21 Search Results for "Gotsman, Alexey"

Document
DOI: 10.4230/LIPIcs.OPODIS.2025.22
Making Democracy Work: Fixing and Simplifying Egalitarian Paxos

Authors: Fedor Ryabinin, Alexey Gotsman, and Pierre Sutra

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

Abstract
Classical state-machine replication protocols, such as Paxos, rely on a distinguished leader process to order commands. Unfortunately, this approach makes the leader a single point of failure and increases the latency for clients that are not co-located with it. As a response to these drawbacks, Egalitarian Paxos [Iulian Moraru et al., 2013] introduced an alternative, leaderless approach, that allows replicas to order commands collaboratively. Not relying on a single leader allows the protocol to maintain non-zero throughput with up to f crashes of any processes out of a total of n = 2f+1. The protocol furthermore allows any process to execute a command c fast, in 2 message delays, provided no more than e = ⌈(f+1)/2⌉ other processes fail, and all concurrently submitted commands commute with c; the latter condition is often satisfied in practical systems. Egalitarian Paxos has served as a foundation for many other replication protocols. But unfortunately, the protocol is very complex, ambiguously specified and suffers from nontrivial bugs. In this paper, we present EPaxos* - a simpler and correct variant of Egalitarian Paxos. Our key technical contribution is a simpler failure-recovery algorithm, which we have rigorously proved correct. Our protocol also generalizes Egalitarian Paxos to cover the whole spectrum of failure thresholds f and e such that n ≥ max{2e+f-1, 2f+1} - the number of processes that we show to be optimal.
Cite as

Fedor Ryabinin, Alexey Gotsman, and Pierre Sutra. Making Democracy Work: Fixing and Simplifying Egalitarian Paxos. In 29th International Conference on Principles of Distributed Systems (OPODIS 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 361, pp. 22:1-22:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{ryabinin_et_al:LIPIcs.OPODIS.2025.22,
  author =	{Ryabinin, Fedor and Gotsman, Alexey and Sutra, Pierre},
  title =	{{Making Democracy Work: Fixing and Simplifying Egalitarian Paxos}},
  booktitle =	{29th International Conference on Principles of Distributed Systems (OPODIS 2025)},
  pages =	{22:1--22:19},
  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.2025.22},
  URN =		{urn:nbn:de:0030-drops-251955},
  doi =		{10.4230/LIPIcs.OPODIS.2025.22},
  annote =	{Keywords: Consensus, state-machine replication, fault tolerance}
}
Document
DOI: 10.4230/LIPIcs.DISC.2025.6
Hierarchical Consensus: Scalability Through Optimism and Weak Liveness

Authors: Pedro Antonino, Antoine Durand, and A. W. Roscoe

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

Abstract
Scalability is a central concern of Byzantine Fault Tolerant (BFT) distributed protocols. The ubiquitous approach to work around the well-known Dolev-Reischuk Ω(n²) communication complexity lower bound is to use a random selection process to draw a hopefully small committee from a population of agents to run the communication-heavy protocol. We propose a notion of hierarchical consensus that combines two sub-protocols: an optimistic primary sub-protocol that can tolerate less than 1/2 failures and a fallback secondary protocol that can tolerate less than 1/3 failures; we achieve the higher failure threshold by requiring a weaker notion of liveness for the primary. This distinction between the level of fault tolerance between primary and secondary is reflected in the size of committees implementing these protocols. For a population of agents with close to 2/3 of honest agents, we need to select a committee with hundreds of agents to reach the level of tolerance expected for the primary, whereas we need thousands to reach the level expected for the secondary with a very small probability of error ε. Our hierarchical construct is such that if the primary comes to a decision, it can simply propagate it to the secondary protocol, so it does not need to properly engage in an agreement protocol independently. Our architecture is flexible and allows us to use our technique for most protocols that are based on random sampling. By studying hierarchical protocols, we discovered new theoretical results of independent interest. Specifically, the ability to handover from a primary protocol requires a new Justifiability property that allows agents to pre-decide on a value, such that if the protocol decides, it must be on that pre-decided value.
Cite as

Pedro Antonino, Antoine Durand, and A. W. Roscoe. Hierarchical Consensus: Scalability Through Optimism and Weak Liveness. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 6:1-6:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{antonino_et_al:LIPIcs.DISC.2025.6,
  author =	{Antonino, Pedro and Durand, Antoine and Roscoe, A. W.},
  title =	{{Hierarchical Consensus: Scalability Through Optimism and Weak Liveness}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{6:1--6:20},
  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.2025.6},
  URN =		{urn:nbn:de:0030-drops-248232},
  doi =		{10.4230/LIPIcs.DISC.2025.6},
  annote =	{Keywords: Hierarchical, Handover, Justifiability, Consensus, Distributed Systems, Blockchain}
}
Document
DOI: 10.4230/LIPIcs.DISC.2025.39
TEE Is Not a Healer: Rollback-Resistant Reliable Storage

Authors: Sadegh Keshavarzi, Gregory Chockler, and Alexey Gotsman

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

Abstract
Recent advances in secure hardware technologies, such as Intel SGX or ARM TrustZone, offer an opportunity to substantially reduce the costs of Byzantine fault-tolerance by placing the program code and state within a secure enclave known as a Trusted Execution Environment (TEE). However, the protection offered by a TEE only applies during program execution. Once power is switched off, the non-volatile portion of the program state becomes vulnerable to rollback attacks wherein it is undetectably reverted to an older version. In this paper we consider the problem of implementing reliable read/write registers out of failure-prone replicas subject to state rollbacks. To this end, we introduce a new unified model that captures multiple failure types that can affect a TEE-based system and establish tight bounds on the fault-tolerance of register constructions in this model. We consider both the static case, where failure thresholds hold throughout the entire execution, and the dynamic case, where any number of replicas can roll back, provided these failures do not occur too often. Our dynamic register emulation algorithm, TEE-Rex , provides the first correct implementation of a distributed state recovery procedure that requires neither durable storage nor specialized hardware, such as trusted monotonic counters.
Cite as

Sadegh Keshavarzi, Gregory Chockler, and Alexey Gotsman. TEE Is Not a Healer: Rollback-Resistant Reliable Storage. In 39th International Symposium on Distributed Computing (DISC 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 356, pp. 39:1-39:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{keshavarzi_et_al:LIPIcs.DISC.2025.39,
  author =	{Keshavarzi, Sadegh and Chockler, Gregory and Gotsman, Alexey},
  title =	{{TEE Is Not a Healer: Rollback-Resistant Reliable Storage}},
  booktitle =	{39th International Symposium on Distributed Computing (DISC 2025)},
  pages =	{39:1--39:18},
  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.2025.39},
  URN =		{urn:nbn:de:0030-drops-248560},
  doi =		{10.4230/LIPIcs.DISC.2025.39},
  annote =	{Keywords: Trusted execution environments, fault tolerance, crash recovery}
}
Document
DOI: 10.4230/LIPIcs.ECOOP.2025.4
Ensuring Convergence and Invariants Without Coordination

Authors: Dina Borrego, Nuno Preguiça, Elisa Gonzalez Boix, and Carla Ferreira

Published in: LIPIcs, Volume 333, 39th European Conference on Object-Oriented Programming (ECOOP 2025)

Abstract
The CAP theorem demonstrates a trade-off between consistency and availability (and, by extension, latency) in systems where network partitions are unavoidable, such as in cloud computing and local-first software. While adopting weak consistency can preserve availability, it may result in inconsistencies that compromise application correctness. Replicated data types provide a principled, coordination-free approach to guarantee convergence but do not consider application invariants. Existing methods for maintaining invariants in replicated systems either rely on coordination - undermining the benefits of weak consistency - or suffer from limited applicability. This paper introduces the No-Op framework, a generic approach for enforcing consistency without coordination while guaranteeing both convergence and invariant preservation. The core idea of the No-Op approach is to resolve conflicts among concurrent operations by prioritising one operation over the other according to programmer-defined conflict resolution policies. This prioritisation transforms the less-preferred operation into a no-side-effect operation, ensuring conflict-free execution. We formalise the model underlying the No-Op framework and introduce a replication protocol built upon it, accompanied by a formal proof of correctness for both the framework and the protocol. Furthermore, we demonstrate the framework’s applicability by showcasing the design of widely used replicated data types and the preservation of a wide range of application invariants.
Cite as

Dina Borrego, Nuno Preguiça, Elisa Gonzalez Boix, and Carla Ferreira. Ensuring Convergence and Invariants Without Coordination. In 39th European Conference on Object-Oriented Programming (ECOOP 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 333, pp. 4:1-4:29, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{borrego_et_al:LIPIcs.ECOOP.2025.4,
  author =	{Borrego, Dina and Pregui\c{c}a, Nuno and Gonzalez Boix, Elisa and Ferreira, Carla},
  title =	{{Ensuring Convergence and Invariants Without Coordination}},
  booktitle =	{39th European Conference on Object-Oriented Programming (ECOOP 2025)},
  pages =	{4:1--4:29},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-373-7},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{333},
  editor =	{Aldrich, Jonathan and Silva, Alexandra},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2025.4},
  URN =		{urn:nbn:de:0030-drops-232978},
  doi =		{10.4230/LIPIcs.ECOOP.2025.4},
  annote =	{Keywords: distributed systems, conflict resolution, RDTs, invariant preservation}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2024.4
AMECOS: A Modular Event-Based Framework for Concurrent Object Specification

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

Published in: LIPIcs, Volume 324, 28th International Conference on Principles of Distributed Systems (OPODIS 2024)

Abstract
In this work, we introduce a modular framework for specifying distributed systems that we call AMECOS. Specifically, our framework departs from the traditional use of sequential specification, which presents limitations both on the specification expressiveness and implementation efficiency of inherently concurrent objects, as documented by Castañeda, Rajsbaum and Raynal in CACM 2023. Our framework focuses on the interactions between the various system components, specified as concurrent objects. Interactions are described with sequences of object events. This provides a modular way of specifying distributed systems and separates legality (object semantics) from other issues, such as consistency. We demonstrate the usability of our framework by (i) specifying various well-known concurrent objects, such as registers, shared memory, message-passing, reliable broadcast, and consensus, (ii) providing hierarchies of ordering semantics (namely, consistency hierarchy, memory hierarchy, and reliable broadcast hierarchy), and (iii) presenting a novel axiomatic proof of the impossibility of the well-known Consensus problem.
Cite as

Timothé Albouy, Antonio Fernández Anta, Chryssis Georgiou, Mathieu Gestin, Nicolas Nicolaou, and Junlang Wang. AMECOS: A Modular Event-Based Framework for Concurrent Object Specification. In 28th International Conference on Principles of Distributed Systems (OPODIS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 324, pp. 4:1-4:29, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{albouy_et_al:LIPIcs.OPODIS.2024.4,
  author =	{Albouy, Timoth\'{e} and Fern\'{a}ndez Anta, Antonio and Georgiou, Chryssis and Gestin, Mathieu and Nicolaou, Nicolas and Wang, Junlang},
  title =	{{AMECOS: A Modular Event-Based Framework for Concurrent Object Specification}},
  booktitle =	{28th International Conference on Principles of Distributed Systems (OPODIS 2024)},
  pages =	{4:1--4:29},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-360-7},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{324},
  editor =	{Bonomi, Silvia and Galletta, Letterio and Rivi\`{e}re, Etienne and Schiavoni, Valerio},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2024.4},
  URN =		{urn:nbn:de:0030-drops-225409},
  doi =		{10.4230/LIPIcs.OPODIS.2024.4},
  annote =	{Keywords: Concurrency, Object specification, Consistency conditions, Consensus impossibility}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2024.8
Optimal Multilevel Slashing for Blockchains

Authors: Kenan Wood, Hammurabi Mendes, and Jonad Pulaj

Published in: LIPIcs, Volume 324, 28th International Conference on Principles of Distributed Systems (OPODIS 2024)

Abstract
We present the notion of multilevel slashing, where proof-of-stake blockchain validators can obtain gradual levels of assurance that a certain block is bound to be finalized in a global consensus procedure, unless an increasing and optimally large number of Byzantine processes have their staked assets slashed - that is, deducted - due to provably incorrect behavior. Our construction is a highly parameterized generalization of combinatorial intersection systems based on finite projective spaces, with asymptotic high availability and optimal slashing properties. Even under weak conditions, we show that our construction has asymptotically optimal slashing properties with respect to message complexity and validator load; this result also illustrates a fundamental trade off between message complexity, load, and slashing. In addition, we show that any intersection system whose ground elements are disjoint subsets of nodes (e.g. "committees" in committee-based consensus protocols) has asymptotic high availability under similarly weak conditions. Finally, our multilevel construction gives the flexibility to blockchain validators to decide how many "levels" of finalization assurance they wish to obtain. This functionality can be seen either as (i) a form of an early, slashing-based block finalization; or (ii) a service to support reorg tolerance.
Cite as

Kenan Wood, Hammurabi Mendes, and Jonad Pulaj. Optimal Multilevel Slashing for Blockchains. In 28th International Conference on Principles of Distributed Systems (OPODIS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 324, pp. 8:1-8:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{wood_et_al:LIPIcs.OPODIS.2024.8,
  author =	{Wood, Kenan and Mendes, Hammurabi and Pulaj, Jonad},
  title =	{{Optimal Multilevel Slashing for Blockchains}},
  booktitle =	{28th International Conference on Principles of Distributed Systems (OPODIS 2024)},
  pages =	{8:1--8:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-360-7},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{324},
  editor =	{Bonomi, Silvia and Galletta, Letterio and Rivi\`{e}re, Etienne and Schiavoni, Valerio},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2024.8},
  URN =		{urn:nbn:de:0030-drops-225445},
  doi =		{10.4230/LIPIcs.OPODIS.2024.8},
  annote =	{Keywords: Blockchains, Finality, Slashablility, Committees, Availability}
}
Document
DOI: 10.4230/LIPIcs.DISC.2024.10
Vertical Atomic Broadcast and Passive Replication

Authors: Manuel Bravo, Gregory Chockler, Alexey Gotsman, Alejandro Naser-Pastoriza, and Christian Roldán

Published in: LIPIcs, Volume 319, 38th International Symposium on Distributed Computing (DISC 2024)

Abstract
Atomic broadcast is a reliable communication abstraction ensuring that all processes deliver the same set of messages in a common global order. It is a fundamental building block for implementing fault-tolerant services using either active (aka state-machine) or passive (aka primary-backup) replication. We consider the problem of implementing reconfigurable atomic broadcast, which further allows users to dynamically alter the set of participating processes, e.g., in response to failures or changes in the load. We give a complete safety and liveness specification of this communication abstraction and propose a new protocol implementing it, called Vertical Atomic Broadcast, which uses an auxiliary service to facilitate reconfiguration. In contrast to prior proposals, our protocol significantly reduces system downtime when reconfiguring from a functional configuration by allowing it to continue processing messages while agreement on the next configuration is in progress. Furthermore, we show that this advantage can be maintained even when our protocol is modified to support a stronger variant of atomic broadcast required for passive replication.
Cite as

Manuel Bravo, Gregory Chockler, Alexey Gotsman, Alejandro Naser-Pastoriza, and Christian Roldán. Vertical Atomic Broadcast and Passive Replication. In 38th International Symposium on Distributed Computing (DISC 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 319, pp. 10:1-10:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{bravo_et_al:LIPIcs.DISC.2024.10,
  author =	{Bravo, Manuel and Chockler, Gregory and Gotsman, Alexey and Naser-Pastoriza, Alejandro and Rold\'{a}n, Christian},
  title =	{{Vertical Atomic Broadcast and Passive Replication}},
  booktitle =	{38th International Symposium on Distributed Computing (DISC 2024)},
  pages =	{10:1--10:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-352-2},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{319},
  editor =	{Alistarh, Dan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2024.10},
  URN =		{urn:nbn:de:0030-drops-212363},
  doi =		{10.4230/LIPIcs.DISC.2024.10},
  annote =	{Keywords: Reconfiguration, consensus, replication}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2023.21
Fault-Tolerant Computing with Unreliable Channels

Authors: Alejandro Naser-Pastoriza, Gregory Chockler, and Alexey Gotsman

Published in: LIPIcs, Volume 286, 27th International Conference on Principles of Distributed Systems (OPODIS 2023)

Abstract
We study implementations of basic fault-tolerant primitives, such as consensus and registers, in message-passing systems subject to process crashes and a broad range of communication failures. Our results characterize the necessary and sufficient conditions for implementing these primitives as a function of the connectivity constraints and synchrony assumptions. Our main contribution is a new algorithm for partially synchronous consensus that is resilient to process crashes and channel failures and is optimal in its connectivity requirements. In contrast to prior work, our algorithm assumes the most general model of message loss where faulty channels are flaky, i.e., can lose messages without any guarantee of fairness. This failure model is particularly challenging for consensus algorithms, as it rules out standard solutions based on leader oracles and failure detectors. To circumvent this limitation, we construct our solution using a new variant of the recently proposed view synchronizer abstraction, which we adapt to the crash-prone setting with flaky channels.
Cite as

Alejandro Naser-Pastoriza, Gregory Chockler, and Alexey Gotsman. Fault-Tolerant Computing with Unreliable Channels. In 27th International Conference on Principles of Distributed Systems (OPODIS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 286, pp. 21:1-21:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{naserpastoriza_et_al:LIPIcs.OPODIS.2023.21,
  author =	{Naser-Pastoriza, Alejandro and Chockler, Gregory and Gotsman, Alexey},
  title =	{{Fault-Tolerant Computing with Unreliable Channels}},
  booktitle =	{27th International Conference on Principles of Distributed Systems (OPODIS 2023)},
  pages =	{21:1--21:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-308-9},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{286},
  editor =	{Bessani, Alysson and D\'{e}fago, Xavier and Nakamura, Junya and Wada, Koichi and Yamauchi, Yukiko},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2023.21},
  URN =		{urn:nbn:de:0030-drops-195118},
  doi =		{10.4230/LIPIcs.OPODIS.2023.21},
  annote =	{Keywords: Consensus, network partitions, liveness, synchronizers}
}
Document
DOI: 10.4230/LIPIcs.DISC.2022.12
Liveness and Latency of Byzantine State-Machine Replication

Authors: Manuel Bravo, Gregory Chockler, and Alexey Gotsman

Published in: LIPIcs, Volume 246, 36th International Symposium on Distributed Computing (DISC 2022)

Abstract
Byzantine state-machine replication (SMR) ensures the consistency of replicated state in the presence of malicious replicas and lies at the heart of the modern blockchain technology. Byzantine SMR protocols often guarantee safety under all circumstances and liveness only under synchrony. However, guaranteeing liveness even under this assumption is nontrivial. So far we have lacked systematic ways of incorporating liveness mechanisms into Byzantine SMR protocols, which often led to subtle bugs. To close this gap, we introduce a modular framework to facilitate the design of provably live and efficient Byzantine SMR protocols. Our framework relies on a view abstraction generated by a special SMR synchronizer primitive to drive the agreement on command ordering. We present a simple formal specification of an SMR synchronizer and its bounded-space implementation under partial synchrony. We also apply our specification to prove liveness and analyze the latency of three Byzantine SMR protocols via a uniform methodology. In particular, one of these results yields what we believe is the first rigorous liveness proof for the algorithmic core of the seminal PBFT protocol.
Cite as

Manuel Bravo, Gregory Chockler, and Alexey Gotsman. Liveness and Latency of Byzantine State-Machine Replication. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 12:1-12:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{bravo_et_al:LIPIcs.DISC.2022.12,
  author =	{Bravo, Manuel and Chockler, Gregory and Gotsman, Alexey},
  title =	{{Liveness and Latency of Byzantine State-Machine Replication}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{12:1--12:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.12},
  URN =		{urn:nbn:de:0030-drops-172037},
  doi =		{10.4230/LIPIcs.DISC.2022.12},
  annote =	{Keywords: Replication, blockchain, partial synchrony, liveness}
}
Document
DOI: 10.4230/LIPIcs.DISC.2022.14
Byzantine Consensus Is Θ(n²): The Dolev-Reischuk Bound Is Tight Even in Partial Synchrony!

Authors: Pierre Civit, Muhammad Ayaz Dzulfikar, Seth Gilbert, Vincent Gramoli, Rachid Guerraoui, Jovan Komatovic, and Manuel Vidigueira

Published in: LIPIcs, Volume 246, 36th International Symposium on Distributed Computing (DISC 2022)

Abstract
The Dolev-Reischuk bound says that any deterministic Byzantine consensus protocol has (at least) quadratic communication complexity in the worst case. While it has been shown that the bound is tight in synchronous environments, it is still unknown whether a consensus protocol with quadratic communication complexity can be obtained in partial synchrony. Until now, the most efficient known solutions for Byzantine consensus in partially synchronous settings had cubic communication complexity (e.g., HotStuff, binary DBFT). This paper closes the existing gap by introducing SQuad, a partially synchronous Byzantine consensus protocol with quadratic worst-case communication complexity. In addition, SQuad is optimally-resilient and achieves linear worst-case latency complexity. The key technical contribution underlying SQuad lies in the way we solve view synchronization, the problem of bringing all correct processes to the same view with a correct leader for sufficiently long. Concretely, we present RareSync, a view synchronization protocol with quadratic communication complexity and linear latency complexity, which we utilize in order to obtain SQuad.
Cite as

Pierre Civit, Muhammad Ayaz Dzulfikar, Seth Gilbert, Vincent Gramoli, Rachid Guerraoui, Jovan Komatovic, and Manuel Vidigueira. Byzantine Consensus Is Θ(n²): The Dolev-Reischuk Bound Is Tight Even in Partial Synchrony!. In 36th International Symposium on Distributed Computing (DISC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 246, pp. 14:1-14:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{civit_et_al:LIPIcs.DISC.2022.14,
  author =	{Civit, Pierre and Dzulfikar, Muhammad Ayaz and Gilbert, Seth and Gramoli, Vincent and Guerraoui, Rachid and Komatovic, Jovan and Vidigueira, Manuel},
  title =	{{Byzantine Consensus Is \Theta(n²): The Dolev-Reischuk Bound Is Tight Even in Partial Synchrony!}},
  booktitle =	{36th International Symposium on Distributed Computing (DISC 2022)},
  pages =	{14:1--14:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-255-6},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{246},
  editor =	{Scheideler, Christian},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2022.14},
  URN =		{urn:nbn:de:0030-drops-172059},
  doi =		{10.4230/LIPIcs.DISC.2022.14},
  annote =	{Keywords: Optimal Byzantine consensus, Communication complexity, Latency complexity}
}
Document
DOI: 10.4230/LIPIcs.DISC.2020.23
Making Byzantine Consensus Live

Authors: Manuel Bravo, Gregory Chockler, and Alexey Gotsman

Published in: LIPIcs, Volume 179, 34th International Symposium on Distributed Computing (DISC 2020)

Abstract
Partially synchronous Byzantine consensus protocols typically structure their execution into a sequence of views, each with a designated leader process. The key to guaranteeing liveness in these protocols is to ensure that all correct processes eventually overlap in a view with a correct leader for long enough to reach a decision. We propose a simple view synchronizer abstraction that encapsulates the corresponding functionality for Byzantine consensus protocols, thus simplifying their design. We present a formal specification of a view synchronizer and its implementation under partial synchrony, which runs in bounded space despite tolerating message loss during asynchronous periods. We show that our synchronizer specification is strong enough to guarantee liveness for single-shot versions of several well-known Byzantine consensus protocols, including HotStuff, Tendermint, PBFT and SBFT. We furthermore give precise latency bounds for these protocols when using our synchronizer. By factoring out the functionality of view synchronization we are able to specify and analyze the protocols in a uniform framework, which allows comparing them and highlights trade-offs.
Cite as

Manuel Bravo, Gregory Chockler, and Alexey Gotsman. Making Byzantine Consensus Live. In 34th International Symposium on Distributed Computing (DISC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 179, pp. 23:1-23:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{bravo_et_al:LIPIcs.DISC.2020.23,
  author =	{Bravo, Manuel and Chockler, Gregory and Gotsman, Alexey},
  title =	{{Making Byzantine Consensus Live}},
  booktitle =	{34th International Symposium on Distributed Computing (DISC 2020)},
  pages =	{23:1--23:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-168-9},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{179},
  editor =	{Attiya, Hagit},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2020.23},
  URN =		{urn:nbn:de:0030-drops-131013},
  doi =		{10.4230/LIPIcs.DISC.2020.23},
  annote =	{Keywords: Byzantine consensus, blockchain, partial synchrony, liveness}
}
Document
DOI: 10.4230/LIPIcs.DISC.2020.24
Leaderless State-Machine Replication: Specification, Properties, Limits

Authors: Tuanir França Rezende and Pierre Sutra

Published in: LIPIcs, Volume 179, 34th International Symposium on Distributed Computing (DISC 2020)

Abstract
Modern Internet services commonly replicate critical data across several geographical locations using state-machine replication (SMR). Due to their reliance on a leader replica, classical SMR protocols offer limited scalability and availability in this setting. To solve this problem, recent protocols follow instead a leaderless approach, in which each replica is able to make progress using a quorum of its peers. In this paper, we study this new emerging class of SMR protocols and states some of their limits. We first propose a framework that captures the essence of leaderless state-machine replication (Leaderless SMR). Then, we introduce a set of desirable properties for these protocols: (R)eliability, (O)ptimal (L)atency and (L)oad Balancing. We show that protocols matching all of the ROLL properties are subject to a trade-off between performance and reliability. We also establish a lower bound on the message delay to execute a command in protocols optimal for the ROLL properties. This lower bound explains the persistent chaining effect observed in experimental results.
Cite as

Tuanir França Rezende and Pierre Sutra. Leaderless State-Machine Replication: Specification, Properties, Limits. In 34th International Symposium on Distributed Computing (DISC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 179, pp. 24:1-24:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{francarezende_et_al:LIPIcs.DISC.2020.24,
  author =	{Fran\c{c}a Rezende, Tuanir and Sutra, Pierre},
  title =	{{Leaderless State-Machine Replication: Specification, Properties, Limits}},
  booktitle =	{34th International Symposium on Distributed Computing (DISC 2020)},
  pages =	{24:1--24:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-168-9},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{179},
  editor =	{Attiya, Hagit},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2020.24},
  URN =		{urn:nbn:de:0030-drops-131024},
  doi =		{10.4230/LIPIcs.DISC.2020.24},
  annote =	{Keywords: Fault Tolerance, State Machine Replication, Consensus}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2019.5
Deconstructing Stellar Consensus

Authors: Álvaro García-Pérez and Maria A. Schett

Published in: LIPIcs, Volume 153, 23rd International Conference on Principles of Distributed Systems (OPODIS 2019)

Abstract
Some of the recent blockchain proposals, such as Stellar and Ripple, allow for open membership while using quorum-like structures typical for classical Byzantine consensus with closed membership. This is achieved by constructing quorums in a decentralised way: each participant independently chooses whom to trust, and quorums arise from these individual decisions. Unfortunately, the consensus protocols underlying such blockchains are poorly understood, and their correctness has not been rigorously investigated. In this paper we rigorously prove correct the Stellar Consensus Protocol (SCP), with our proof giving insights into the protocol structure and its use of lower-level abstractions. To this end, we first propose an abstract version of SCP that uses as a black box Stellar’s federated voting primitive (analogous to reliable Byzantine broadcast), previously investigated by García-Pérez and Gotsman [Álvaro García-Pérez and Alexey Gotsman, 2018]. The abstract consensus protocol highlights a modular structure in Stellar and can be proved correct by reusing the previous results on federated voting. However, it is unsuited for realistic implementations, since its processes maintain infinite state. We thus establish a refinement between the abstract protocol and the concrete SCP that uses only finite state, thereby carrying over the result about the correctness of former to the latter. Our results help establish the theoretical foundations of decentralised blockchains like Stellar and gain confidence in their correctness.
Cite as

Álvaro García-Pérez and Maria A. Schett. Deconstructing Stellar Consensus. In 23rd International Conference on Principles of Distributed Systems (OPODIS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 153, pp. 5:1-5:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{garciaperez_et_al:LIPIcs.OPODIS.2019.5,
  author =	{Garc{\'\i}a-P\'{e}rez, \'{A}lvaro and Schett, Maria A.},
  title =	{{Deconstructing Stellar Consensus}},
  booktitle =	{23rd International Conference on Principles of Distributed Systems (OPODIS 2019)},
  pages =	{5:1--5:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-133-7},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{153},
  editor =	{Felber, Pascal and Friedman, Roy and Gilbert, Seth and Miller, Avery},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2019.5},
  URN =		{urn:nbn:de:0030-drops-117910},
  doi =		{10.4230/LIPIcs.OPODIS.2019.5},
  annote =	{Keywords: Blockchain, Consensus protocol, Stellar, Byzantine quorum systems}
}
Document
DOI: 10.4230/LIPIcs.DISC.2019.24
Privatization-Safe Transactional Memories

Authors: Artem Khyzha, Hagit Attiya, and Alexey Gotsman

Published in: LIPIcs, Volume 146, 33rd International Symposium on Distributed Computing (DISC 2019)

Abstract
Transactional memory (TM) facilitates the development of concurrent applications by letting the programmer designate certain code blocks as atomic. Programmers using a TM often would like to access the same data both inside and outside transactions, and would prefer their programs to have a strongly atomic semantics, which allows transactions to be viewed as executing atomically with respect to non-transactional accesses. Since guaranteeing such semantics for arbitrary programs is prohibitively expensive, researchers have suggested guaranteeing it only for certain data-race free (DRF) programs, particularly those that follow the privatization idiom: from some point on, threads agree that a given object can be accessed non-transactionally. In this paper we show that a variant of Transactional DRF (TDRF) by Dalessandro et al. is appropriate for a class of privatization-safe TMs, which allow using privatization idioms. We prove that, if such a TM satisfies a condition we call privatization-safe opacity and a program using the TM is TDRF under strongly atomic semantics, then the program indeed has such semantics. We also present a method for proving privatization-safe opacity that reduces proving this generalization to proving the usual opacity, and apply the method to a TM based on two-phase locking and a privatization-safe version of TL2. Finally, we establish the inherent cost of privatization-safety: we prove that a TM cannot be progressive and have invisible reads if it guarantees strongly atomic semantics for TDRF programs.
Cite as

Artem Khyzha, Hagit Attiya, and Alexey Gotsman. Privatization-Safe Transactional Memories. In 33rd International Symposium on Distributed Computing (DISC 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 146, pp. 24:1-24:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{khyzha_et_al:LIPIcs.DISC.2019.24,
  author =	{Khyzha, Artem and Attiya, Hagit and Gotsman, Alexey},
  title =	{{Privatization-Safe Transactional Memories}},
  booktitle =	{33rd International Symposium on Distributed Computing (DISC 2019)},
  pages =	{24:1--24:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-126-9},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{146},
  editor =	{Suomela, Jukka},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.DISC.2019.24},
  URN =		{urn:nbn:de:0030-drops-113310},
  doi =		{10.4230/LIPIcs.DISC.2019.24},
  annote =	{Keywords: Transactional memory, privatization, observational refinement}
}
Document
DOI: 10.4230/LIPIcs.OPODIS.2018.17
Federated Byzantine Quorum Systems

Authors: Álvaro García-Pérez and Alexey Gotsman

Published in: LIPIcs, Volume 125, 22nd International Conference on Principles of Distributed Systems (OPODIS 2018)

Abstract
Some of the recent blockchain proposals, such as Stellar and Ripple, use quorum-like structures typical for Byzantine consensus while allowing for open membership. This is achieved by constructing quorums in a decentralised way: each participant independently chooses whom to trust, and quorums arise from these individual decisions. Unfortunately, the theoretical foundations underlying such blockchains have not been thoroughly investigated. To close this gap, in this paper we study decentralised quorum construction by means of federated Byzantine quorum systems, used by Stellar. We rigorously prove the correctness of basic broadcast abstractions over federated quorum systems and establish their relationship to the classical Byzantine quorum systems. In particular, we prove correctness in the realistic setting where Byzantine nodes may lie about their trust choices. We show that this setting leads to a novel variant of Byzantine quorum systems where different nodes may have different understanding of what constitutes a quorum.
Cite as

Álvaro García-Pérez and Alexey Gotsman. Federated Byzantine Quorum Systems. In 22nd International Conference on Principles of Distributed Systems (OPODIS 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 125, pp. 17:1-17:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{garciaperez_et_al:LIPIcs.OPODIS.2018.17,
  author =	{Garc{\'\i}a-P\'{e}rez, \'{A}lvaro and Gotsman, Alexey},
  title =	{{Federated Byzantine Quorum Systems}},
  booktitle =	{22nd International Conference on Principles of Distributed Systems (OPODIS 2018)},
  pages =	{17:1--17:16},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-098-9},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{125},
  editor =	{Cao, Jiannong and Ellen, Faith and Rodrigues, Luis and Ferreira, Bernardo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.OPODIS.2018.17},
  URN =		{urn:nbn:de:0030-drops-100772},
  doi =		{10.4230/LIPIcs.OPODIS.2018.17},
  annote =	{Keywords: Blockchain, Stellar, Byzantine quorum systems}
}
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