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1. Churn-tolerant Leader Election Protocols

   Jiangran Wang, Indranil Gupta (July 2023, Proceedings 43rd IEEE International Conference on Distributed Computing Systems)

   Classical leader election protocols typically assume complete and correct knowledge of underlying membership lists at all participating nodes. Yet many edge and IoT settings are dynamic, with nodes joining, leaving, and failing continuously—a phenomenon called churn. This implies that in any membership protocol, a given node’s membership list may have entries that are missing (e.g., false positive detections, or newly joined nodes whose information has not spread yet) or stale (e.g., failed nodes that are undetected)—these would render classical election protocols incorrect. We present a family of four leader election protocols that are churn-tolerant (or c-tolerant). The key ideas are to: i) involve the minimum number of nodes necessary to achieve safety; ii) use optimism so that decisions are made faster when churn is low; iii) incorporate a preference for electing healthier nodes as leaders. We prove the correctness and safety of our c-tolerant protocols and show their message complexity is optimal. We present experimental results from both a trace- driven simulation as well as our implementation atop Raspberry Pi devices, including a comparison against Zookeeper. 

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2. HotStuff-1: Linear Consensus with One-Phase Speculation

   https://doi.org/10.1145/3725308  

   Kang, Dakai; Gupta, Suyash; Malkhi, Dahlia; Sadoghi, Mohammad (June 2025, Proceedings of the ACM on Management of Data)

   This paper introduces HotStuff-1, a BFT consensus protocol that improves the latency of HotStuff-1 by two network hops while maintaining linear communication complexity against faults. Furthermore, HotStuff-1 incorporates an incentive-compatible leader rotation design that motivates leaders to propose transactions promptly. HotStuff-1 achieves a reduction of two network hops byspeculativelysending clients early finality confirmations, after one phase of the protocol. Introducing speculation into streamlined protocols is challenging because, unlike stable-leader protocols, these protocols cannot stop the consensus and recover from failures. Thus, we identifyprefix speculation dilemmain the context of streamlined protocols; HotStuff-1 is the first streamlined protocol to resolve it. HotStuff-1 embodies an additional mechanism,slotting, that thwarts delays caused by (1) rationally-incentivized leaders and (2) malicious leaders inclined to sabotage others' progress. The slotting mechanism allows leaders to dynamically drive as many decisions as allowed by network transmission delays before view timers expire, thus mitigating both threats. 

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3. Time-Space Trade-offs in Population Protocols

   https://doi.org/10.1137/1.9781611974782.169  

   Alistarh, Dan; Aspnes, James; Eisenstat, David; Gelashvili, Rati; Rivest, Ronald L. (January 2017, Proceedings of the Twenty-Eighth Annual ACM-SIAM Symposium on Discrete Algorithms)

   Population protocols are a popular model of distributed computing, in which randomly-interacting agents with little computational power cooperate to jointly perform computational tasks. Inspired by developments in molecular computation, and in particular DNA computing, recent algorithmic work has focused on the complexity of solving simple yet fundamental tasks in the population model, such as leader election (which requires convergence to a single agent in a special “leader” state), and majority (in which agents must converge to a decision as to which of two possible initial states had higher initial count). Known results point towards an inherent trade-off between the time complexity of such algorithms, and the space complexity, i.e. size of the memory available to each agent. In this paper, we explore this trade-off and provide new upper and lower bounds for majority and leader election. First, we prove a unified lower bound, which relates the space available per node with the time complexity achievable by a protocol: for instance, our result implies that any protocol solving either of these tasks for n agents using O(log log n) states must take Ω(n/polylogn) expected time. This is the first result to characterize time complexity for protocols which employ super-constant number of states per node, and proves that fast, poly-logarithmic running times require protocols to have relatively large space costs. On the positive side, we give algorithms showing that fast, poly-logarithmic convergence time can be achieved using O (log2 n) space per node, in the case of both tasks. Overall, our results highlight a time complexity separation between O (log log n) and Θ(log2 n) state space size for both majority and leader election in population protocols, and introduce new techniques, which should be applicable more broadly. 

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4. RT-ZooKeeper: Taming the Recovery Latency of a Coordination Service

   https://doi.org/10.1145/3477034  

   Li, Haoran; Lu, Chenyang; Gill, Christopher D. (October 2021, ACM Transactions on Embedded Computing Systems)

   Fault-tolerant coordination services have been widely used in distributed applications in cloud environments. Recent years have witnessed the emergence of time-sensitive applications deployed in edge computing environments, which introduces both challenges and opportunities for coordination services. On one hand, coordination services must recover from failures in a timely manner. On the other hand, edge computing employs local networked platforms that can be exploited to achieve timely recovery. In this work, we first identify the limitations of the leader election and recovery protocols underlying Apache ZooKeeper, the prevailing open-source coordination service. To reduce recovery latency from leader failures, we then design RT-Zookeeper with a set of novel features including a fast-convergence election protocol, a quorum channel notification mechanism, and a distributed epoch persistence protocol. We have implemented RT-Zookeeper based on ZooKeeper version 3.5.8. Empirical evaluation shows that RT-ZooKeeper achieves 91% reduction in maximum recovery latency in comparison to ZooKeeper. Furthermore, a case study demonstrates that fast failure recovery in RT-ZooKeeper can benefit a common messaging service like Kafka in terms of message latency. 

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5. DAG of DAGs: Order-Fairness Made Practical

   https://doi.org/10.1145/3769777  

   Nagda, Heena; Sankhe, Sidharth; Sinha, Sakshi; Attarha, Keon; Amiri, Mohammad Javad; Loo, Boon Thau (December 2025, Proceedings of the ACM on Management of Data)

   Ensuring order-fairness in distributed data management systems deployed in untrustworthy environments is crucial to prevent adversarial manipulation of transaction ordering, particularly in unpredictable markets where transaction order directly influences financial outcomes. While Byzantine Fault-Tolerant (BFT) consensus protocols guarantee safety and liveness, they inherently lack mechanisms to enforce order-fairness, exposing distributed systems to attacks such as frontrunning and sandwiching. Previous attempts to integrate order-fairness have often introduced substantial performance overhead, largely due to limitations of the underlying consensus protocols. This paper presents DAG of DAGs (DoD), a high-performance order-fairness protocol designed on top of DAG-based BFT consensus protocols. By leveraging the high throughput and resilience of DAG-based protocols, DoD addresses the performance limitations of existing order-fairness solutions. DoD's novel DAG of DAGs architecture enables seamless integration of order fairness with BFT consensus protocols. Through concurrent block proposals and a wave-based leader election mechanism, DoD significantly improves resilience against adversarial manipulation. A prototype implementation and experimental evaluation demonstrate that DoD effectively provides order fairness with minimal performance overhead. 

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