Right On Time Distributed Shared Memory

The demand for real-time data storage in distributed control systems (DCSs) is growing. Yet, providing real- time DCS guarantees is challenging, especially when more and more sensor and actuator devices are connected to industrial plants and message loss needs to be taken into account. In this paper, we investigate how to build a shared memory abstraction for DCSs as a first step towards implementing different shared storage systems in a DCS context. We first prove that, in the presence of host crashes and message losses, the necessary guarantees of such an abstraction are impossible to implement using a traditional approach that has no access to the internals of existing DCS services, e.g., a modular approach where algorithms are built on top of existing software blocks like failure detectors. We propose a white-box approach that utilizes messages of existing services in any DCS as the sole means of communication. More precisely, we present TapeWorm, an algorithm that attaches itself to the heartbeat messages of the failure detector component in DCSs. We prove that TapeWorm implements the desired shared memory guarantees for applications running on a DCS. We also analyze the performance of TapeWorm and we showcase ways of adapting TapeWorm to various application needs and workloads

Never Say Never Probabilistic & Temporal Failure Detectors (Extended)

The failure detector approach for solving distributed computing problems has been celebrated for its modularity. This approach allows the construction of algorithms using abstract failure detection mechanisms, defined by axiomatic properties, as building blocks. The minimal synchrony assumptions on communication, which enable to implement the failure detection mechanism, are studied separately. Such synchrony assumptions are typically expressed as eventual guarantees that need to hold, after some point in time, forever and deterministically. But in practice, they never do. Synchrony assumptions may hold only probabilistically and temporarily. In this paper, we study failure detectors in a realistic distributed system N, with asynchrony inflicted by probabilistic synchronous communication. We address the following paradox about the weakest failure detector to solve the consensus problem (and many equivalent problems), i.e., S: an implementation of “consensus with probability 1” is possible in N without using randomness in the algorithm itself, while an implementation of “S with probability 1” is impossible to achieve in N. We circumvent this paradox by introducing a new failure detector S*, a variant of S with probabilistic and temporal accuracy. We prove that S* is implementable in N and we provide an optimal S* implementation. Interestingly, we show that S* can replace S , in several existing deterministic consensus algorithms using S, to yield an algorithm that solves “consensus with probability 1”. In fact, we show that such result holds for all decisive problems (not only consensus) and also for failure detector P (not only S). The resulting algorithms combine the modularity of distributed computing practices with the practicality of networking ones

Simultaneous Acoustic Localization of Multiple Smartphones with Euclidean Distance Matrices

In this paper, we present an acoustic localization system for multiple devices. In contrast to systems which localize a device relative to one or several anchor points, we focus on the joint localization of several devices relative to each other. We present a prototype of our system on off-the-shelf smartphones. No user interaction is required, the phones emit acoustic pulses according to a precomputed schedule. Using the elapsed time between two times of arrivals (ETOA) method with sample counting, distances between the devices are estimated. These, possibly incomplete, distances are the input to an efficient and robust multi-dimensional scaling algorithm returning a position for each phone. We evaluated our system in real-world scenarios, achieving error margins of 15 cm in an office environment