An algorithm for clock synchronization with the gradient property in sensor networks

We introduce a distributed algorithm for clock synchronization in sensor networks. Our algorithm assumes that nodes in the network only know their immediate neighborhoods and an upper bound on the network's diameter. Clock-synchronization messages are only sent as part of the communication, assumed reasonably frequent, that already takes place among nodes. The algorithm has the gradient property of [2], achieving an O(1) worst-case skew between the logical clocks of neighbors. As in the case of [3,8], the algorithm's actions are such that no constant lower bound exists on the rate at which logical clocks progress in time, and for this reason the lower bound of [2,5] that forbids constant skew between neighbors does not apply

Why logical clocks are easy

Tracking causality should not be ignored. It is important in the design of many distributed algorithms. And not respecting causality can lead to strange behaviors for users. The most commonly used mechanisms for tracking causality, vector clocks and version vectors, are simply optimized representations of causal histories, which are easy to understand. By building on the notion of causal histories, users can begin to see the logic ehind these mechanisms, to identify how they differ, and even consider possible optimizations. When confronted with an unfamiliar causality tracking mechanism, or when trying to design a new system that requires it, readers should ask two simple questions, which events need tracking and how does the mechanism translate back to a simple causal history.We would like to thank Rodrigo Rodrigues, Marc Shapiro, Russell Brown, Sean Cribbs, and Justin Sheehy for their feedback. This work was partially supported by EU FP7 SyncFree project (609551) and FCT/MCT projects UID/CEC/04516/2013 and UID/EEA/50014/2013.info:eu-repo/semantics/publishedVersio

Logical clocks for could databases

Dissertação de mestrado em Engenharia de InformáticaCloud computing environments, particularly cloud databases, are rapidly increasing in importance, acceptance and usage in major (web) applications, that need the partition-tolerance and availability for scalability purposes, thus sacrificing the consistency side (CAP theorem). With this approach, use of paradigms such as Eventual Consistency became more widespread. In these environments, a large number of users access data stored in highly available storage systems. To provide good performance to geographically disperse users and allow operation even in the presence of failures or network partitions, these systems often rely on optimistic replication solutions that guarantee only eventual consistency. In this scenario, it is important to be able to accurately and e ciently identify updates executed concurrently. In this dissertation we review, and expose problems with current approaches to causality tracking in optimistic replication: these either lose information about causality or do not scale, as they require replicas to maintain information that grows linearly with the number of clients or updates. Then, we propose Dotted Version Vectors (DVV), a novel mechanism for dealing with data versioning in eventual consistent systems, that allows both accurate causality tracking and scalability both in the number of clients and servers, while limiting vector size to replication degree. We conclude with the challenges faced when implementing DVV in Riak (a distributed key-value store), the evaluation of its behavior and performance, and discuss the advantages and disadvantages of it.Ambientes de computação na nuvem, em especial sistemas de base de dados na nuvem, estão rapidamente a aumentar em importância, aceitação e utilização entre as grandes aplicações (web), que precisam de alta disponibilidade e tolerância a partições por razões de escalabilidade, para isso sacrificando o lado da coerência (teorema de CAP). Com esta abordagem, o uso de paradigmas como a Coerência Inevitável tornou-se generalizado. Nestes sistemas, um grande número de utilizadores têm acesso aos dados presentes em sistemas de dados de alta disponibilidade. Para fornecer bom desempenho para utilizadores geograficamente dispersos e permitir a realização de operações mesmo em presença de partições ou falhas de nós, estes sistemas usam técnicas de replicação optimista que garantem apenas uma coerência inevitável. Nestes cenários, é importante que a identificação de escritas concorrentes de dados, seja o mais exata e eficiente possível. Nesta dissertação, revemos os problemas com as abordagens atuais para o registo da causalidade na replicação optimista: estes ou perdem informação sobre a causalidade ou não escalam, já que obrigam as réplicas a manter informação que cresce linearmente com o número de clientes ou escritas. Propomos então, os Dotted Version Vectors (DVV), um novo mecanismo para lidar com o versionamento de dados em ambientes com coerência inevitável, que permite tanto um registo exato e correto da causalidade, bem como escalabilidade em relação ao número de clientes e número de servidores, limitando o seu tamanho ao factor de replicação. Concluímos com os desafios surgidos na implementação dos DVV no Riak (uma base de dados distribuída de chave/valor), a sua avaliação de comportamento e de desempenho, acabando com uma análise das vantagens e desvantagens da mesma

Adaptive Synchronization of Robotic Sensor Networks

The main focus of recent time synchronization research is developing power-efficient synchronization methods that meet pre-defined accuracy requirements. However, an aspect that has been often overlooked is the high dynamics of the network topology due to the mobility of the nodes. Employing existing flooding-based and peer-to-peer synchronization methods, are networked robots still be able to adapt themselves and self-adjust their logical clocks under mobile network dynamics? In this paper, we present the application and the evaluation of the existing synchronization methods on robotic sensor networks. We show through simulations that Adaptive Value Tracking synchronization is robust and efficient under mobility. Hence, deducing the time synchronization problem in robotic sensor networks into a dynamic value searching problem is preferable to existing synchronization methods in the literature.Comment: First International Workshop on Robotic Sensor Networks part of Cyber-Physical Systems Week, Berlin, Germany, 14 April 201

A Decidable Characterization of a Graphical Pi-calculus with Iterators

This paper presents the Pi-graphs, a visual paradigm for the modelling and verification of mobile systems. The language is a graphical variant of the Pi-calculus with iterators to express non-terminating behaviors. The operational semantics of Pi-graphs use ground notions of labelled transition and bisimulation, which means standard verification techniques can be applied. We show that bisimilarity is decidable for the proposed semantics, a result obtained thanks to an original notion of causal clock as well as the automatic garbage collection of unused names.Comment: In Proceedings INFINITY 2010, arXiv:1010.611

Monitoring Partially Synchronous Distributed Systems using SMT Solvers

In this paper, we discuss the feasibility of monitoring partially synchronous distributed systems to detect latent bugs, i.e., errors caused by concurrency and race conditions among concurrent processes. We present a monitoring framework where we model both system constraints and latent bugs as Satisfiability Modulo Theories (SMT) formulas, and we detect the presence of latent bugs using an SMT solver. We demonstrate the feasibility of our framework using both synthetic applications where latent bugs occur at any time with random probability and an application involving exclusive access to a shared resource with a subtle timing bug. We illustrate how the time required for verification is affected by parameters such as communication frequency, latency, and clock skew. Our results show that our framework can be used for real-life applications, and because our framework uses SMT solvers, the range of appropriate applications will increase as these solvers become more efficient over time.Comment: Technical Report corresponding to the paper accepted at Runtime Verification (RV) 201