Update Consistency for Wait-free Concurrent Objects

In large scale systems such as the Internet, replicating data is an essential feature in order to provide availability and fault-tolerance. Attiya and Welch proved that using strong consistency criteria such as atomicity is costly as each operation may need an execution time linear with the latency of the communication network. Weaker consistency criteria like causal consistency and PRAM consistency do not ensure convergence. The different replicas are not guaranteed to converge towards a unique state. Eventual consistency guarantees that all replicas eventually converge when the participants stop updating. However, it fails to fully specify the semantics of the operations on shared objects and requires additional non-intuitive and error-prone distributed specification techniques. This paper introduces and formalizes a new consistency criterion, called update consistency, that requires the state of a replicated object to be consistent with a linearization of all the updates. In other words, whereas atomicity imposes a linearization of all of the operations, this criterion imposes this only on updates. Consequently some read operations may return out-dated values. Update consistency is stronger than eventual consistency, so we can replace eventually consistent objects with update consistent ones in any program. Finally, we prove that update consistency is universal, in the sense that any object can be implemented under this criterion in a distributed system where any number of nodes may crash.Comment: appears in International Parallel and Distributed Processing Symposium, May 2015, Hyderabad, Indi

Causal Consistency: Beyond Memory

In distributed systems where strong consistency is costly when not impossible, causal consistency provides a valuable abstraction to represent program executions as partial orders. In addition to the sequential program order of each computing entity, causal order also contains the semantic links between the events that affect the shared objects -- messages emission and reception in a communication channel , reads and writes on a shared register. Usual approaches based on semantic links are very difficult to adapt to other data types such as queues or counters because they require a specific analysis of causal dependencies for each data type. This paper presents a new approach to define causal consistency for any abstract data type based on sequential specifications. It explores, formalizes and studies the differences between three variations of causal consistency and highlights them in the light of PRAM, eventual consistency and sequential consistency: weak causal consistency, that captures the notion of causality preservation when focusing on convergence ; causal convergence that mixes weak causal consistency and convergence; and causal consistency, that coincides with causal memory when applied to shared memory.Comment: 21st ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming, Mar 2016, Barcelone, Spai

Time-Efficient Read/Write Register in Crash-prone Asynchronous Message-Passing Systems

GDD_HCERES2020The atomic register is certainly the most basic object of computing science. Its implementation on top of an n-process asynchronous message-passing system has received a lot of attention. It has been shown that t < n/2 (where t is the maximal number of processes that may crash) is a necessary and sufficient requirement to build an atomic register on top of a crash-prone asynchronous message-passing system. Considering such a context, this paper visits the notion of a fast implementation of an atomic register, and presents a new time-efficient asynchronous algorithm. Its time-efficiency is measured according to two different underlying synchrony assumptions. Whatever this assumption, a write operation always costs a round-trip delay, while a read operation costs always a round-trip delay in favorable circumstances (intuitively, when it is not concurrent with a write). When designing this algorithm, the design spirit was to be as close as possible to the one of the famous ABD algorithm (proposed by Attiya, Bar-Noy, and Dolev)

Brief Announcement: Wait-Free Universality of Consensus in the Infinite Arrival Model

International audienceIn classical asynchronous distributed systems composed of a fixed number n of processes where some proportion may fail by crashing, many objects do not have a wait-free linearizable implementation (e.g. stacks, queues, etc.). It has been proved that consensus is universal in such systems, which means that this system augmented with consensus objects allows to implement any object that has a sequential specification. In this paper, we consider a more general system model called infinite arrival model where infinitely many processes may arrive and leave or crash during a run. We prove that consensus is still universal in this more general model. For that, we propose a universal construction based on a weak log that can be implementated using consensus objects

Asynchronous Byzantine Systems: From Multivalued to Binary Consensus with t < n/3, O(n 2 ) Messages, O(1) Time, and no Signature

This paper presents a new algorithm that reduces multivalued consensus to binary consensus in an asynchronous message-passing system made up of n processes where up to t may commit Byzantine failures. This algorithm has the following noteworthy properties: it assumes t < n/3 (and is consequently optimal from a resilience point of view), uses O(n 2) messages, has a constant time complexity, and does not use signatures. The design of this reduction algorithm relies on two new all-to-all communication abstractions. The first one allows the non-faulty processes to reduce the number of proposed values to c, where c is a small constant. The second communication abstraction allows each non-faulty process to compute a set of (proposed) values such that, if the set of a non-faulty process contains a single value, then this value belongs to the set of any non-faulty process. Both communication abstractions have an O(n 2) message complexity and a constant time complexity. The reduction of multivalued Byzantine consensus to binary Byzantine consensus is then a simple sequential use of these communication abstractions. To the best of our knowledge, this is the first asynchronous message-passing algorithm that reduces multivalued consensus to binary consensus with O(n 2) messages and constant time complexity (measured with the longest causal chain of messages) in the presence of up to t < n/3 Byzantine processes, and without using cryptography techniques. Moreover, this reduction algorithm tolerates message re-ordering by Byzantine processes. Une réduction du consensus multivalué au consensus binaire en présence d'asynchronisme, de t < n/3 processus byzantins, avec un temps constant, O(n 2) messages, et pas de signatures Résumé : Cet article présente un algorithme réparti qui, dans un système asynchrone de n processus qui communiquent par passage de messages, et qui comprend jusqu'à t processus byzantins, ramène le problème du consensus multivalué au problème du consensus binaire. Cette réduction est optimale par rapport à t (t < n/3), requiert un temps constant et O(n 2) messages, et n'utilise aucun élément cryptographique (i.e., pas de signatures). Elle considère donc un adversaire donc la la puissance de calcul peut être illimitée

Two-Bit Messages are Sufficient to Implement Atomic Read/Write Registers in Crash-prone Systems

International audienceAtomic registers are certainly the most basic objects of computing science. Their implementation on top of an n-process asynchronous message-passing system has received a lot of attention. It has been shown that t < n/2 (where t is the maximal number of processes that may crash) is a necessary and sufficient requirement to build an atomic register on top of a crash-prone asynchronous message-passing system. Considering such a context, this paper presents an algorithm which implements a single-writer multi-reader atomic register with four message types only, and where no message needs to carry control information in addition to its type. Hence, two bits are sufficient to capture all the control information carried by all the implementation messages. Moreover, the messages of two types need to carry a data value while the messages of the two other types carry no value at all. As far as we know, this algorithm is the first with such an optimality property on the size of control information carried by messages. It is also particularly efficient from a time complexity point of view

The unscripted image

This article examines realism in the films of Ken Loach and Mike Leigh, comparing and relating the two director's distinct processes to the procedure of making a painting that represents everyday life. In particular, the article positions painting from found photographs as akin to making a film based on a very tight script and predetermined narrative (Loach). Leigh's mode of arriving at narrative, subject and meaning through devising is compared to painting from memory and experience without a constraint on outcome. The article references Ricoeur's proposals that narrative structures impose linear histories and Carel Weight's narrative paintings

Looking for Efficient Implementations of Concurrent Objects

As introduced by Taubenfeld, a contention-sensitive implementation of a concurrent object is an implementation such that the overhead introduced by locking is eliminated in the common cases, i.e., when there is no contention or when the operations accessing concurrently the object are non-interfering. This paper, that can be considered as an introductory paper to this topic, presents a methodological construction of a contention-sensitive implementation of a concurrent stack. In a contention-free context a push or pop operation does not rest on a lock mechanism and needs only six accesses to the shared memory. In case of concurrency a single lock is required. Moreover, the implementation is starvation-free (any operation is eventually executed). The paper, that presents the algorithms in an incremental way, visits also a family of liveness conditions and important concurrency-related concepts such as the notion of an abortable object