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Earth observation Low Earth Orbit (LEO) satellites collect enormous amounts of data that needs to be transferred first to ground stations and then to the cloud, for storage and processing. Satellites today transmit data greedily to ground stations, with full utilization of bandwidth during each contact period. We show that due to the layout of ground stations and orbital characteristics, this approach overloads some ground stations and underloads others, leading to lost throughput and large end-to-end latency for images. We present a new end-to-end scheduler system called Umbra, which plans transfers from large satellite constellations through ground stations to the cloud, by accounting for both spatial and temporal factors, i.e., orbital dynamics, bandwidth constraints, and queue sizes. At the heart of Umbra is a new class of scheduling algorithms called withhold scheduling, wherein the sender (i.e., satellite) selectively under-utilizes some links to ground stations. We show that Umbra’s counter-intuitive approach increases throughput by 13-31% & reduces P90 latency by 3-6 X. 

Classical leader election protocols typically assume complete and correct knowledge of underlying membership lists at all participating nodes. Yet many edge and IoT settings are dynamic, with nodes joining, leaving, and failing continuously—a phenomenon called churn. This implies that in any membership protocol, a given node’s membership list may have entries that are missing (e.g., false positive detections, or newly joined nodes whose information has not spread yet) or stale (e.g., failed nodes that are undetected)—these would render classical election protocols incorrect. We present a family of four leader election protocols that are churn-tolerant (or c-tolerant). The key ideas are to: i) involve the minimum number of nodes necessary to achieve safety; ii) use optimism so that decisions are made faster when churn is low; iii) incorporate a preference for electing healthier nodes as leaders. We prove the correctness and safety of our c-tolerant protocols and show their message complexity is optimal. We present experimental results from both a trace- driven simulation as well as our implementation atop Raspberry Pi devices, including a comparison against Zookeeper. 

With the increasing adoption of smart home devices, users rely on device automation to control their homes. This automation commonly comes in the form of smart home routines, an abstraction available via major vendors. Yet, questions remain about how a system should best handle conflicts in which different routines access the same devices simultaneously. In particular---among the myriad ways a smart home system could handle conflicts, which of them are currently utilized by existing systems, and which ones result in the highest user satisfaction? We investigate the first question via a survey of existing literature and find a set of conditions, modifications, and system strategies related to handling conflicts. We answer the second question via a scenario-based Mechanical-Turk survey of users interested in owning smart home devices and current smart home device owners (N=197). We find that: (i) there is no context-agnostic strategy that always results in high user satisfaction, and (ii) users' personal values frequently form the basis for shaping their expectations of how routines should execute. 

Smart environments (homes, factories, hospitals, buildings) contain an increasing number of IoT devices, making them complex to manage. Today, in smart homes when users or triggers initiate routines (i.e., a sequence of commands), concurrent routines and device failures can cause incongruent outcomes. We describe SafeHome, a system that provides notions of atomicity and serial equivalence for smart homes. Due to the human-facing nature of smart homes, SafeHome offers a spectrum of visibility models which trade off between responsiveness vs. isolation of the smart home. We implemented SafeHome and performed workload-driven experiments. We find that a weak visibility model, called eventual visibility, is almost as fast as today's status quo (up to 23% slower) and yet guarantees serially-equivalent end states. 

Efficient and correct operation of an IoT network requires the presence of a failure detector and membership protocol amongst the IoT nodes. This paper presents a new failure de- tector for IoT settings where nodes are connected via a wire- less ad-hoc network. This failure detector, which we name Medley, is fully decentralized, allows IoT nodes to maintain a local membership list of other alive nodes, detects failures quickly (and updates the membership list), and incurs low communication overhead in the underlying ad-hoc network. In order to minimize detection time and communication, we adapt a failure detector originally proposed for datacenters (SWIM), for the IoT environment. In Medley each node picks a medley of ping targets in a randomized and skewed manner, preferring nearer nodes. Via analysis and NS-3 simulation we show the right mix of pinging probabilities that simulta- neously optimize detection time and communication traffic. We have also implemented Medley for Raspberry Pis, and present deployment results. 

As smart home environments get more complex and denser, they are becoming harder to manage. We present our ongoing work on the design and implementation of ``SafeHome'', a system for management and coordination inside a smart home. SafeHome offers users and programmers the flexibility to specify safety properties in a declarative way, and to specify routines of commands in an imperative way. SafeHome includes mechanisms which ensure that under concurrent routines and device failures, the smart home behavior is consistent (e.g., serializable) and safety properties are always guaranteed. SafeHome is intended to run on edge machines co-located with the smart home. Our design space opens the opportunity to borrow and adapt rich ideas and mechanisms from related areas such as databases and compilers. Paper available (Open Access) at: https://www.usenix.org/conference/hotedge19/presentation/ahsan