32 Search Results for "Gafni, Eli"

Document
DOI: 10.4230/LIPIcs.OPODIS.2025.4
A Hierarchy of Unrestricted Deterministic Objects with Consensus Number 1

Authors: Warren Zhu

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
The consensus number of a shared object is the maximum number of processes that can solve consensus in a wait-free manner using copies of the object and registers. In 2016, to prove that an object is not fully characterized by its consensus number, Afek, Ellen and Gafni showed that for all integers n ≥ 2, there exists an infinite sequence of deterministic objects of consensus number n with strictly increasing computational power. In 2018, Daian, Losa, Afek, and Gafni constructed an infinite sequence of deterministic objects of consensus number 1 with strictly decreasing computational power, but the single operation that each of these objects supports is restricted in how it can be used during an execution. As restrictions can have an effect on an object’s consensus number, it was left as an open question whether the same result holds without this restriction. In this paper, we construct an infinite sequence of unrestricted deterministic objects with strictly decreasing computational power. All objects in this sequence have consensus number 1.
Cite as

Warren Zhu. A Hierarchy of Unrestricted Deterministic Objects with Consensus Number 1. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 4:1-4:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{zhu:LIPIcs.OPODIS.2025.4,
  author =	{Zhu, Warren},
  title =	{{A Hierarchy of Unrestricted Deterministic Objects with Consensus Number 1}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{4:1--4:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.4},
  URN =		{urn:nbn:de:0030-drops-251778},
  doi =		{10.4230/LIPIcs.OPODIS.2025.4},
  annote =	{Keywords: Shared Memory, Wait-free, Set Agreement, Consensus Hierarchy}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.5
Tight Conditions for Binary-Output Tasks Under Crashes

Authors: Timothé Albouy, Antonio Fernández Anta, Chryssis Georgiou, Nicolas Nicolaou, and Junlang Wang

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
This paper explores necessary and sufficient system conditions to solve distributed tasks with binary outputs (i.e., tasks with output values in {0,1}). We focus on the distinct output sets of values a task can produce (intentionally disregarding validity and value multiplicity), considering that some processes may output no value. In a distributed system with n processes, of which up to t ≤ n can crash, we provide a complete characterization of the tight conditions on n and t under which every class of tasks with binary outputs is solvable, for both synchronous and asynchronous systems. This output-set approach yields highly general results: it unifies multiple distributed computing problems, such as binary consensus and symmetry breaking, and it produces impossibility proofs that hold for stronger task formulations, including those that consider validity, account for value multiplicity, or move beyond binary outputs.
Cite as

Timothé Albouy, Antonio Fernández Anta, Chryssis Georgiou, Nicolas Nicolaou, and Junlang Wang. Tight Conditions for Binary-Output Tasks Under Crashes. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 5:1-5:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{albouy_et_al:LIPIcs.OPODIS.2025.5,
  author =	{Albouy, Timoth\'{e} and Fern\'{a}ndez Anta, Antonio and Georgiou, Chryssis and Nicolaou, Nicolas and Wang, Junlang},
  title =	{{Tight Conditions for Binary-Output Tasks Under Crashes}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{5:1--5:24},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.5},
  URN =		{urn:nbn:de:0030-drops-251786},
  doi =		{10.4230/LIPIcs.OPODIS.2025.5},
  annote =	{Keywords: Distributed solvability, Asynchrony, Synchrony, Impossibility proofs, Binary-output tasks, Crash tolerance, Disagreement}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.8
Weaker Assumptions for Asymmetric Trust

Authors: Ignacio Amores-Sesar, Christian Cachin, Simon Holmgaard Kamp, and Juan Villacis

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
In distributed systems with asymmetric trust, each participant is free to make its own trust assumptions about others, captured by an asymmetric quorum system. This contrasts with ordinary, symmetric quorum systems and threshold models, where trust assumptions are uniformly shared among participants. Fundamental problems like reliable broadcast and consensus are unsolvable in the asymmetric model if quorum systems satisfy only the classical properties of consistency and availability. Existing approaches overcome this by introducing stronger assumptions. We show that some of these assumptions are overly restrictive, so much so that they effectively eliminate the benefits of asymmetric trust. To address this, we propose a new approach to characterize asymmetric problems and, building upon it, present algorithms for reliable broadcast and consensus that require weaker assumptions than previous solutions. Our methods are general and can be extended to other core problems in systems with asymmetric trust.
Cite as

Ignacio Amores-Sesar, Christian Cachin, Simon Holmgaard Kamp, and Juan Villacis. Weaker Assumptions for Asymmetric Trust. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 8:1-8:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{amoressesar_et_al:LIPIcs.OPODIS.2025.8,
  author =	{Amores-Sesar, Ignacio and Cachin, Christian and Kamp, Simon Holmgaard and Villacis, Juan},
  title =	{{Weaker Assumptions for Asymmetric Trust}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{8:1--8:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.8},
  URN =		{urn:nbn:de:0030-drops-251812},
  doi =		{10.4230/LIPIcs.OPODIS.2025.8},
  annote =	{Keywords: Asymmetric Trust, Quorum Systems, Reliable Broadcast, Consensus}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.9
Contention-Aware Cooperation

Authors: Timothé Albouy, Davide Frey, Mathieu Gestin, Michel Raynal, and François Taïani

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
As shown by Reliable Broadcast and Consensus, cooperation among a set of independent computing entities (sequential processes) is crucial in fault-tolerant distributed computing. Considering n-process asynchronous message-passing systems where some processes may be Byzantine, this paper introduces a novel cooperation abstraction, Contention-Aware Cooperation (CAC). While Reliable Broadcast is a one-to-n cooperation abstraction and Consensus is an n-to-n cooperation abstraction, CAC is a d-to-n cooperation abstraction where d (1 ≤ d ≤ n) varies with each run and remains unknown to the processes. Correct processes accept the same set of 𝓁 pairs ⟨ v,i ⟩ (v is the value proposed by p_i) from the d proposer processes, where 1 ≤ 𝓁 ≤ d and (as d) 𝓁 remains unknown to the processes (except in specific cases). Those 𝓁 values are accepted one at a time, potentially in different orders at each process. In addition, CAC provides each process with an imperfect oracle that provides insights into the values that they may accept in the future. Interestingly, the CAC abstraction is particularly efficient in favorable circumstances, when the oracle becomes accurate, which processes can detect. To illustrate its practical utility, the paper details two applications leveraging CAC: a fast consensus implementation optimized for low contention (named Cascading Consensus), and a novel naming problem that can be solved under full asynchrony. All algorithms presented require signatures.
Cite as

Timothé Albouy, Davide Frey, Mathieu Gestin, Michel Raynal, and François Taïani. Contention-Aware Cooperation. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 9:1-9:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{albouy_et_al:LIPIcs.OPODIS.2025.9,
  author =	{Albouy, Timoth\'{e} and Frey, Davide and Gestin, Mathieu and Raynal, Michel and Ta\"{i}ani, Fran\c{c}ois},
  title =	{{Contention-Aware Cooperation}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{9:1--9:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.9},
  URN =		{urn:nbn:de:0030-drops-251823},
  doi =		{10.4230/LIPIcs.OPODIS.2025.9},
  annote =	{Keywords: Agreement, Asynchronous message-passing system, Byzantine processes, Conflict detection, Consensus, Cooperation abstraction, Distributed computing, Fault tolerance, Optimistically terminating consensus, Short-naming}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.15
Time-Optimal and Energy-Efficient Deterministic Consensus

Authors: Shachar Meir, Hugo Mirault, David Peleg, and Peter Robinson

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
We study fault-tolerant consensus in a variant of the synchronous message passing model, where, in each round, every node can choose to be awake or asleep. This is known as the sleeping model (Chatterjee, Gmyr, Pandurangan PODC 2020) and defines the awake complexity (also called energy complexity), which measures the maximum number of rounds that any node is awake throughout the execution. Only awake nodes can send and receive messages in a given round and all messages sent to sleeping nodes are lost. We present new deterministic consensus algorithms that tolerate up to f < n crash failures, where n is the number of nodes. Our algorithms match the optimal time complexity lower bound of f+1 rounds. For multi-value consensus, where the input values are chosen from some possibly large set, we achieve an energy complexity of 𝒪(⌈ f² / n ⌉) rounds, whereas for binary consensus, we show an algorithm to achieve 𝒪(⌈ f / √n ⌉) energy complexity.
Cite as

Shachar Meir, Hugo Mirault, David Peleg, and Peter Robinson. Time-Optimal and Energy-Efficient Deterministic Consensus. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 15:1-15:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{meir_et_al:LIPIcs.OPODIS.2025.15,
  author =	{Meir, Shachar and Mirault, Hugo and Peleg, David and Robinson, Peter},
  title =	{{Time-Optimal and Energy-Efficient Deterministic Consensus}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{15:1--15:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.15},
  URN =		{urn:nbn:de:0030-drops-251881},
  doi =		{10.4230/LIPIcs.OPODIS.2025.15},
  annote =	{Keywords: Distributed computing, Crash faults, Consensus, Energy complexity, Sleeping model}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.21
Solving Tasks with Fewer Registers Than Processes

Authors: Eli Gafni, Giuliano Losa, Michel Raynal, and Gadi Taubenfeld

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
This paper studies distributed-computing tasks through the lens of space complexity in the read/write wait-free model, defined as the number of multi-reader-multi-writer atomic read/write registers needed to solve a task using a wait-free algorithm. Surprisingly, even though the read/write wait-free model is at the foundation of distributed computing, previous work on space complexity has focused on synchronization primitives stronger than read/write registers or on weaker progress conditions. The paper reveals that the read/write wait-free model offers a rich space-complexity landscape: (1) assuming non-anonymous processes, it shows that there is an infinite hierarchy of tasks of increasing space complexity; (2) it shows that space complexity separates anonymous from non-anonymous memory; (3) regardless of process or register anonymity, it exhibits a task of space complexity two, which is the minimal non-trivial space complexity; (4) finally, it shows that subcases of the adopt-commit task have different space complexity in non-anonymous memory under bounded wait-freedom.
Cite as

Eli Gafni, Giuliano Losa, Michel Raynal, and Gadi Taubenfeld. Solving Tasks with Fewer Registers Than Processes. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 21:1-21:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{gafni_et_al:LIPIcs.OPODIS.2025.21,
  author =	{Gafni, Eli and Losa, Giuliano and Raynal, Michel and Taubenfeld, Gadi},
  title =	{{Solving Tasks with Fewer Registers Than Processes}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{21:1--21:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.21},
  URN =		{urn:nbn:de:0030-drops-251947},
  doi =		{10.4230/LIPIcs.OPODIS.2025.21},
  annote =	{Keywords: Asynchrony, Read/write registers, Wait-freedom, Tasks, Covering argument, Lower bound, Space complexity, Anonymous Processes, Anonymous Memory}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.13
A General Input-Dependent Colorless Computability Theorem and Applications to Core-Dependent Adversaries

Authors: Yannis Coutouly and Emmanuel Godard

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
Distributed computing tasks can be presented with a triple (ℐ,𝒪,Δ). The solvability of a colorless task on the Iterated Immediate Snapshot model (IIS) has been characterized by the Colorless Computability Theorem [Maurice Herlihy et al., 2013]. A recent paper [Yannis Coutouly and Emmanuel Godard, 2024] generalizes this theorem for any message adversaries ℳ ⊆ IIS by geometric methods. In 2001, Mostéfaoui, Rajsbaum, Raynal, and Roy [Achour Mostéfaoui et al., 2002] introduced condition-based adversaries. This setting considers a particular adversary that will be applied only to a subset of input configurations. In this setting, they studied the k-set agreement task with condition-based t-resilient adversaries and obtained a sufficient condition on the conditions that make k-Set Agreement solvable. In this paper we have three contributions: 1) We generalize the characterization of [Yannis Coutouly and Emmanuel Godard, 2024] to input-dependent adversaries, which means that the adversaries can change depending on the input configuration. 2) We show that core-resilient adversaries of IIS_n have the same computability power as the core-resilient adversaries of IIS_n where crashes only happen at the start. 3) Using the two previous contributions, we provide a necessary and sufficient characterization of the condition-based, core-dependent adversaries that can solve k-Set Agreement. We also distinguish four settings that may appear when presenting a distributed task as (ℐ,𝒪,Δ). Finally, in a later section, we present structural properties on the carrier map Δ. Such properties allow simpler proof, without changing the computability power of the task. Most of the proofs in this article leverage the topological framework used in distributed computing by using simple geometric constructions.
Cite as

Yannis Coutouly and Emmanuel Godard. A General Input-Dependent Colorless Computability Theorem and Applications to Core-Dependent Adversaries. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 13:1-13:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{coutouly_et_al:LIPIcs.OPODIS.2025.13,
  author =	{Coutouly, Yannis and Godard, Emmanuel},
  title =	{{A General Input-Dependent Colorless Computability Theorem and Applications to Core-Dependent Adversaries}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{13:1--13:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.13},
  URN =		{urn:nbn:de:0030-drops-251862},
  doi =		{10.4230/LIPIcs.OPODIS.2025.13},
  annote =	{Keywords: colorless task, topological methods, geometric simplicial complex, k-set-agreement, t-resilient model, condition-based computability}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.24
On Time-Optimal, Fault-Tolerant Algorithms for Connected Consensus Beyond Grade Two

Authors: Alan Ernesto Arteaga Vázquez

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
A common question in the asynchronous model is whether some given notion of agreement between processes is achievable. Usually, we formalise such agreement notions in the form of agreement problems. Some of these problems also receive the name of coordination primitives. Several fault-tolerant algorithms in asynchronous systems rely upon the use of different primitives as building blocks, such as adopt-commit, crusader agreement, or graded broadcast. Recently, the connected consensus problem - a form of agreement over a specific family of graphs parametrised by a positive integer R- was introduced. This problem unifies the three mentioned primitives while extending them for multi-valued inputs. Moreover, the problem is equipped with a security condition called binding, which limits the effect of malicious processes over the decision of correct parties. While fault-tolerant connected consensus algorithms for R = 1 and R = 2 are known, the existence of algorithmic solutions for any positive integer parameter remained an open question. In this work, we introduce a pair of fault-tolerant algorithms for connected consensus when the R parameter is any positive integer. We introduce a crash-resilient algorithm, which is optimal with respect to the maximum number of possible faulty processes. Our second algorithm is resilient to Byzantine failures; whose failure-resilience is optimal for a specific class of algorithms. Both algorithms satisfy the binding property and match the best known time complexities achieved for the R = 1 and R = 2 cases, further achieving time optimality for the general case in the crash-failure setting, and asymptotic time optimality in the Byzantine scenario.
Cite as

Alan Ernesto Arteaga Vázquez. On Time-Optimal, Fault-Tolerant Algorithms for Connected Consensus Beyond Grade Two. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 24:1-24:28, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{arteagavazquez:LIPIcs.OPODIS.2025.24,
  author =	{Arteaga V\'{a}zquez, Alan Ernesto},
  title =	{{On Time-Optimal, Fault-Tolerant Algorithms for Connected Consensus Beyond Grade Two}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{24:1--24:28},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.24},
  URN =		{urn:nbn:de:0030-drops-251973},
  doi =		{10.4230/LIPIcs.OPODIS.2025.24},
  annote =	{Keywords: Approximate Agreement, Binding, Connected Consensus}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.29
How Exhaustive Does an Extension-Based Proof Need to Be?

Authors: Faith Ellen, Shihao Liu, Leqi Zhu, Eli Gafni, and Rati Gelashvili

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
The class of extension-based proofs encompasses traditional valency arguments. It has been shown that they are insufficient to establish the impossibility of (n-1)-set agreement among n ≥ 3 processes in an asynchronous system with crash failures. We generalize this definition to k-exhaustive extension-based proofs, in which a prover can learn the maximum length of all executions involving a set of at most k processes from a specified configuration (which may be infinite). An upper bound on the length of these executions enables the prover to determine the outputs of all these executions. When k = n, this enables the prover to perform an exhaustive search of all reachable configurations, so it knows everything about the protocol. On the other hand, extension based proofs are as powerful as 1-exhaustive extension-based proofs. For any task with no deterministic, wait-free solution among n ≥ 2 processes, we show that there is an (n-1)-exhaustive extension-based proof of its impossibility. This is done using a new characterization of such tasks. In contrast, we prove that for 1 ≤ k ≤ n-2, there is no k-exhaustive extension-based proof of the impossibility of (n-1)-set agreement.
Cite as

Faith Ellen, Shihao Liu, Leqi Zhu, Eli Gafni, and Rati Gelashvili. How Exhaustive Does an Extension-Based Proof Need to Be?. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 29:1-29:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{ellen_et_al:LIPIcs.OPODIS.2025.29,
  author =	{Ellen, Faith and Liu, Shihao and Zhu, Leqi and Gafni, Eli and Gelashvili, Rati},
  title =	{{How Exhaustive Does an Extension-Based Proof Need to Be?}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{29:1--29:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.29},
  URN =		{urn:nbn:de:0030-drops-252020},
  doi =		{10.4230/LIPIcs.OPODIS.2025.29},
  annote =	{Keywords: Extension-based proof, set agreement, valency argument, zero-one exclusion}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.31
Fast Re-Routing in Networks: On the Complexity of Perfect Resilience

Authors: Matthias Bentert, Esra Ceylan, Valentin Hübner, Stefan Schmid, and Jiří Srba

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
To achieve fast recovery from link failures, most modern communication networks feature fully decentralized fast re-routing mechanisms. These re-routing mechanisms rely on pre-installed static re-routing rules at the nodes (the routers), which depend only on local failure information, namely on the failed links incident to the node. Ideally, a network is perfectly resilient: the re-routing rules ensure that packets are always successfully routed to their destinations as long as the source and the destination are still physically connected in the underlying network after the failures. Unfortunately, there are examples where achieving perfect resilience is not possible. Surprisingly, only very little is known about the algorithmic aspect of when and how perfect resilience can be achieved. We investigate the computational complexity of analyzing such local fast re-routing mechanisms. Our main result is a negative one: we show that even checking whether a given set of static re-routing rules ensures perfect resilience is coNP-complete. Additionally, we investigate other fundamental variations of the problem. In particular, we show that our coNP-completeness proof also applies to scenarios where the re-routing rules have specific patterns (known as skipping in the literature). On the positive side, for scenarios where nodes do not have information about the link from which a packet arrived (the so-called in-port), we present a linear-time algorithm to realize perfect resilience whenever possible (which we show can also be determined in linear time).
Cite as

Matthias Bentert, Esra Ceylan, Valentin Hübner, Stefan Schmid, and Jiří Srba. Fast Re-Routing in Networks: On the Complexity of Perfect Resilience. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 31:1-31:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{bentert_et_al:LIPIcs.OPODIS.2025.31,
  author =	{Bentert, Matthias and Ceylan, Esra and H\"{u}bner, Valentin and Schmid, Stefan and Srba, Ji\v{r}{\'\i}},
  title =	{{Fast Re-Routing in Networks: On the Complexity of Perfect Resilience}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{31:1--31:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.31},
  URN =		{urn:nbn:de:0030-drops-252040},
  doi =		{10.4230/LIPIcs.OPODIS.2025.31},
  annote =	{Keywords: routing in computer networks, fast re-route, perfect resilience, complexity}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2025.33
Resolving Conflicts with Grace: Dynamically Concurrent Universality

Authors: Petr Kuznetsov and Nathan Josia Schrodt

Published in: LIPIcs, Volume 361, 29th International Conference on Principles of Distributed Systems (OPODIS 2025)

Abstract
Synchronization is the major obstacle to scalability in distributed computing. Concurrent operations on the shared data engage in synchronization when they encounter a conflict, i.e., their effects depend on the order in which they are applied. Ideally, one would like to detect conflicts in a dynamic manner, i.e., adjusting to the current system state. Indeed, it is very common that two concurrent operations conflict only in some rarely occurring states. In this paper, we define the notion of dynamic concurrency: an operation employs strong synchronization primitives only if it has to arbitrate with concurrent operations, given the current system state. We then present a dynamically concurrent universal construction.
Cite as

Petr Kuznetsov and Nathan Josia Schrodt. Resolving Conflicts with Grace: Dynamically Concurrent Universality. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 33:1-33:29, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{kuznetsov_et_al:LIPIcs.OPODIS.2025.33,
  author =	{Kuznetsov, Petr and Schrodt, Nathan Josia},
  title =	{{Resolving Conflicts with Grace: Dynamically Concurrent Universality}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{33:1--33:29},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-409-3},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{361},
  editor =	{Arusoaie, Andrei and Onica, Emanuel and Spear, Michael and Tucci-Piergiovanni, Sara},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2025.33},
  URN =		{urn:nbn:de:0030-drops-252068},
  doi =		{10.4230/LIPIcs.OPODIS.2025.33},
  annote =	{Keywords: Universal Construction, Consensus, Dynamic Concurrency}
}
Document
DOI: 10.4230/LIPIcs.DISC.2025.3
An Almost-Logarithmic Lower Bound for Leader Election with Bounded Value Contention

Authors: Dan Alistarh, Faith Ellen, and Alexander Fedorov

Published in: LIPIcs, Volume 356, 39th International Symposium on Distributed Computing (DISC 2025)

Abstract
We investigate the step complexity of the Leader Election problem (and implementing the corresponding test-and-set object) in asynchronous shared memory, where processes communicate through registers supporting atomic read and write and must coordinate so that a single process becomes the leader. Determining tight step complexity bounds for solving this problem is one of the key open problems in the theory of shared memory distributed computing. The best known algorithm is a randomized tournament-tree, which has worst-case expected step complexity O(log N) for N processes. There are provably no deterministic wait-free algorithms, and only restricted lower bounds are known for obstruction-free and randomized wait-free algorithms. We introduce a new lower bound that establishes an Ω((log N)/(log log N + log Q)) step complexity for any obstruction-free Leader Election algorithm, where N is the number of processes, and 2 ≤ Q ≤ N is a bound on the value contention, which we define as the maximum number of different values that processes can be simultaneously poised to write to the same register in any execution of the algorithm. Our result is strictly stronger than previous bounds based on write contention. In particular, it implies new lower bounds on step complexity that depend on register size.
Cite as

Dan Alistarh, Faith Ellen, and Alexander Fedorov. An Almost-Logarithmic Lower Bound for Leader Election with Bounded Value Contention. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 3:1-3:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{alistarh_et_al:LIPIcs.DISC.2025.3,
  author =	{Alistarh, Dan and Ellen, Faith and Fedorov, Alexander},
  title =	{{An Almost-Logarithmic Lower Bound for Leader Election with Bounded Value Contention}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{3:1--3:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-402-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{356},
  editor =	{Kowalski, Dariusz R.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2025.3},
  URN =		{urn:nbn:de:0030-drops-248204},
  doi =		{10.4230/LIPIcs.DISC.2025.3},
  annote =	{Keywords: Leader Election, Test-and-Set, Shared Memory, Lower Bounds}
}
Document
DOI: 10.4230/LIPIcs.DISC.2025.30
Team Formation and Applications

Authors: Yuval Emek, Shay Kutten, Ido Rafael, and Gadi Taubenfeld

Published in: LIPIcs, Volume 356, 39th International Symposium on Distributed Computing (DISC 2025)

Abstract
A novel long-lived distributed problem, called Team Formation (TF), is introduced together with a message- and time-efficient randomized algorithm. The problem is defined over the asynchronous model with a complete communication graph, using bounded size messages, where a certain fraction of the nodes may experience a generalized, strictly stronger, version of initial failures. The goal of a TF algorithm is to assemble tokens injected by the environment, in a distributed manner, into teams of size σ, where σ is a parameter of the problem. The usefulness of TF is demonstrated by using it to derive efficient algorithms for many distributed problems. Specifically, we show that various (one-shot as well as long-lived) distributed problems reduce to TF. This includes well-known (and extensively studied) distributed problems such as several versions of leader election and threshold detection. For example, we are the first to break the linear message complexity bound for asynchronous implicit leader election. We also improve the time complexity of message-optimal algorithms for asynchronous explicit leader election. Other distributed problems that reduce to TF are new ones, including matching players in online gaming platforms, a generalization of gathering, constructing a perfect matching in an induced subgraph of the complete graph, and more. To complement our positive contribution, we establish a tight lower bound on the message complexity of TF algorithms.
Cite as

Yuval Emek, Shay Kutten, Ido Rafael, and Gadi Taubenfeld. Team Formation and Applications. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 30:1-30:25, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{emek_et_al:LIPIcs.DISC.2025.30,
  author =	{Emek, Yuval and Kutten, Shay and Rafael, Ido and Taubenfeld, Gadi},
  title =	{{Team Formation and Applications}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{30:1--30:25},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-402-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{356},
  editor =	{Kowalski, Dariusz R.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2025.30},
  URN =		{urn:nbn:de:0030-drops-248474},
  doi =		{10.4230/LIPIcs.DISC.2025.30},
  annote =	{Keywords: asynchronous message-passing, complete communication graph, initial failures, leader election, matching}
}
Document
DOI: 10.4230/LIPIcs.DISC.2025.29
Strong Linearizability Without Compare&Swap: The Case of Bags

Authors: Faith Ellen and Gal Sela

Published in: LIPIcs, Volume 356, 39th International Symposium on Distributed Computing (DISC 2025)

Abstract
Because strongly-linearizable objects provide stronger guarantees than linearizability, they serve as valuable building blocks for the design of concurrent data structures. Yet, many objects that have linearizable implementations from base objects weaker than compare&swap objects do not have strongly-linearizable implementations from the same base objects. We focus on one such object: the bag, a multiset from which processes can take unspecified elements. We present the first lock-free, strongly-linearizable implementation of a bag from interfering objects (specifically, registers and test&set objects). This may be surprising, since there are provably no such implementations of stacks or queues. Since a bag can contain arbitrarily many elements, an unbounded amount of space must be used to implement it. Hence, it makes sense to also consider a bag with a bound on its capacity. However, like stacks and queues, a bag with capacity b shared by more than 2b processes has no lock-free, strongly-linearizable implementation from interfering objects. If we further restrict a bounded bag so that only one process can insert into it, we are able to obtain a lock-free, strongly-linearizable implementation from O(b+n) interfering objects, where n is the number of processes. Our goal is to understand the circumstances under which strongly-linearizable implementations of bags exist and, more generally, to understand the power of interfering objects.
Cite as

Faith Ellen and Gal Sela. Strong Linearizability Without Compare&Swap: The Case of Bags. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 29:1-29:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{ellen_et_al:LIPIcs.DISC.2025.29,
  author =	{Ellen, Faith and Sela, Gal},
  title =	{{Strong Linearizability Without Compare\&Swap: The Case of Bags}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{29:1--29:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-402-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{356},
  editor =	{Kowalski, Dariusz R.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2025.29},
  URN =		{urn:nbn:de:0030-drops-248464},
  doi =		{10.4230/LIPIcs.DISC.2025.29},
  annote =	{Keywords: Strong-Linearizability, Bag, Concurrent Data Structures, Wait-Freedom, Lock-Freedom}
}
Document
DOI: 10.4230/LIPIcs.DISC.2025.28
Byzantine Consensus in the Random Asynchronous Model

Authors: George Danezis, Jovan Komatovic, Lefteris Kokoris-Kogias, Alberto Sonnino, and Igor Zablotchi

Published in: LIPIcs, Volume 356, 39th International Symposium on Distributed Computing (DISC 2025)

Abstract
We propose a novel relaxation of the classic asynchronous network model, called the random asynchronous model, which removes adversarial message scheduling while preserving unbounded message delays and Byzantine faults. Instead of an adversary dictating message order, delivery follows a random schedule. We analyze Byzantine consensus at different resilience thresholds (n = 3f+1, n = 2f+1, and n = f+2) and show that our relaxation allows consensus with probabilistic guarantees which are impossible in the standard asynchronous model or even the partially synchronous model. We complement these protocols with corresponding impossibility results, establishing the limits of consensus in the random asynchronous model.
Cite as

George Danezis, Jovan Komatovic, Lefteris Kokoris-Kogias, Alberto Sonnino, and Igor Zablotchi. Byzantine Consensus in the Random Asynchronous Model. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 28:1-28:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{danezis_et_al:LIPIcs.DISC.2025.28,
  author =	{Danezis, George and Komatovic, Jovan and Kokoris-Kogias, Lefteris and Sonnino, Alberto and Zablotchi, Igor},
  title =	{{Byzantine Consensus in the Random Asynchronous Model}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{28:1--28:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-402-4},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{356},
  editor =	{Kowalski, Dariusz R.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2025.28},
  URN =		{urn:nbn:de:0030-drops-248457},
  doi =		{10.4230/LIPIcs.DISC.2025.28},
  annote =	{Keywords: network model, asynchronous, random scheduler, Byzantine consensus}
}
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