Space Complexity of Fault-Tolerant Register Emulations

Driven by the rising popularity of cloud storage, the costs associated with implementing reliable storage services from a collection of fault-prone servers have recently become an actively studied question. The well-known ABD result shows that an f-tolerant register can be emulated using a collection of 2f + 1 fault-prone servers each storing a single read-modify-write object type, which is known to be optimal. In this paper we generalize this bound: we investigate the inherent space complexity of emulating reliable multi-writer registers as a fucntion of the type of the base objects exposed by the underlying servers, the number of writers to the emulated register, the number of available servers, and the failure threshold. We establish a sharp separation between registers, and both max-registers (the base object types assumed by ABD) and CAS in terms of the resources (i.e., the number of base objects of the respective types) required to support the emulation; we show that no such separation exists between max-registers and CAS. Our main technical contribution is lower and upper bounds on the resources required in case the underlying base objects are fault-prone read/write registers. We show that the number of required registers is directly proportional to the number of writers and inversely proportional to the number of servers.Comment: Conference version appears in Proceedings of PODC '1

Exploring Key-Value Stores in Multi-Writer Byzantine-Resilient Register Emulations

Resilient register emulation is a fundamental technique to implement dependable storage and distributed systems. In data-centric models, where servers are modeled as fail-prone base objects, classical solutions achieve resilience by using fault-tolerant quorums of read-write registers or read-modify-write objects. Recently, this model has attracted renewed interest due to the popularity of cloud storage providers (e.g., Amazon S3), that can be modeled as key-value stores (KVSs) and combined for providing secure and dependable multi-cloud storage services. In this paper we present three novel wait-free multi-writer multi-reader regular register emulations on top of Byzantine-prone KVSs. We implemented and evaluated these constructions using five existing cloud storage services and show that their performance matches or surpasses existing data-centric register emulations

Brief Announcement: Space Bounds for Reliable Multi-Writer Data Store:Inherent Cost of Read/Write Primitives

LNCS v. 9363 entitled: Distributed Computing: 29th International Symposium, DISC 2015, Tokyo, Japan, October 7-9, 2015, ProceedingsBack Matter pp. 647-678We consider a complete graph of n nodes, any pair of which can communicate with each other directly through one of F available wireless channels. n is not known to the nodes. Time is divided into synchronous rounds. In each round, a node can select at most one channel to listen to or transmit on. Transmission is successful if there is exactly one node transmitting on a channel (and one or more nodes listening). If two or more nodes transmit on the same channel, a collision occurs and their transmissions fail. Nodes can detect collisions, i.e., can distinguish collision from silence. We study distributed solutions to the information exchange problem: given initially k nodes each holding a packet, the task is to disseminate these k packets to all n nodes as quickly as possible. We assume that multiple packets can be packed in a single message. Recently, due to the advent of mobile devices that can operate on multiple channels, some attention has been given to studying the effect of multiple channels on improving communication [1–4]. However, all existing works require prior knowledge of n. In ad hoc networks, to make n known to all the nodes in fact can be a tough task. Moreover, in ad hoc networks, the value of n could change sporadically or even frequently due to nodes leaving and joining. Hence, there is practical need for designing uniform protocols that do not require any prior information about the network including n and k. Not knowing the parameters n or k greatly increases the difficulty of designing fast algorithms, especially in the case where different nodes can operate on different channels, as it is hard to manage the transmission probabilities over the distributed set of nodes

Integrated Bounds for Disintegrated Storage

We point out a somewhat surprising similarity between non-authenticated Byzantine storage, coded storage, and certain emulations of shared registers from smaller ones. A common characteristic in all of these is the inability of reads to safely return a value obtained in a single atomic access to shared storage. We collectively refer to such systems as disintegrated storage, and show integrated space lower bounds for asynchronous regular wait-free emulations in all of them. In a nutshell, if readers are invisible, then the storage cost of such systems is inherently exponential in the size of written values; otherwise, it is at least linear in the number of readers. Our bounds are asymptotically tight to known algorithms, and thus justify their high costs

Tame the Wild with Byzantine Linearizability: Reliable Broadcast, Snapshots, and Asset Transfer

We formalize Byzantine linearizability, a correctness condition that specifies whether a concurrent object with a sequential specification is resilient against Byzantine failures. Using this definition, we systematically study Byzantine-tolerant emulations of various objects from registers. We focus on three useful objects- reliable broadcast, atomic snapshot, and asset transfer. We prove that there exist n-process f-resilient Byzantine linearizable implementations of such objects from registers if and only if f < n/2

On the diagnostic emulation technique and its use in the AIRLAB

An aid is presented for understanding and judging the relevance of the diagnostic emulation technique to studies of highly reliable, digital computing systems for aircraft. A short review is presented of the need for and the use of the technique as well as an explanation of its principles of operation and implementation. Details that would be needed for operational control or modification of existing versions of the technique are not described