Current systems are very often based on large-scale, unpredictable, and unreliable infrastructures. However, users of these systems increasingly require services with timeliness properties. This creates a difficult-to-solve contradiction with regard to the adequate time model: synchronous, or asynchronous? In this paper, we propose an architectural construct and programming model which address this problem. We assume the existence of a component that is capable of executing timely functions, however asynchronous the rest of the system may be. We call this component the Timely Computing Base and it can be used by the other components to execute a set of simple but crucial time-related services. We also show how to use it to build dependable and timely applications exhibiting varying degrees of timeliness assurance, under several synchrony models.
In open and heterogeneous environments, where an unpredictable number of applications compete for a limited amount of resources, executions can be affected by also unpredictable delays, which may not even be bounded. Since many of these applications have timeliness requirements, they can only be implemented if they are able to adapt to the existing conditions. Adaptation can be done by several ways, taking into account many different factors, but an obvious factor of success is knowing what they have to adapt to. In this paper we present a novel approach, called Dependable QoS adaptation, which can only be achieved if the environment is accurately and reliably observed.Dependable QoS adaptation is based on the Timely Computing Base (TCB) model. The TCB model is a partial synchrony model that adequately characterizes environments of uncertain synchrony and allows, at the same time, the specification and verification of timeliness requirements. We introduce the coverage stability property and show that adaptive applications can use the TCB to dependably adapt and enjoy this property. We describe the characteristics and the interface of a QoS coverage service and discuss its implementation details.
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Wireless sensor networks are being increasingly used in several application areas, particularly to collect data and monitor physical processes. Non-functional requirements, like reliability, security or availability, are often important and must be accounted for in the application development. For that purpose, there is a large body of knowledge on dependability techniques for distributed systems, which provide a good basis to understand how to satisfy these non-functional requirements of WSN-based monitoring applications. Given the data-centric nature of monitoring applications, it is of particular importance to ensure that data are reliable or, more generically, that they have the necessary quality. In this survey, we look into the problem of ensuring the desired quality of data for dependable monitoring using WSNs. We take a dependability-oriented perspective, reviewing the possible impairments to dependability and the prominent existing solutions to solve or mitigate these impairments. Despite the variety of components that may form a WSN-based monitoring system, we give particular attention to understanding which faults can affect sensors, how they can affect the quality of the information and how this quality can be improved and quantified.
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