of dynamic atomic read/write storage in terms of the interface available to the users of such storage. We discover that, perhaps surprisingly, dynamic R/W storage is solvable in a completely asynchronous system: we present DynaStore, an algorithm that solves this problem. Our result implies that atomic R/W storage is in fact easier than consensus, even in dynamic systems.

More and more users store data in "clouds" that are accessed remotely over the Internet. We survey well-known cryptographic tools for providing integrity and consistency for data stored in clouds and discuss recent research in cryptography and distributed computing addressing these problems.