Solving k-Set Agreement with Stable Skeleton Graphs

In this paper we consider the k-set agreement problem in distributed message-passing systems using a round-based approach: Both synchrony of communication and failures are captured just by means of the messages that arrive within a round, resulting in round-by-round communication graphs that can be characterized by simple communication predicates. We introduce the weak communication predicate PSources(k) and show that it is tight for k-set agreement, in the following sense: We (i) prove that there is no algorithm for solving (k-1)-set agreement in systems characterized by PSources(k), and (ii) present a novel distributed algorithm that achieves k-set agreement in runs where PSources(k) holds. Our algorithm uses local approximations of the stable skeleton graph, which reflects the underlying perpetual synchrony of a run. We prove that this approximation is correct in all runs, regardless of the communication predicate, and show that graph-theoretic properties of the stable skeleton graph can be used to solve k-set agreement if PSources(k) holds.Comment: to appear in 16th IEEE Workshop on Dependable Parallel, Distributed and Network-Centric System

A Characterization of Consensus Solvability for Closed Message Adversaries

Distributed computations in a synchronous system prone to message loss can be modeled as a game between a (deterministic) distributed algorithm versus an omniscient message adversary. The latter determines, for each round, the directed communication graph that specifies which messages can reach their destination. Message adversary definitions range from oblivious ones, which pick the communication graphs arbitrarily from a given set of candidate graphs, to general message adversaries, which are specified by the set of sequences of communication graphs (called admissible communication patterns) that they may generate. This paper provides a complete characterization of consensus solvability for closed message adversaries, where every inadmissible communication pattern has a finite prefix that makes all (infinite) extensions of this prefix inadmissible. Whereas every oblivious message adversary is closed, there are also closed message adversaries that are not oblivious. We provide a tight non-topological, purely combinatorial characterization theorem, which reduces consensus solvability to a simple condition on prefixes of the communication patterns. Our result not only non-trivially generalizes the known combinatorial characterization of the consensus solvability for oblivious message adversaries by Coulouma, Godard, and Peters (Theor. Comput. Sci., 2015), but also provides the first combinatorial characterization for this important class of message adversaries that is formulated directly on the prefixes of the communication patterns

Unfaithful Glitch Propagation in Existing Binary Circuit Models

We show that no existing continuous-time, binary value-domain model for digital circuits is able to correctly capture glitch propagation. Prominent examples of such models are based on pure delay channels (P), inertial delay channels (I), or the elaborate PID channels proposed by Bellido-D\'iaz et al. We accomplish our goal by considering the solvability/non-solvability border of a simple problem called Short-Pulse Filtration (SPF), which is closely related to arbitration and synchronization. On one hand, we prove that SPF is solvable in bounded time in any such model that provides channels with non-constant delay, like I and PID. This is in opposition to the impossibility of solving bounded SPF in real (physical) circuit models. On the other hand, for binary circuit models with constant-delay channels, we prove that SPF cannot be solved even in unbounded time; again in opposition to physical circuit models. Consequently, indeed none of the binary value-domain models proposed so far (and that we are aware of) faithfully captures glitch propagation of real circuits. We finally show that these modeling mismatches do not hold for the weaker eventual SPF problem.Comment: 23 pages, 15 figure

Constitution and narrative: peculiarities of rhetoric and genre in the foundational laws of the USSR and the Russian federation

Constitutions are not just legal texts but form a narrative with an engaging plot, a hierarchy of actors and a distinct ideology. They can be read and interpreted as literary texts. The four constitutions in 20th century Russia (1924, 1936, 1977, 1993) can be attributed to specific genres (drama, fairy tale, gospel, performance). Moreover, they interact closely with the official culture of their time (painting, collage, film, literature). The constitutions serve an important task in the cultural self-definition of Russian society which as a rule occurred in moments of ideological crisis. The case of Russia is especially intriguing since the utopian project of a just society needed in every stage of its evolution (revolution, consolidation, "developed socialism”, postcommunism) a new convincing design which was able to guarantee the citizens' loyalty to the stat

The average CRI-length of a controlled ALOHA collision resolution algorithm

AbstractWe investigate the expected CRI-length (collision resolution interval) of a hybrid collision resolution algorithm based on slotted ALOHA with controlled retransmission probability, e.g., we study the average number of slots necessary for the resolution of an initial collision of multiplicity n. An algorithm similar to binary exponential backoff for adjusting the retransmission probability w.r.t. the channel load is used prior to the application of the ALOHA resolution algorithm, thus operating it in the region of nonexponential behaviour. Mellin-transform techniques are used for the derivation of an asymptotic expression for the desired quantity, which turns out to be O(n log n)

Nationality Policy: Russian Nation vs. Russian People?

In January 2020, President Putin initiated a major constitutional reform. One of the changes concerns the question of the national identity of Russia's multi-ethnic state and more specifically, the role of the ethnic Russians in the context of all peoples in the Federation. Article 68 of the constitution defines Russian as the official state language, but the new version adds as an explanation for this special status that Russian is the language of the "state-forming people." This noteworthy wording corroborates a development which has been going on for ten years. The changes include the concept of Russkii Mir, the idea of the multi-national nation of the "Russian Federation with an ethnic Russian cultural core" and the frequent use in official contexts of the phrase "ethnic Russian (russkii)" instead of "multi-national Russian (rossiiskii).

A large neighbourhood based heuristic for two-echelon routing problems

In this paper, we address two optimisation problems arising in the context of city logistics and two-level transportation systems. The two-echelon vehicle routing problem and the two-echelon location routing problem seek to produce vehicle itineraries to deliver goods to customers, with transits through intermediate facilities. To efficiently solve these problems, we propose a hybrid metaheuristic which combines enumerative local searches with destroy-and-repair principles, as well as some tailored operators to optimise the selections of intermediate facilities. We conduct extensive computational experiments to investigate the contribution of these operators to the search performance, and measure the performance of the method on both problem classes. The proposed algorithm finds the current best known solutions, or better ones, for 95% of the two-echelon vehicle routing problem benchmark instances. Overall, for both problems, it achieves high-quality solutions within short computing times. Finally, for future reference, we resolve inconsistencies between different versions of benchmark instances, document their differences, and provide them all online in a unified format