eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
1
580
10.4230/LIPIcs.OPODIS.2021
article
LIPIcs, Volume 217, OPODIS 2021, Complete Volume
Bramas, Quentin
1
https://orcid.org/0000-0003-0612-5616
Gramoli, Vincent
2
3
https://orcid.org/0000-0001-5632-8572
Milani, Alessia
4
University of Strasbourg, France
University of Sydney, Australia
EPFL, Switzerland
LIS UMR 7020 CNRS,Aix-Marseille University, France
LIPIcs, Volume 217, OPODIS 2021, Complete Volume
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021/LIPIcs.OPODIS.2021.pdf
LIPIcs, Volume 217, OPODIS 2021, Complete Volume
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
0:i
0:xvi
10.4230/LIPIcs.OPODIS.2021.0
article
Front Matter, Table of Contents, Preface, Conference Organization
Bramas, Quentin
1
https://orcid.org/0000-0003-0612-5616
Gramoli, Vincent
2
3
https://orcid.org/0000-0001-5632-8572
Milani, Alessia
4
University of Strasbourg, France
University of Sydney, Australia
EPFL, Switzerland
LIS UMR 7020 CNRS,Aix-Marseille University, France
Front Matter, Table of Contents, Preface, Conference Organization
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.0/LIPIcs.OPODIS.2021.0.pdf
Front Matter
Table of Contents
Preface
Conference Organization
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
1:1
1:1
10.4230/LIPIcs.OPODIS.2021.1
article
Distributed Algorithms: A Challenging Playground for Model Checking (Invited Talk)
Bertrand, Nathalie
1
https://orcid.org/0000-0002-9957-5394
Univ Rennes, Inria, CNRS, IRISA, France
Distributed computing is increasingly spreading, in advanced technological applications as well as in our daily life. Failures in distributed algorithms can have important human and financial consequences, so that is is crucial to develop rigorous techniques to verify their correctness. Model checking is a model-based approach to formal verification, dating back the 80’s. It has been successfully applied first to hardware, and later to software verification.
Distributed computing raises new challenges for the model checking community, and calls for the development of new verification techniques and tools. In particular, the parameterized verification paradigm is nowadays blooming to help proving automatically the correctness of distributed algorithms. In this invited talk, we present recent parameterized verification developments to automatically prove properties of some classical distributed algorithms.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.1/LIPIcs.OPODIS.2021.1.pdf
Verification
Distributed algorithms
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
2:1
2:1
10.4230/LIPIcs.OPODIS.2021.2
article
Accountable Distributed Computing (Invited Talk)
Kuznetsov, Petr
1
LTCI, Télécom Paris, Institut Polytechnique de Paris, France
There are two major ways to deal with failures in distributed computing: fault-tolerance and accountability. Fault-tolerance intends to anticipate failures by investing into replication and synchronization, so that the system’s correctness is not affected by faulty components. In contrast, accountability enables detecting failures a posteriori and raising undeniable evidences against faulty components.
In this talk, we discuss how accountability can be achieved, both in generic and application-specific ways. We begin with an overview of fault detection mechanisms used in benign, crash-prone system, with a focus on the weakest failure detector question. We then consider the fault detection problem in systems with general, Byzantine failures and explore which classes of misbehavior can be detected and which - cannot. We then study the mechanism of application-specific accountability that, intuitively, only accounts for instances of misbehavior that affect particular correctness criteria. Finally, we discuss how fault detection can be combined with reconfiguration, opening an avenue of "self-healing" systems that seamlessly replace faulty system components with correct ones.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.2/LIPIcs.OPODIS.2021.2.pdf
Fault-tolerance
fault detection
accountability
application-specific
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
3:1
3:1
10.4230/LIPIcs.OPODIS.2021.3
article
A Fresh Look at the Design and Implementation of Communication Paradigms (Invited Talk)
van Renesse, Robbert
1
https://orcid.org/0000-0003-3598-0283
Cornell University, Ithaca, NY, USA
Datacenter applications consist of many communicating components and evolve organically as requirements develop over time. In this talk I will present two projects that try to support such organic growth. The first project, Escher, recognizes that components of a distributed systems may themselves be distributed systems. Escher introduces a communication abstraction that hides the internals of a distributed component, and in particular how to communicate with it, from other components. Using Escher, a replicated server can invoke another replicated server without either server having to even know that the servers are replicated. The second project, Scalog, presents a datacenter scale totally ordered logging service. Logs are increasingly a central component in many datacenter applications, but log configurations can lead to significant hiccups in the performance of those applications. Scalog has seamless reconfiguration operations that allow it to scale up and down without any downtime.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.3/LIPIcs.OPODIS.2021.3.pdf
Distributed systems
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
4:1
4:17
10.4230/LIPIcs.OPODIS.2021.4
article
Arbitrarily Accurate Aggregation Scheme for Byzantine SGD
Maurer, Alexandre
1
School of Computer Science, UM6P, Ben Guerir, Morocco
A very common optimization technique in Machine Learning is Stochastic Gradient Descent (SGD). SGD can easily be distributed: several workers try to estimate the gradient of a loss function, and a central parameter server gathers these estimates. When all workers behave correctly, the more workers we have, the more accurate the gradient estimate is. We call this the Arbitrary Aggregation Accuracy (AAA) property.
However, in practice, some workers may be Byzantine (i.e., have an arbitrary behavior). Interestingly, when a fixed fraction of workers is assumed to be Byzantine (e.g. 20%), no existing aggregation scheme has the AAA property. In this paper, we propose the first aggregation scheme that has this property despite a fixed fraction of Byzantine workers (less than 50%). We theoretically prove this property, and then illustrate it with simulations.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.4/LIPIcs.OPODIS.2021.4.pdf
distributed machine learning
Byzantine failures
stochastic gradient descent
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
5:1
5:20
10.4230/LIPIcs.OPODIS.2021.5
article
Good-Case and Bad-Case Latency of Unauthenticated Byzantine Broadcast: A Complete Categorization
Abraham, Ittai
1
Ren, Ling
2
Xiang, Zhuolun
2
VMware Research, Herzliya, Israel
University of Illinois at Urbana-Champaign, IL, USA
This paper studies the good-case latency of unauthenticated Byzantine fault-tolerant broadcast, which measures the time it takes for all non-faulty parties to commit given a non-faulty broadcaster. For both asynchrony and synchrony, we show that n ≥ 4f is the tight resilience threshold that separates good-case 2 rounds and 3 rounds. For asynchronous Byzantine reliable broadcast (BRB), we also investigate the bad-case latency for all non-faulty parties to commit when the broadcaster is faulty but some non-faulty party commits. We provide matching upper and lower bounds on the resilience threshold of bad-case latency for BRB protocols with optimal good-case latency of 2 rounds. In particular, we show 2 impossibility results and propose 4 asynchronous BRB protocols.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.5/LIPIcs.OPODIS.2021.5.pdf
Byzantine broadcast
asynchrony
synchrony
latency
good-case
optimal
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
6:1
6:19
10.4230/LIPIcs.OPODIS.2021.6
article
On Finality in Blockchains
Anceaume, Emmanuelle
1
https://orcid.org/0000-0003-4158-149X
Del Pozzo, Antonella
2
https://orcid.org/0000-0003-0913-2141
Rieutord, Thibault
2
Tucci-Piergiovanni, Sara
2
https://orcid.org/0000-0001-9738-9021
CNRS, Univ Rennes, Inria, IRISA, Rennes, France
CEA-List, Université Paris-Saclay, Palaiseau, France
This paper focuses on blockchain finality, which refers to the time when it becomes impossible to remove a block that has previously been appended to the blockchain. Blockchain finality can be deterministic or probabilistic, immediate or eventual. To favor availability against consistency in the face of partitions, most blockchains only offer probabilistic eventual finality: blocks may be revoked after being appended to the blockchain, yet with decreasing probability as they sink deeper into the chain. Other blockchains favor consistency by leveraging the immediate finality of Consensus - a block appended is never revoked - at the cost of additional synchronization.
The quest for "good" deterministic finality properties for blockchains is still in its infancy, though. Our motivation is to provide a thorough study of several possible deterministic finality properties and explore their solvability. This is achieved by introducing the notion of bounded revocation, which informally says that the number of blocks that can be revoked from the current blockchain is bounded. Based on the requirements we impose on this revocation number, we provide reductions between different forms of eventual finality, Consensus and Eventual Consensus. From these reductions, we show some related impossibility results in presence of Byzantine processes, and provide non-trivial results. In particular, we provide an algorithm that solves a weak form of eventual finality in an asynchronous system in presence of an unbounded number of Byzantine processes. We also provide an algorithm that solves eventual finality with a bounded revocation number in an eventually synchronous environment in presence of less than half of Byzantine processes. The simplicity of the arguments should better guide blockchain designs and link them to clear formal properties of finality.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.6/LIPIcs.OPODIS.2021.6.pdf
Blockchain
consistency properties
Byzantine tolerant implementations
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
7:1
7:29
10.4230/LIPIcs.OPODIS.2021.7
article
Twins: BFT Systems Made Robust
Bano, Shehar
1
Sonnino, Alberto
1
Chursin, Andrey
2
Perelman, Dmitri
2
Li, Zekun
2
Ching, Avery
2
Malkhi, Dahlia
2
Facebook Novi, London, UK
Facebook Novi, Menlo Park, CA, USA
This paper presents Twins, an automated unit test generator of Byzantine attacks. Twins implements three types of Byzantine behaviors: (i) leader equivocation, (ii) double voting, and (iii) losing internal state such as forgetting "locks" guarding voted values. To emulate interesting attacks by a Byzantine node, it instantiates twin copies of the node instead of one, giving both twins the same identities and network credentials. To the rest of the system, the twins appear indistinguishable from a single node behaving in a "questionable" manner. Twins can systematically generate Byzantine attack scenarios at scale, execute them in a controlled manner, and examine their behavior. Twins scenarios iterate over protocol rounds and vary the communication patterns among nodes. Twins runs in a production setting within DiemBFT where it can execute 44M Twins-generated scenarios daily. Whereas the system at hand did not manifest errors, subtle safety bugs that were deliberately injected for the purpose of validating the implementation of Twins itself were exposed within minutes. Twins can prevent developers from regressing correctness when updating the codebase, introducing new features, or performing routine maintenance tasks. Twins only requires a thin wrapper over DiemBFT, we thus envision other systems using it. Building on this idea, one new attack and several known attacks against other BFT protocols were materialized as Twins scenarios. In all cases, the target protocols break within fewer than a dozen protocol rounds, hence it is realistic for the Twins approach to expose the problems.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.7/LIPIcs.OPODIS.2021.7.pdf
Distributed Systems
Byzantine Fault Tolerance
Real-World Deployment
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
8:1
8:19
10.4230/LIPIcs.OPODIS.2021.8
article
Near-Optimal Dispersion on Arbitrary Anonymous Graphs
Kshemkalyani, Ajay D.
1
https://orcid.org/0000-0003-2451-7306
Sharma, Gokarna
2
https://orcid.org/0000-0002-4930-4609
University of Illinois at Chicago, IL, USA
Kent State University, OH, USA
Given an undirected, anonymous, port-labeled graph of n memory-less nodes, m edges, and degree Δ, we consider the problem of dispersing k ≤ n robots (or tokens) positioned initially arbitrarily on one or more nodes of the graph to exactly k different nodes of the graph, one on each node. The objective is to simultaneously minimize time to achieve dispersion and memory requirement at each robot. If all k robots are positioned initially on a single node, depth first search (DFS) traversal solves this problem in O(min{m,kΔ}) time with Θ(log(k+Δ)) bits at each robot. However, if robots are positioned initially on multiple nodes, the best previously known algorithm solves this problem in O(min{m,kΔ}⋅ log 𝓁) time storing Θ(log(k+Δ)) bits at each robot, where 𝓁 ≤ k/2 is the number of multiplicity nodes in the initial configuration. In this paper, we present a novel multi-source DFS traversal algorithm solving this problem in O(min{m,kΔ}) time with Θ(log(k+Δ)) bits at each robot, improving the time bound of the best previously known algorithm by O(log 𝓁) and matching asymptotically the single-source DFS traversal bounds. This is the first algorithm for dispersion that is optimal in both time and memory in arbitrary anonymous graphs of constant degree, Δ = O(1). Furthermore, the result holds in both synchronous and asynchronous settings.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.8/LIPIcs.OPODIS.2021.8.pdf
Distributed algorithms
Multi-agent systems
Mobile robots
Local communication
Dispersion
Exploration
Time and memory complexity
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
9:1
9:17
10.4230/LIPIcs.OPODIS.2021.9
article
Asynchronous Gathering in a Torus
Kamei, Sayaka
1
Lamani, Anissa
2
Ooshita, Fukuhito
3
Tixeuil, Sébastien
4
Wada, Koichi
5
Hiroshima University, Japan
Strasbourg University, CNRS, ICUBE, France
Nara Institute of Science and Technology, Japan
Sorbonne University, CNRS, LIP6, France
Hosei University, Tokyo, Japan
We consider the gathering problem for asynchronous and oblivious robots that cannot communicate explicitly with each other but are endowed with visibility sensors that allow them to see the positions of the other robots.
Most investigations on the gathering problem on the discrete universe are done on ring shaped networks due to the number of symmetric configurations. We extend in this paper the study of the gathering problem on torus shaped networks assuming robots endowed with local weak multiplicity detection. That is, robots cannot make the difference between nodes occupied by only one robot from those occupied by more than one robot unless it is their current node. Consequently, solutions based on creating a single multiplicity node as a landmark for the gathering cannot be used. We present in this paper a deterministic algorithm that solves the gathering problem starting from any rigid configuration on an asymmetric unoriented torus shaped network.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.9/LIPIcs.OPODIS.2021.9.pdf
Autonomous distributed systems
Robots gathering
Torus
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
10:1
10:20
10.4230/LIPIcs.OPODIS.2021.10
article
Pattern Formation by Robots with Inaccurate Movements
Bose, Kaustav
1
https://orcid.org/0000-0003-3579-1941
Das, Archak
2
https://orcid.org/0000-0002-1630-3052
Sau, Buddhadeb
2
https://orcid.org/0000-0001-7008-6135
Advanced Computing and Microelectronics Unit, Indian Statistical Institute, Kolkata, India
Department of Mathematics, Jadavpur University, Kolkata, India
Arbitrary Pattern Formation is a fundamental problem in autonomous mobile robot systems. The problem asks to design a distributed algorithm that moves a team of autonomous, anonymous and identical mobile robots to form any arbitrary pattern F given as input. In this paper, we study the problem for robots whose movements can be inaccurate. Our movement model assumes errors in both direction and extent of the intended movement. Forming the given pattern exactly is not possible in this setting. So we require that the robots must form a configuration which is close to the given pattern F. We call this the Approximate Arbitrary Pattern Formation problem. With no agreement in coordinate system, the problem is unsolvable, even by fully synchronous robots, if the initial configuration 1) has rotational symmetry and there is no robot at the center of rotation or 2) has reflectional symmetry and there is no robot on the reflection axis. From all other initial configurations, we solve the problem by 1) oblivious, silent and semi-synchronous robots and 2) oblivious, asynchronous robots that can communicate using externally visible lights.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.10/LIPIcs.OPODIS.2021.10.pdf
Distributed Algorithm
Mobile Robots
Movement Error
Approximate Arbitrary Pattern Formation
Look-Compute-Move
Minimum Enclosing Circle
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
11:1
11:23
10.4230/LIPIcs.OPODIS.2021.11
article
Near-Shortest Path Routing in Hybrid Communication Networks
Coy, Sam
1
Czumaj, Artur
1
Feldmann, Michael
2
Hinnenthal, Kristian
2
Kuhn, Fabian
3
Scheideler, Christian
2
Schneider, Philipp
3
Struijs, Martijn
4
University of Warwick, Coventry, UK
Paderborn University, Germany
University of Freiburg, Germany
TU Eindhoven, The Netherlands
Hybrid networks, i.e., networks that leverage different means of communication, become ever more widespread. To allow theoretical study of such networks, [Augustine et al., SODA'20] introduced the HYBRID model, which is based on the concept of synchronous message passing and uses two fundamentally different principles of communication: a local mode, which allows every node to exchange one message per round with each neighbor in a local communication graph; and a global mode where any pair of nodes can exchange messages, but only few such exchanges can take place per round. A sizable portion of the previous research for the HYBRID model revolves around basic communication primitives and computing distances or shortest paths in networks. In this paper, we extend this study to a related fundamental problem of computing compact routing schemes for near-shortest paths in the local communication graph. We demonstrate that, for the case where the local communication graph is a unit-disc graph with n nodes that is realized in the plane and has no radio holes, we can deterministically compute a routing scheme that has constant stretch and uses labels and local routing tables of size O(log n) bits in only O(log n) rounds.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.11/LIPIcs.OPODIS.2021.11.pdf
Hybrid networks
overlay networks
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
12:1
12:21
10.4230/LIPIcs.OPODIS.2021.12
article
Efficient Assignment of Identities in Anonymous Populations
Gąsieniec, Leszek
1
https://orcid.org/0000-0003-1809-9814
Jansson, Jesper
2
https://orcid.org/0000-0001-6859-8932
Levcopoulos, Christos
3
https://orcid.org/0000-0003-0983-7862
Lingas, Andrzej
3
https://orcid.org/0000-0003-4998-9844
Department of Computer Science, University of Liverpool, UK
Graduate School of Informatics, Kyoto University, Japan
Department of Computer Science, Lund University, Sweden
We consider the fundamental problem of assigning distinct labels to agents in the probabilistic model of population protocols. Our protocols operate under the assumption that the size n of the population is embedded in the transition function. Their efficiency is expressed in terms of the number of states utilized by agents, the size of the range from which the labels are drawn, and the expected number of interactions required by our solutions. Our primary goal is to provide efficient protocols for this fundamental problem complemented with tight lower bounds in all the three aspects. W.h.p. (with high probability), our labeling protocols are silent, i.e., eventually each agent reaches its final state and remains in it forever, and they are safe, i.e., never update the label assigned to any single agent. We first present a silent w.h.p. and safe labeling protocol that draws labels from the range [1,2n]. Both the number of interactions required and the number of states used by the protocol are asymptotically optimal, i.e., O(n log n) w.h.p. and O(n), respectively. Next, we present a generalization of the protocol, where the range of assigned labels is [1,(1+ε) n]. The generalized protocol requires O(n log n / ε) interactions in order to complete the assignment of distinct labels from [1,(1+ε) n] to the n agents, w.h.p. It is also silent w.h.p. and safe, and uses (2+ε)n+O(n^c) states, for any positive c < 1. On the other hand, we consider the so-called pool labeling protocols that include our fast protocols. We show that the expected number of interactions required by any pool protocol is ≥ (n²)/(r+1), when the labels range is 1,… , n+r < 2n. Furthermore, we provide a protocol which uses only n+5√ n +O(n^c) states, for any c < 1, and draws labels from the range 1,… ,n. The expected number of interactions required by the protocol is O(n³). Once a unique leader is elected it produces a valid labeling and it is silent and safe. On the other hand, we show that (even if a unique leader is given in advance) any silent protocol that produces a valid labeling and is safe with probability > 1-(1/n), uses ≥ n+√{(n-1)/2}-1 states. Hence, our protocol is almost state-optimal. We also present a generalization of the protocol to include a trade-off between the number of states and the expected number of interactions. Finally, we show that for any silent and safe labeling protocol utilizing n+t < 2n states, the expected number of interactions required to achieve a valid labeling is ≥ (n²)/(t+1).
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.12/LIPIcs.OPODIS.2021.12.pdf
population protocol
state efficiency
time efficiency
one-way epidemics
leader election
agent identities
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
13:1
13:19
10.4230/LIPIcs.OPODIS.2021.13
article
Population Protocols for Graph Class Identification Problems
Yasumi, Hiroto
1
Ooshita, Fukuhito
1
Inoue, Michiko
1
Nara Institute of Science and Technology, Japan
In this paper, we focus on graph class identification problems in the population protocol model. A graph class identification problem aims to decide whether a given communication graph is in the desired class (e.g. whether the given communication graph is a ring graph). Angluin et al. proposed graph class identification protocols with directed graphs and designated initial states under global fairness [Angluin et al., DCOSS2005]. We consider graph class identification problems for undirected graphs on various assumptions such as initial states of agents, fairness of the execution, and initial knowledge of agents. In particular, we focus on lines, rings, k-regular graphs, stars, trees, and bipartite graphs. With designated initial states, we propose graph class identification protocols for k-regular graphs and trees under global fairness, and propose a graph class identification protocol for stars under weak fairness. Moreover, we show that, even if agents know the number of agents n, there is no graph class identification protocol for lines, rings, k-regular graphs, trees, or bipartite graphs under weak fairness, and no graph class identification for lines, rings, k-regular graphs, stars, trees, or bipartite graphs with arbitrary initial states.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.13/LIPIcs.OPODIS.2021.13.pdf
population protocol
graph class identification
distributed protocol
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
14:1
14:18
10.4230/LIPIcs.OPODIS.2021.14
article
Fast Graphical Population Protocols
Alistarh, Dan
1
Gelashvili, Rati
2
Rybicki, Joel
1
https://orcid.org/0000-0002-6432-6646
IST Austria, Klosterneuburg, Austria
Novi Research, Menlo Park, CA, USA
Let G be a graph on n nodes. In the stochastic population protocol model, a collection of n indistinguishable, resource-limited nodes collectively solve tasks via pairwise interactions. In each interaction, two randomly chosen neighbors first read each other’s states, and then update their local states. A rich line of research has established tight upper and lower bounds on the complexity of fundamental tasks, such as majority and leader election, in this model, when G is a clique. Specifically, in the clique, these tasks can be solved fast, i.e., in n polylog n pairwise interactions, with high probability, using at most polylog n states per node.
In this work, we consider the more general setting where G is an arbitrary regular graph, and present a technique for simulating protocols designed for fully-connected networks in any connected regular graph. Our main result is a simulation that is efficient on many interesting graph families: roughly, the simulation overhead is polylogarithmic in the number of nodes, and quadratic in the conductance of the graph. As a sample application, we show that, in any regular graph with conductance φ, both leader election and exact majority can be solved in φ^{-2} ⋅ n polylog n pairwise interactions, with high probability, using at most φ^{-2} ⋅ polylog n states per node. This shows that there are fast and space-efficient population protocols for leader election and exact majority on graphs with good expansion properties. We believe our results will prove generally useful, as they allow efficient technology transfer between the well-mixed (clique) case, and the under-explored spatial setting.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.14/LIPIcs.OPODIS.2021.14.pdf
population protocols
leader election
exact majority
graphs
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
15:1
15:18
10.4230/LIPIcs.OPODIS.2021.15
article
Beyond Distributed Subgraph Detection: Induced Subgraphs, Multicolored Problems and Graph Parameters
Nikabadi, Amir
1
Korhonen, Janne H.
2
ENS de Lyon, France
IST Austria, Klosterneuburg, Austria
Subgraph detection has recently been one of the most studied problems in the CONGEST model of distributed computing. In this work, we study the distributed complexity of problems closely related to subgraph detection, mainly focusing on induced subgraph detection. The main line of this work presents lower bounds and parameterized algorithms w.r.t structural parameters of the input graph:
- On general graphs, we give unconditional lower bounds for induced detection of cycles and patterns of treewidth 2 in CONGEST. Moreover, by adapting reductions from centralized parameterized complexity, we prove lower bounds in CONGEST for detecting patterns with a 4-clique, and for induced path detection conditional on the hardness of triangle detection in the congested clique.
- On graphs of bounded degeneracy, we show that induced paths can be detected fast in CONGEST using techniques from parameterized algorithms, while detecting cycles and patterns of treewidth 2 is hard.
- On graphs of bounded vertex cover number, we show that induced subgraph detection is easy in CONGEST for any pattern graph. More specifically, we adapt a centralized parameterized algorithm for a more general maximum common induced subgraph detection problem to the distributed setting. In addition to these induced subgraph detection results, we study various related problems in the CONGEST and congested clique models, including for multicolored versions of subgraph-detection-like problems.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.15/LIPIcs.OPODIS.2021.15.pdf
distributed algorithms
parameterized distributed complexity
CONGEST model
induced subgraph detection
graph parameters
lower bounds
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
16:1
16:17
10.4230/LIPIcs.OPODIS.2021.16
article
An Improved Random Shift Algorithm for Spanners and Low Diameter Decompositions
Forster, Sebastian
1
https://orcid.org/0000-0002-2191-3381
Grösbacher, Martin
1
de Vos, Tijn
1
https://orcid.org/0000-0002-1417-6387
University of Salzburg, Austria
Spanners have been shown to be a powerful tool in graph algorithms. Many spanner constructions use a certain type of clustering at their core, where each cluster has small diameter and there are relatively few spanner edges between clusters. In this paper, we provide a clustering algorithm that, given k ≥ 2, can be used to compute a spanner of stretch 2k-1 and expected size O(n^{1+1/k}) in k rounds in the CONGEST model. This improves upon the state of the art (by Elkin, and Neiman [TALG'19]) by making the bounds on both running time and stretch independent of the random choices of the algorithm, whereas they only hold with high probability in previous results. Spanners are used in certain synchronizers, thus our improvement directly carries over to such synchronizers. Furthermore, for keeping the total number of inter-cluster edges small in low diameter decompositions, our clustering algorithm provides the following guarantees. Given β ∈ (0,1], we compute a low diameter decomposition with diameter bound O((log n)/β) such that each edge e ∈ E is an inter-cluster edge with probability at most β⋅ w(e) in O((log n)/β) rounds in the CONGEST model. Again, this improves upon the state of the art (by Miller, Peng, and Xu [SPAA'13]) by making the bounds on both running time and diameter independent of the random choices of the algorithm, whereas they only hold with high probability in previous results.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.16/LIPIcs.OPODIS.2021.16.pdf
Spanner
low diameter decomposition
synchronizer
distributed graph algorithms
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
17:1
17:20
10.4230/LIPIcs.OPODIS.2021.17
article
Distributed CONGEST Approximation of Weighted Vertex Covers and Matchings
Faour, Salwa
1
Fuchs, Marc
1
Kuhn, Fabian
1
University of Freiburg, Germany
We provide CONGEST model algorithms for approximating the minimum weighted vertex cover and the maximum weighted matching problem. For bipartite graphs, we show that a (1+ε)-approximate weighted vertex cover can be computed deterministically in poly((log n)/ε) rounds. This generalizes a corresponding result for the unweighted vertex cover problem shown in [Faour, Kuhn; OPODIS '20]. Moreover, we show that in general weighted graph families that are closed under taking subgraphs and in which we can compute an independent set of weight at least λ⋅ w(V) (where w(V) denotes the total weight of all nodes) in polylogarithmic time in the CONGEST model, one can compute a (2-2λ +ε)-approximate weighted vertex cover in poly((log n)/ε) rounds in the CONGEST model. Our result in particular implies that in graphs of arboricity a, one can compute a (2-1/a+ε)-approximate weighted vertex cover problem in poly((log n)/ε) rounds in the CONGEST model.
For maximum weighted matchings, we show that a (1-ε)-approximate solution can be computed deterministically in time 2^{O(1/ε)}⋅ polylog n in the CONGEST model. We also provide a randomized algorithm that with arbitrarily good constant probability succeeds in computing a (1-ε)-approximate weighted matching in time 2^{O(1/ε)}⋅ polylog(Δ W)⋅ log^* n, where W denotes the ratio between the largest and the smallest edge weight. Our algorithm generalizes results of [Lotker, Patt-Shamir, Pettie; SPAA '08] and [Bar-Yehuda, Hillel, Ghaffari, Schwartzman; PODC '17], who gave 2^{O(1/ε)}⋅ log n and 2^{O(1/ε)}⋅ (logΔ)/(log logΔ)-round randomized approximations for the unweighted matching problem.
Finally, we show that even in the LOCAL model and in bipartite graphs of degree ≤ 3, if ε < ε₀ for some constant ε₀ > 0, then computing a (1+ε)-approximation for the unweighted minimum vertex cover problem requires Ω((log n)/ε) rounds. This generalizes a result of [Göös, Suomela; DISC '12], who showed that computing a (1+ε₀)-approximation in such graphs requires Ω(log n) rounds.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.17/LIPIcs.OPODIS.2021.17.pdf
distributed graph algorithms
minimum weighted vertex cover
maximum weighted matching
distributed optimization
CONGEST model
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
18:1
18:23
10.4230/LIPIcs.OPODIS.2021.18
article
Improved Distributed Fractional Coloring Algorithms
Balliu, Alkida
1
Kuhn, Fabian
2
Olivetti, Dennis
1
Gran Sasso Science Institute, L'Aquila, Italy
University of Freiburg, Germany
We prove new bounds on the distributed fractional coloring problem in the LOCAL model. A fractional c-coloring of a graph G = (V,E) is a fractional covering of the nodes of G with independent sets such that each independent set I of G is assigned a fractional value λ_I ∈ [0,1]. The total value of all independent sets of G is at most c, and for each node v ∈ V, the total value of all independent sets containing v is at least 1. Equivalently, fractional c-colorings can also be understood as multicolorings as follows. For some natural numbers p and q such that p/q ≤ c, each node v is assigned a set of at least q colors from {1,…,p} such that adjacent nodes are assigned disjoint sets of colors. The minimum c for which a fractional c-coloring of a graph G exists is called the fractional chromatic number χ_f(G) of G.
Recently, [Bousquet, Esperet, and Pirot; SIROCCO '21] showed that for any constant ε > 0, a fractional (Δ+ε)-coloring can be computed in Δ^{O(Δ)} + O(Δ⋅log^* n) rounds. We show that such a coloring can be computed in only O(log² Δ) rounds, without any dependency on n.
We further show that in O((log n)/ε) rounds, it is possible to compute a fractional (1+ε)χ_f(G)-coloring, even if the fractional chromatic number χ_f(G) is not known. That is, the fractional coloring problem can be approximated arbitrarily well by an efficient algorithm in the LOCAL model. For the standard coloring problem, it is only known that an O((log n)/(log log n))-approximation can be computed in polylogarithmic time in the LOCAL model. We also show that our distributed fractional coloring approximation algorithm is best possible. We show that in trees, which have fractional chromatic number 2, computing a fractional (2+ε)-coloring requires at least Ω((log n)/ε) rounds.
We finally study fractional colorings of regular grids. In [Bousquet, Esperet, and Pirot; SIROCCO '21], it is shown that in regular grids of bounded dimension, a fractional (2+ε)-coloring can be computed in time O(log^* n). We show that such a coloring can even be computed in O(1) rounds in the LOCAL model.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.18/LIPIcs.OPODIS.2021.18.pdf
distributed graph algorithms
distributed coloring
locality
fractional coloring
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
19:1
19:17
10.4230/LIPIcs.OPODIS.2021.19
article
Distributed Recoloring of Interval and Chordal Graphs
Bousquet, Nicolas
1
https://orcid.org/0000-0003-0170-0503
Feuilloley, Laurent
1
https://orcid.org/0000-0002-3994-0898
Heinrich, Marc
2
https://orcid.org/0000-0003-4546-2359
Rabie, Mikaël
3
https://orcid.org/0000-0001-6782-7625
Univ. Lyon, Université Lyon 1, LIRIS UMR CNRS 5205, F-69621, Lyon, France
University of Leeds, UK
Université de Paris, CNRS, IRIF, F-75013, Paris, France
One of the fundamental and most-studied algorithmic problems in distributed computing on networks is graph coloring, both in bounded-degree and in general graphs. Recently, the study of this problem has been extended in two directions. First, the problem of recoloring, that is computing an efficient transformation between two given colorings (instead of computing a new coloring), has been considered, both to model radio network updates, and as a useful subroutine for coloring. Second, as it appears that general graphs and bounded-degree graphs do not model real networks very well (with, respectively, pathological worst-case topologies and too strong assumptions), coloring has been studied in more specific graph classes. In this paper, we study the intersection of these two directions: distributed recoloring in two relevant graph classes, interval and chordal graphs.
More formally, the question of recoloring a graph is as follows: we are given a network, an input coloring α and a target coloring β, and we want to find a schedule of colorings to reach β starting from α. In a distributed setting, the schedule needs to be found within the LOCAL model, where nodes communicate with their direct neighbors synchronously. The question we want to answer is: how many rounds of communication {are} needed to produce a schedule, and what is the length of this schedule?
In the case of interval and chordal graphs, we prove that, if we have less than 2ω colors, ω being the size of the largest clique, extra colors will be needed in the intermediate colorings. For interval graphs, we produce a schedule after O(poly(Δ)log*n) rounds of communication, and for chordal graphs, we need O(ω²Δ²log n) rounds to get one.
Our techniques also improve classic coloring algorithms. Namely, we get ω+1-colorings of interval graphs in O(ωlog*n) rounds and of chordal graphs in O(ωlog n) rounds, which improves on previous known algorithms that use ω+2 colors for the same running times.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.19/LIPIcs.OPODIS.2021.19.pdf
Distributed coloring
distributed recoloring
interval graphs
chordal graphs
intersection graphs
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
20:1
20:25
10.4230/LIPIcs.OPODIS.2021.20
article
Non-Blocking Dynamic Unbounded Graphs with Worst-Case Amortized Bounds
Chatterjee, Bapi
1
https://orcid.org/0000-0002-2742-4028
Peri, Sathya
2
https://orcid.org/0000-0002-3471-7929
Sa, Muktikanta
3
https://orcid.org/0000-0002-7070-8210
Manogna, Komma
2
Indraprastha Institute of Information Technology Delhi, India
Indian Institute of Technology Hyderabad, India
Télécom SudParis - Institut Polytechnique de Paris, France
Today’s graph-based analytics tasks in domains such as blockchains, social networks, biological networks, and several others demand real-time data updates at high speed. The real-time updates are efficiently ingested if the data structure naturally supports dynamic addition and removal of both edges and vertices. These dynamic updates are best facilitated by concurrency in the underlying data structure. Unfortunately, the existing dynamic graph frameworks broadly refurbish the static processing models using approaches such as versioning and incremental computation. Consequently, they carry their original design traits such as high memory footprint and batch processing that do not honor the real-time changes. At the same time, multi-core processors-a natural setting for concurrent data structures-are now commonplace, and the analytics tasks are moving closer to data sources over lightweight devices. Thus, exploring a fresh design approach for real-time graph analytics is significant.
This paper reports a novel concurrent graph data structure that provides breadth-first search, single-source shortest-path, and betweenness centrality with concurrent dynamic updates of both edges and vertices. We evaluate the proposed data structure theoretically - by an amortized analysis - and experimentally via a C++ implementation. The experimental results show that (a) our algorithm outperforms the current state-of-the-art by a throughput speed-up of up to three orders of magnitude in several cases, and (b) it offers up to 80x lighter memory-footprint compared to existing methods. The experiments include several counterparts: Stinger, Ligra and GraphOne. We prove that the presented concurrent algorithms are non-blocking and linearizable.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.20/LIPIcs.OPODIS.2021.20.pdf
concurrent data structure
linearizability
non-blocking
directed graph
breadth-first search
single-source shortest-path
betweenness centrality
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
21:1
21:22
10.4230/LIPIcs.OPODIS.2021.21
article
Explicit Space-Time Tradeoffs for Proof Labeling Schemes in Graphs with Small Separators
Fischer, Orr
1
Oshman, Rotem
1
Shamir, Dana
1
Tel-Aviv University, Israel
In distributed verification, our goal is to verify that the network configuration satisfies some desired property, using pre-computed information stored at each network node. This is formally modeled as a proof labeling scheme (PLS): a prover assigns to each node a certificate, and then the nodes exchange their certificates with their neighbors and decide whether to accept or reject the configuration. Subsequent work has shown that in some specific cases, allowing more rounds of communication - so that nodes can communicate further across the network - can yield shorter certificates, trading off the space required to store the certificate against the time required for verification. Such tradeoffs were previously known for trees, cycles, and grids, or for proof labeling schemes where all nodes receive the same certificate.
In this work we show that in large classes of graphs, every one-round PLS can be transformed into a multi-round PLS with shorter certificates. We give two constructions: given a 1-round PLS with certificates of 𝓁 bits, in graphs families with balanced edge separators of size s(n), we construct a t-round PLS with certificates of size Õ(𝓁 ⋅ s(n) / t), and in graph families with an excluded minor and maximum degree Δ, we construct a t-round PLS with certificates of size Õ(𝓁 ⋅ Δ / √t). Our constructions are explicit, and we use erasure codes to exploit the larger neighborhood viewed by each node in a t-round PLS.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.21/LIPIcs.OPODIS.2021.21.pdf
proof-labeling schemes
space-time tradeoffs
families with excluded minor
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
22:1
22:17
10.4230/LIPIcs.OPODIS.2021.22
article
Local Certification of Graph Decompositions and Applications to Minor-Free Classes
Bousquet, Nicolas
1
https://orcid.org/0000-0003-0170-0503
Feuilloley, Laurent
1
https://orcid.org/0000-0002-3994-0898
Pierron, Théo
1
https://orcid.org/0000-0002-5586-5613
Univ. Lyon, Université Lyon 1, LIRIS UMR CNRS 5205, F-69621, Lyon, France
Local certification consists in assigning labels to the nodes of a network to certify that some given property is satisfied, in such a way that the labels can be checked locally. In the last few years, certification of graph classes received a considerable attention. The goal is to certify that a graph G belongs to a given graph class 𝒢. Such certifications with labels of size O(log n) (where n is the size of the network) exist for trees, planar graphs and graphs embedded on surfaces. Feuilloley et al. ask if this can be extended to any class of graphs defined by a finite set of forbidden minors.
In this work, we develop new decomposition tools for graph certification, and apply them to show that for every small enough minor H, H-minor-free graphs can indeed be certified with labels of size O(log n). We also show matching lower bounds using a new proof technique.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.22/LIPIcs.OPODIS.2021.22.pdf
Local certification
proof-labeling schemes
locally checkable proofs
graph decompositions
minor-free graphs
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
23:1
23:16
10.4230/LIPIcs.OPODIS.2021.23
article
RandSolomon: Optimally Resilient Random Number Generator with Deterministic Termination
Freitas de Souza, Luciano
1
2
Tonkikh, Andrei
2
Tucci-Piergiovanni, Sara
1
Sirdey, Renaud
1
Stan, Oana
1
Quero, Nicolas
1
Kuznetsov, Petr
2
CEA LIST, Université de Paris-Saclay, Gif-sur-Yvette, France
LTCI, Télécom Paris, Institut Polytechnique de Paris, France
Multi-party random number generation is a key building-block in many practical protocols. While straightforward to solve when all parties are trusted to behave correctly, the problem becomes much more difficult in the presence of faults. This paper presents RandSolomon, a partially synchronous protocol that allows a system of N processes to produce an unpredictable common random number shared by correct participants. The protocol is optimally resilient, as it allows up to f = ⌊(N-1)/3⌋ of the processes to behave arbitrarily, ensures deterministic termination and, contrary to prior solutions, does not, at any point, expect faulty processes to be responsive.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.23/LIPIcs.OPODIS.2021.23.pdf
Byzantine Fault Tolerance
Partially Synchronous
Deterministic Termination
Randomness Beacon
Multi Party Computation
BFT-RNG
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
24:1
24:12
10.4230/LIPIcs.OPODIS.2021.24
article
Optimal Space Lower Bound for Deterministic Self-Stabilizing Leader Election Algorithms
Blin, Lélia
1
https://orcid.org/0000-0003-0342-9243
Feuilloley, Laurent
2
https://orcid.org/0000-0002-3994-0898
Le Bouder, Gabriel
3
Sorbonne Université, Université d’Evry-Val-d’Essonne, CNRS, LIP6 UMR 7606, 4 place Jussieu, 75005 Paris, France
Univ. Lyon, Université Lyon 1, LIRIS UMR CNRS 5205, F-69621, Lyon, France
Sorbonne Université, CNRS, INRIA, LIP6 UMR 7606, 4 place Jussieu, 75005 Paris, France
Given a boolean predicate Π on labeled networks (e.g., proper coloring, leader election, etc.), a self-stabilizing algorithm for Π is a distributed algorithm that can start from any initial configuration of the network (i.e., every node has an arbitrary value assigned to each of its variables), and eventually converge to a configuration satisfying Π. It is known that leader election does not have a deterministic self-stabilizing algorithm using a constant-size register at each node, i.e., for some networks, some of their nodes must have registers whose sizes grow with the size n of the networks. On the other hand, it is also known that leader election can be solved by a deterministic self-stabilizing algorithm using registers of O(log log n) bits per node in any n-node bounded-degree network. We show that this latter space complexity is optimal. Specifically, we prove that every deterministic self-stabilizing algorithm solving leader election must use Ω(log log n)-bit per node registers in some n-node networks. In addition, we show that our lower bounds go beyond leader election, and apply to all problems that cannot be solved by anonymous algorithms.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.24/LIPIcs.OPODIS.2021.24.pdf
Space lower bound
memory tight bound
self-stabilization
leader election
anonymous
identifiers
state model
ring topology
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
25:1
25:23
10.4230/LIPIcs.OPODIS.2021.25
article
Accountability and Reconfiguration: Self-Healing Lattice Agreement
Freitas de Souza, Luciano
1
Kuznetsov, Petr
1
Rieutord, Thibault
2
Tucci-Piergiovanni, Sara
2
https://orcid.org/0000-0001-9738-9021
LTCI, Télécom Paris, Institut Polytechnique de Paris, France
CEA-List, Université Paris-Saclay, Palaiseau, France
An accountable distributed system provides means to detect deviations of system components from their expected behavior. It is natural to complement fault detection with a reconfiguration mechanism, so that the system could heal itself, by replacing malfunctioning parts with new ones. In this paper, we describe a framework that can be used to implement a large class of accountable and reconfigurable replicated services. We build atop the fundamental lattice agreement abstraction lying at the core of storage systems and cryptocurrencies.
Our asynchronous implementation of accountable lattice agreement ensures that every violation of consistency is followed by an undeniable evidence of misbehavior of a faulty replica. The system can then be seamlessly reconfigured by evicting faulty replicas, adding new ones and merging inconsistent states. We believe that this paper opens a direction towards asynchronous "self-healing" systems that combine accountability and reconfiguration.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.25/LIPIcs.OPODIS.2021.25.pdf
Reconfiguration
accountability
asynchronous
lattice agreement
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
26:1
26:16
10.4230/LIPIcs.OPODIS.2021.26
article
Design and Analysis of a Logless Dynamic Reconfiguration Protocol
Schultz, William
1
Zhou, Siyuan
2
Dardik, Ian
1
Tripakis, Stavros
3
Northeastern University, Boston, MA, USA
MongoDB, New York, NY, USA
Northeastern University, Boston, MA,USA
Distributed replication systems based on the replicated state machine model have become ubiquitous as the foundation of modern database systems. To ensure availability in the presence of faults, these systems must be able to dynamically replace failed nodes with healthy ones via dynamic reconfiguration. MongoDB is a document oriented database with a distributed replication mechanism derived from the Raft protocol. In this paper, we present MongoRaftReconfig, a novel dynamic reconfiguration protocol for the MongoDB replication system. MongoRaftReconfig utilizes a logless approach to managing configuration state and decouples the processing of configuration changes from the main database operation log. The protocol’s design was influenced by engineering constraints faced when attempting to redesign an unsafe, legacy reconfiguration mechanism that existed previously in MongoDB. We provide a safety proof of MongoRaftReconfig, along with a formal specification in TLA+. To our knowledge, this is the first published safety proof and formal specification of a reconfiguration protocol for a Raft-based system. We also present results from model checking the safety properties of MongoRaftReconfig on finite protocol instances. Finally, we discuss the conceptual novelties of MongoRaftReconfig, how it can be understood as an optimized and generalized version of the single server reconfiguration algorithm of Raft, and present an experimental evaluation of how its optimizations can provide performance benefits for reconfigurations.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.26/LIPIcs.OPODIS.2021.26.pdf
Fault Tolerance
Dynamic Reconfiguration
State Machine Replication
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
27:1
27:19
10.4230/LIPIcs.OPODIS.2021.27
article
Optimal Good-Case Latency for Rotating Leader Synchronous BFT
Abraham, Ittai
1
Nayak, Kartik
2
Shrestha, Nibesh
3
VMware Research, Herzliya, Israel
Duke University, Durham, NC, USA
Rochester Institute of Technology, NY, USA
This paper explores the good-case latency of synchronous Byzantine Fault Tolerant (BFT) consensus protocols in the rotating leader setting. We first present a lower bound that relates the latency of a broadcast when the sender is honest and the latency of switching to the next sender. We then present a matching upper bound with a latency of 2Δ (Δ is the pessimistic synchronous delay) with an optimistically responsive change to the next sender. The results imply that both our lower and upper bounds are tight. We implement and evaluate our protocol and show that our protocol obtains similar latency compared to state-of-the-art stable-leader protocol Sync HotStuff while allowing optimistically responsive leader rotation.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.27/LIPIcs.OPODIS.2021.27.pdf
Distributed Computing
Byzantine Fault Tolerance
Synchrony
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
28:1
28:20
10.4230/LIPIcs.OPODIS.2021.28
article
Strongly Linearizable Linked List and Queue
Hwang, Steven Munsu
1
Woelfel, Philipp
1
University of Calgary, Canada
Strong linearizability is a correctness condition conceived to address the inadequacies of linearzability when using implemented objects in randomized algorithms. Due to its newfound nature, not many strongly linearizable implementations of data structures are known. In particular, very little is known about what can be achieved in terms of strong linearizability with strong primitives that are available in modern systems, such as the compare-and-swap (CAS) operation.
This paper kick-starts the research into filling this gap. We show that Harris’s linked list and Michael and Scott’s queue, two well-known lock-free, linearizable data structures, are not strongly linearizable. In addition, we give modifications to these data structures to make them strongly linearizable while maintaining lock-freedom. The algorithms we describe are the first instances of non-trivial, strongly linearizable data structures of their type not derived by a universal construction.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.28/LIPIcs.OPODIS.2021.28.pdf
Strong linearizability
compare-and-swap
linked list
queue
lock-freedom
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
29:1
29:17
10.4230/LIPIcs.OPODIS.2021.29
article
Recoverable and Detectable Fetch&Add
Nahum, Liad
1
https://orcid.org/0000-0003-4190-8309
Attiya, Hagit
2
https://orcid.org/0000-0002-8017-6457
Ben-Baruch, Ohad
1
https://orcid.org/0000-0003-3390-2951
Hendler, Danny
1
https://orcid.org/0000-0001-7152-7828
Department of Computer Science, Ben Gurion University, Beer Sheva, Israel
Department of Computer Science, Technion, Haifa, Israel
The emergence of systems with non-volatile main memory (NVRAM) increases the need for persistent concurrent objects. Of specific interest are recoverable implementations that, in addition to being robust to crash-failures, are also detectable. Detectability ensures that upon recovery, it is possible to infer whether the failed operation took effect or not and, in the former case, obtain its response.
This work presents two recoverable detectable Fetch&Add (FAA) algorithms that are self-implementations, i.e, use only a fetch&add base object, in addition to read/write registers. The algorithms target two different models for recovery: the global-crash model and the individual-crash model. In both algorithms, operations are wait-free when there are no crashes, but the recovery code may block if there are repeated failures. We also prove that in the individual-crash model, there is no implementation of recoverable and detectable FAA using only read, write and fetch&add primitives in which all operations, including recovery, are lock-free.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.29/LIPIcs.OPODIS.2021.29.pdf
Multi-core algorithms
persistent memory
non-volatile memory
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
30:1
30:17
10.4230/LIPIcs.OPODIS.2021.30
article
Using Nesting to Push the Limits of Transactional Data Structure Libraries
Assa, Gal
1
Meir, Hagar
2
Golan-Gueta, Guy
3
Keidar, Idit
1
Spiegelman, Alexander
4
Technion - Israel Institute of Technology, Haifa, Israel
IBM Research, Haifa, Israel
Independent researcher, Israel
Novi Research, USA
Transactional data structure libraries (TDSL) combine the ease-of-programming of transactions with the high performance and scalability of custom-tailored concurrent data structures. They can be very efficient thanks to their ability to exploit data structure semantics in order to reduce overhead, aborts, and wasted work compared to general-purpose software transactional memory. However, TDSLs were not previously used for complex use-cases involving long transactions and a variety of data structures.
In this paper, we boost the performance and usability of a TDSL, towards allowing it to support complex applications. A key idea is nesting. Nested transactions create checkpoints within a longer transaction, so as to limit the scope of abort, without changing the semantics of the original transaction. We build a Java TDSL with built-in support for nested transactions over a number of data structures. We conduct a case study of a complex network intrusion detection system that invests a significant amount of work to process each packet. Our study shows that our library outperforms publicly available STMs twofold without nesting, and by up to 16x when nesting is used.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.30/LIPIcs.OPODIS.2021.30.pdf
Transactional Libraries
Nesting
eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2022-02-28
217
31:1
31:20
10.4230/LIPIcs.OPODIS.2021.31
article
Asynchronous Rumor Spreading in Dynamic Graphs
Mans, Bernard
1
https://orcid.org/0000-0001-7897-2043
Pourmiri, Ali
2
https://orcid.org/0000-0003-1173-2883
Macquarie University, Sydney, Australia
UNSW, Sydney, Australia
We study asynchronous rumor spreading algorithm in dynamic and static graphs. In the asynchronous rumor spreading, for a given underlying graph, each node is equipped with an exponential time clock of rate 1. When a node’s clock ticks, the node calls a random neighbor in order to exchange a rumor, if at least one of them knows it. Assuming a single node knows a rumor, we apply a differential equation-based technique to obtain an upper bound for the spread time of the algorithm in general dynamic graphs, which is the first time when all nodes get informed with high probability. In particular, we derive an upper bound for the spread time of the algorithm in a discrete version of a geometric mobile network, introduced by Clementi et al. [Andrea E. F. Clementi et al., 2011]. In this model, a set of n agents independently performs random walks on a √n× √n plane and every two agents are able to communicate if they are within Euclidean distance at most R, where f(n)√{log n} ⩽ R ⩽ √n and f(n) is a slowly growing function in n. Here, we show that the algorithm spreads a rumor through the network in 𝒪(log n+√n/R) time, with high probability. Although we only show an upper bound the spread time of the algorithm in a 2 dimensional space, the framework can be also applied for geometric mobile networks defined over higher dimensional space and other random dynamic evolving networks such as stationary edge-Markovian model. Besides these synchronous and discrete dynamic models, we also consider the spreading time in dynamical Erdős-Rényi graphs.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol217-opodis2021/LIPIcs.OPODIS.2021.31/LIPIcs.OPODIS.2021.31.pdf
randomized rumor spreading
push/pull
asynchronous rumor spreading