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A number of structured peer-to-peer (P2P) lookup protocols have been proposed recently. A P2P lookup protocol routes a lookup request to its target node in a P2P distributed system. Existing protocols achieve balanced routing traffic among nodes by assuming that lookup requests are evenly targeted at every node. However, when lookup requests concentrate on a few nodes simultaneously, these nodes become hot spots. Due to uneven routing patterns in existing protocols, hot spots cause unbalanced routing traffic which leads to routing bottlenecks. In this paper, we present a novel structured P2P lookup protocol called SCALLOP that delivers balanced routing and avoids routing bottlenecks at occurrences of hot spots. Among existing protocols, SCALLOP is the first one to accomplish this goal at the fundamental nature of a routing protocol. SCALLOP achieves balanced routing by uniquely constructing a balanced lookup tree for each node. The balanced tree evenly distributes routing traffic among sibling nodes and, therefore, avoids or reduces routing bottlenecks. In addition, as a load-balanced protocol, SCALLOP delivers asymptotically optimal lookup performance at the tradeoff between routing path and routing table size. We conducted a set of simulations to demonstrate the effectiveness of SCALLOP. The results show that, compared with a most-referenced and representative structured P2P lookup protocol and a graph-based extension of this protocol, SCALLOP significantly reduces routing bottlenecks while all three protocols deliver comparable lookup performance.

This paper presents a model-based approach to developing easily configurable and customizable and safe-touse UCAADS. The acronym UCAADS stands for user-centric automation and assistive devices and systems, which include automation tools used by individuals and care-providing institutions for purposes of improving their quality of life and quality of care. The UCAADS model underlying the development process of such devices incorporates workflow elements with goals, objectives, methods, and selections elements, and is executable. This unique combination enables the device behavior, user actions, and userdevice interactions to be specified in terms of a model for requirement capture and design assessment purposes early in the development process. The workflow elements in the device model become its implementation when the required programs and other resources become available. Reusable models, resource components, tools, and runtime environment needed to support this process are provided by UCAADS simulation environment and embedded workflow framework also described in this paper.Index Terms-Model-based development, simulation, system specification and evaluation, workflow.

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