6,134 research outputs found
Randomized protocols for asynchronous consensus
The famous Fischer, Lynch, and Paterson impossibility proof shows that it is
impossible to solve the consensus problem in a natural model of an asynchronous
distributed system if even a single process can fail. Since its publication,
two decades of work on fault-tolerant asynchronous consensus algorithms have
evaded this impossibility result by using extended models that provide (a)
randomization, (b) additional timing assumptions, (c) failure detectors, or (d)
stronger synchronization mechanisms than are available in the basic model.
Concentrating on the first of these approaches, we illustrate the history and
structure of randomized asynchronous consensus protocols by giving detailed
descriptions of several such protocols.Comment: 29 pages; survey paper written for PODC 20th anniversary issue of
Distributed Computin
A Prescription for Partial Synchrony
Algorithms in message-passing distributed systems often require partial synchrony to tolerate crash failures. Informally, partial synchrony refers to systems where timing bounds on communication and computation may exist, but the knowledge of such bounds is limited. Traditionally, the foundation for the theory of partial synchrony has been real time: a time base measured by counting events external to the system, like the vibrations of Cesium atoms or piezoelectric crystals.
Unfortunately, algorithms that are correct relative to many real-time based models of partial synchrony may not behave correctly in empirical distributed systems. For example, a set of popular theoretical models, which we call M_*, assume (eventual) upper bounds on message delay and relative process speeds, regardless of message size
and absolute process speeds. Empirical systems with bounded channel capacity and bandwidth cannot realize such assumptions either natively, or through algorithmic
constructions. Consequently, empirical deployment of the many M_*-based algorithms risks anomalous behavior.
As a result, we argue that real time is the wrong basis for such a theory. Instead, the appropriate foundation for partial synchrony is fairness: a time base measured
by counting events internal to the system, like the steps executed by the processes. By way of example, we redefine M_* models with fairness-based bounds and provide algorithmic techniques to implement fairness-based M_* models on a significant subset of the empirical systems. The proposed techniques use failure detectors — system
services that provide hints about process crashes — as intermediaries that preserve the fairness constraints native to empirical systems. In effect, algorithms that are correct in M_* models are now proved correct in such empirical systems as well.
Demonstrating our results requires solving three open problems. (1) We propose the first unified mathematical framework based on Timed I/O Automata to specify empirical systems, partially synchronous systems, and algorithms that execute within the aforementioned systems. (2) We show that crash tolerance capabilities of popular distributed systems can be denominated exclusively through fairness constraints. (3) We specify exemplar system models that identify the set of weakest system models to implement popular failure detectors
Liveness and Latency of Byzantine State-Machine Replication
Byzantine state-machine replication (SMR) ensures the consistency of replicated state in the presence of malicious replicas and lies at the heart of the modern blockchain technology. Byzantine SMR protocols often guarantee safety under all circumstances and liveness only under synchrony. However, guaranteeing liveness even under this assumption is nontrivial. So far we have lacked systematic ways of incorporating liveness mechanisms into Byzantine SMR protocols, which often led to subtle bugs. To close this gap, we introduce a modular framework to facilitate the design of provably live and efficient Byzantine SMR protocols. Our framework relies on a view abstraction generated by a special SMR synchronizer primitive to drive the agreement on command ordering. We present a simple formal specification of an SMR synchronizer and its bounded-space implementation under partial synchrony. We also apply our specification to prove liveness and analyze the latency of three Byzantine SMR protocols via a uniform methodology. In particular, one of these results yields what we believe is the first rigorous liveness proof for the algorithmic core of the seminal PBFT protocol
Byzantine fault-tolerant agreement protocols for wireless Ad hoc networks
Tese de doutoramento, Informática (Ciências da Computação), Universidade de Lisboa, Faculdade de Ciências, 2010.The thesis investigates the problem of fault- and intrusion-tolerant consensus
in resource-constrained wireless ad hoc networks. This is a fundamental
problem in distributed computing because it abstracts the need
to coordinate activities among various nodes. It has been shown to be a
building block for several other important distributed computing problems
like state-machine replication and atomic broadcast.
The thesis begins by making a thorough performance assessment of existing
intrusion-tolerant consensus protocols, which shows that the performance
bottlenecks of current solutions are in part related to their system
modeling assumptions. Based on these results, the communication failure
model is identified as a model that simultaneously captures the reality
of wireless ad hoc networks and allows the design of efficient protocols.
Unfortunately, the model is subject to an impossibility result stating that
there is no deterministic algorithm that allows n nodes to reach agreement
if more than n2 omission transmission failures can occur in a communication
step. This result is valid even under strict timing assumptions (i.e.,
a synchronous system).
The thesis applies randomization techniques in increasingly weaker variants
of this model, until an efficient intrusion-tolerant consensus protocol
is achieved. The first variant simplifies the problem by restricting the
number of nodes that may be at the source of a transmission failure at
each communication step. An algorithm is designed that tolerates f dynamic
nodes at the source of faulty transmissions in a system with a total
of n 3f + 1 nodes.
The second variant imposes no restrictions on the pattern of transmission
failures. The proposed algorithm effectively circumvents the Santoro-
Widmayer impossibility result for the first time. It allows k out of n nodes
to decide despite dn
2 e(nk)+k2 omission failures per communication
step. This algorithm also has the interesting property of guaranteeing
safety during arbitrary periods of unrestricted message loss.
The final variant shares the same properties of the previous one, but relaxes
the model in the sense that the system is asynchronous and that a
static subset of nodes may be malicious. The obtained algorithm, called
Turquois, admits f < n
3 malicious nodes, and ensures progress in communication
steps where dnf
2 e(n k f) + k 2. The algorithm is
subject to a comparative performance evaluation against other intrusiontolerant
protocols. The results show that, as the system scales, Turquois
outperforms the other protocols by more than an order of magnitude.Esta tese investiga o problema do consenso tolerante a faltas acidentais
e maliciosas em redes ad hoc sem fios. Trata-se de um problema fundamental
que captura a essência da coordenação em actividades envolvendo
vários nós de um sistema, sendo um bloco construtor de outros importantes
problemas dos sistemas distribuÃdos como a replicação de máquina
de estados ou a difusão atómica.
A tese começa por efectuar uma avaliação de desempenho a protocolos
tolerantes a intrusões já existentes na literatura. Os resultados mostram
que as limitações de desempenho das soluções existentes estão em parte
relacionadas com o seu modelo de sistema. Baseado nestes resultados, é
identificado o modelo de falhas de comunicação como um modelo que simultaneamente
permite capturar o ambiente das redes ad hoc sem fios e
projectar protocolos eficientes. Todavia, o modelo é restrito por um resultado
de impossibilidade que afirma não existir algoritmo algum que permita
a n nós chegaram a acordo num sistema que admita mais do que n2
transmissões omissas num dado passo de comunicação. Este resultado é
válido mesmo sob fortes hipóteses temporais (i.e., em sistemas sÃncronos)
A tese aplica técnicas de aleatoriedade em variantes progressivamente
mais fracas do modelo até ser alcançado um protocolo eficiente e tolerante
a intrusões. A primeira variante do modelo, de forma a simplificar
o problema, restringe o número de nós que estão na origem de transmissões
faltosas. É apresentado um algoritmo que tolera f nós dinâmicos na
origem de transmissões faltosas em sistemas com um total de n 3f + 1
nós.
A segunda variante do modelo não impõe quaisquer restrições no padrão
de transmissões faltosas. É apresentado um algoritmo que contorna efectivamente
o resultado de impossibilidade Santoro-Widmayer pela primeira
vez e que permite a k de n nós efectuarem progresso nos passos de comunicação
em que o número de transmissões omissas seja dn
2 e(n
k) + k 2. O algoritmo possui ainda a interessante propriedade de tolerar
perÃodos arbitrários em que o número de transmissões omissas seja
superior a .
A última variante do modelo partilha das mesmas caracterÃsticas da variante
anterior, mas com pressupostos mais fracos sobre o sistema. Em particular,
assume-se que o sistema é assÃncrono e que um subconjunto estático
dos nós pode ser malicioso. O algoritmo apresentado, denominado
Turquois, admite f < n
3 nós maliciosos e assegura progresso nos passos
de comunicação em que dnf
2 e(n k f) + k 2. O algoritmo é
sujeito a uma análise de desempenho comparativa com outros protocolos
na literatura. Os resultados demonstram que, à medida que o número de
nós no sistema aumenta, o desempenho do protocolo Turquois ultrapassa
os restantes em mais do que uma ordem de magnitude.FC
Towards Quantum Repeaters with Solid-State Qubits: Spin-Photon Entanglement Generation using Self-Assembled Quantum Dots
In this chapter we review the use of spins in optically-active InAs quantum
dots as the key physical building block for constructing a quantum repeater,
with a particular focus on recent results demonstrating entanglement between a
quantum memory (electron spin qubit) and a flying qubit (polarization- or
frequency-encoded photonic qubit). This is a first step towards demonstrating
entanglement between distant quantum memories (realized with quantum dots),
which in turn is a milestone in the roadmap for building a functional quantum
repeater. We also place this experimental work in context by providing an
overview of quantum repeaters, their potential uses, and the challenges in
implementing them.Comment: 51 pages. Expanded version of a chapter to appear in "Engineering the
Atom-Photon Interaction" (Springer-Verlag, 2015; eds. A. Predojevic and M. W.
Mitchell
DOTA: A Large-scale Dataset for Object Detection in Aerial Images
Object detection is an important and challenging problem in computer vision.
Although the past decade has witnessed major advances in object detection in
natural scenes, such successes have been slow to aerial imagery, not only
because of the huge variation in the scale, orientation and shape of the object
instances on the earth's surface, but also due to the scarcity of
well-annotated datasets of objects in aerial scenes. To advance object
detection research in Earth Vision, also known as Earth Observation and Remote
Sensing, we introduce a large-scale Dataset for Object deTection in Aerial
images (DOTA). To this end, we collect aerial images from different
sensors and platforms. Each image is of the size about 4000-by-4000 pixels and
contains objects exhibiting a wide variety of scales, orientations, and shapes.
These DOTA images are then annotated by experts in aerial image interpretation
using common object categories. The fully annotated DOTA images contains
instances, each of which is labeled by an arbitrary (8 d.o.f.)
quadrilateral To build a baseline for object detection in Earth Vision, we
evaluate state-of-the-art object detection algorithms on DOTA. Experiments
demonstrate that DOTA well represents real Earth Vision applications and are
quite challenging.Comment: Accepted to CVPR 201
Strategies of development and maintenance in supervision, control, synchronization, data acquisition and processing in light sources
Programa Oficial de Doutoramento en TecnoloxÃas da Información e as Comunicacións. 5032V01[Resumo]
Os aceleradores de partÃculas e fontes de luz sincrotrón, evolucionan constantemente para estar
na vangarda da tecnoloxÃa, levando os lÃmites cada vez mais lonxe para explorar novos
dominios e universos. Os sistemas de control son unha parte crucial desas instalacións
cientÃficas e buscan logra-la flexibilidade de manobra para poder facer experimentos moi
variados, con configuracións diferentes que engloban moitos tipos de detectores,
procedementos, mostras a estudar e contornas.
As propostas de experimento son cada vez máis ambiciosas e van sempre un paso por diante
do establecido. PrecÃsanse detectores cada volta máis rápidos e eficientes, con máis ancho de
banda e con máis resolución. Tamén é importante a operación simultánea de varios detectores
tanto escalares como mono ou bidimensionáis, con mecanismos de sincronización de precisión
que integren as singularidades de cada un.
Este traballo estuda as solucións existentes no campo dos sistemas de control e adquisición de
datos nos aceleradores de partÃculas e fontes de luz e raios X, ó tempo que explora novos
requisitos e retos no que respecta á sincronización e velocidade de adquisición de datos para
novos experimentos, a optimización do deseño, soporte, xestión de servizos e custos de
operación. Tamén se estudan diferentes solucións adaptadas a cada contorna.[Resumen] Los aceleradores de partÃculas y fuentes de luz sincrotrón, evolucionan constantemente para
estar en la vanguardia de la tecnologÃa, y poder explorar nuevos dominios. Los sistemas de
control son una parte fundamental de esas instalaciones cientÃficas y buscan lograr la máxima
flexibilidad para poder llevar a cabo experimentos más variados, con configuraciones
diferentes que engloban varios tipos de detectores, procedimientos, muestras a estudiar y
entornos.
Los experimentos se proponen cada vez más ambiciosos y en ocasiones más allá de los lÃmites
establecidos. Se necesitan detectores cada vez más rápidos y eficientes, con más resolución y
ancho de banda, que puedan sincronizarse simultáneamente con otros detectores tanto escalares
como mono y bidimensionales, integrando las singularidades de cada uno y homogeneizando
la adquisición de datos.
Este trabajo estudia los sistemas de control y adquisición de datos de aceleradores de partÃculas
y fuentes de luz y rayos X, y explora nuevos requisitos y retos en lo que respecta a la
sincronización y velocidad de adquisición de datos, optimización y costo-eficiencia en el
diseño, operación soporte, mantenimiento y gestión de servicios. También se estudian diferentes soluciones adaptadas a cada entorno.[Abstract]
Particle accelerators and photon sources are constantly evolving, attaining the cutting-edge
technologies to push the limits forward and explore new domains. The control systems are a crucial
part of these installations and are required to provide flexible solutions to the new
challenging experiments, with different kinds of detectors, setups, sample environments and
procedures.
Experiment proposals are more and more ambitious at each call and go often a step beyond the
capabilities of the instrumentation. Detectors shall be faster, with higher efficiency,
more resolution, more bandwidth and able to synchronize with other detectors of all kinds; scalars,
one or two-dimensional, taking into account their singularities and homogenizing the
data acquisition.
This work examines the control and data acquisition systems for particle accelerators and X- ray /
light sources and explores new requirements and challenges regarding synchronization and data
acquisition bandwidth, optimization and cost-efficiency in the design / operation / support. It
also studies different solutions depending on the environment
A Middleware Framework for Constraint-Based Deployment and Autonomic Management of Distributed Applications
We propose a middleware framework for deployment and subsequent autonomic
management of component-based distributed applications. An initial deployment
goal is specified using a declarative constraint language, expressing
constraints over aspects such as component-host mappings and component
interconnection topology. A constraint solver is used to find a configuration
that satisfies the goal, and the configuration is deployed automatically. The
deployed application is instrumented to allow subsequent autonomic management.
If, during execution, the manager detects that the original goal is no longer
being met, the satisfy/deploy process can be repeated automatically in order to
generate a revised deployment that does meet the goal.Comment: Submitted to Middleware 0
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