Archiving scientific data

We present an archiving technique for hierarchical data with key structure. Our approach is based on the notion of timestamps whereby an element appearing in multiple versions of the database is stored only once along with a compact description of versions in which it appears. The basic idea of timestamping was discovered by Driscoll et. al. in the context of persistent data structures where one wishes to track the sequences of changes made to a data structure. We extend this idea to develop an archiving tool for XML data that is capable of providing meaningful change descriptions and can also efficiently support a variety of basic functions concerning the evolution of data such as retrieval of any specific version from the archive and querying the temporal history of any element. This is in contrast to diff-based approaches where such operations may require undoing a large number of changes or significant reasoning with the deltas. Surprisingly, our archiving technique does not incur any significant space overhead when contrasted with other approaches. Our experimental results support this and also show that the compacted archive file interacts well with other compression techniques. Finally, another useful property of our approach is that the resulting archive is also in XML and hence can directly leverage existing XML tools

Improved access point selection

This paper presents Virgil, an automatic access point discovery and selection system. Unlike existing systems that select access points based entirely on received signal strength, Virgil scans for all available APs at a location, quickly associates to each, and runs a battery of tests to estimate the quality of each AP’s connection to the Internet. Virgil also probes for blocked or redirected ports, to guide AP selection in favor of preserving application services that are currently in use. Results of our evaluation across five neighborhoods in three cities show Virgil finds a usable connection from 22% to 100 % more often than selecting based on signal strength alone. By caching AP test results, Virgil both improves performance and success rate. Our overhead is acceptable and is shown to be faster than manually selecting an AP with Windows XP

Congestion Control in a Reliable Scalable Message-Oriented Middleware

This paper presents congestion control mechanisms for reliable and scalable message-oriented middleware following the publish/subscribe communication model. We identify the key requirements of congestion control in this environment, how it differs from congestion control for the Internet, and propose a combination of two congestion control mechanisms, (1) driven by a publisher hosting broker (PDCC), (2) driven by a subscriber hosting broker (SDCC). SDCC decouples the notion of a receive window and a NACK window, and is used by subscriber hosting brokers in recovery mode. PDCC implements a scalable and low latency feedback loop between a publisher hosting broker and all subscriber hosting brokers, which is used to adjust the rate of publishing new messages, to allow brokers in recovery to eventually catch up, and other brokers to keep up. We present a detailed experimental evaluation of our implementation of these mechanisms in the Gryphon system by injecting network failures and link congestion

Implementing dynamic querying search in k-ary DHT-based overlays

Distributed Hash Tables (DHTs) provide scalable mechanisms for implementing resource discovery services in structured Peer-to-Peer (P2P) networks. However, DHT-based lookups do not support some types of queries which are fundamental in several classes of applications. A way to support arbitrary queries in structured P2P networks is implementing unstructured search techniques on top of DHT-based overlays. This approach has been exploited in the design of DQDHT, a P2P search algorithm that combines the dynamic querying (DQ) technique used in unstructured networks with an algorithm for efficient broadcast over a DHT. Similarly to DQ, DQ-DHT dynamically adapts the search extent on the basis of the desired number of results and the popularity of the resource to be found. Differently from DQ, DQ-DHT exploits the structural constraints of the DHT to avoid message duplications. The original DQ-DHT algorithm has been implemented using Chord as basic overlay. In this paper we focus on extending DQ-DHT to work in k-ary DHT-based overlays. In a k-ary DHT, broadcast takes only O(logkN) hops using O(logkN) pointers per node. We exploit this “k-ary principle” in DQ-DHT to improve the search time with respect to the original Chord-based implementation. This paper describes the implementation of DQ-DHT over a k-ary DHT and analyzes its performance in terms of search time and generated number of messages in different configuration scenarios

Mapping a Single Assignment Programming Language to Recon gurable Systems

Abstract. This paper presents the high level, machine independent, algorithmic, single-assignment programming language SA-C and its optimizing compiler targeting recon gurable systems. SA-C is intended for Image Processing applications. Language features are introduced and discussed. The intermediate forms DDCF, DFG and AHA, used in the optimization and code-generation phases are described. Conventional and recon gurable system speci c optimizations are introduced. The code generation process is described. The performance for these systems is analyzed, using a range of applications from simple Image Processing Library functions to more comprehensive applications, such as the ARAGTAP target acquisition prescreener