5638908
Invalidation Clues for Database Scalability Services
For their scalability needs, data-intensive Web applications can use a database scalability service (DBSS), which caches applications' query results and answers queries on their behalf. One way for applications to address their security/privacy concerns when using a DBSS is to encrypt all data that passes through the DBSS. Doing so, however, causes the DBSS to invalidate large regions of its cache when data updates occur. To invalidate more precisely, the DBSS needs help in order to know which results to invalidate; such help inevitably reveals some properties about the data. In this paper, we present invalidation clues, a general technique that enables applications to reveal little data to the DBSS, yet limit the number of unnecessary invalidations. Compared with previous approaches, invalidation clues provide applications significantly improved tradeoffs between security/privacy and scalability. Our experiments using three Web application benchmarks, on a prototype DBSS we have built, confirm that invalidation clues are indeed a low-overhead, effective, and general technique for applications to balance their privacy and scalability needs.
14427569
Design and evaluation of a continuous consistency model for replicated services
The tradeoffs between consistency, performance, and availability are well understood. Traditionally, however, designers of replicated systems have been forced to choose from either strong consistency guarantees or none at all. This paper explores the semantic space between traditional strong and optimistic consistency models for replicated services. We argue that an important class of applications can tolerate relaxed consistency, but benefit from bounding the maximum rate of inconsistent access in an application-specific manner. Thus, we develop a set of metrics, Numerical Error, Order Error, and Staleness, to capture the consistency spectrum. We then present the design and implementation of TACT, a middleware layer that enforces arbitrary consistency bounds among replicas using these metrics. Finally, we show that three replicated applications demonstrate significant semantic and performance benefits from using our framework.
8892885
The costs and limits of availability for replicated services
As raw system and network performance continues to improve at exponential rates, the utility of many services is increasingly limited by availability rather than performance. A key approach to improving availability involves replicating the service across multiple, wide-area sites. However, replication introduces well-known tradeoffs between service consistency and availability. Thus, this paper explores the benefits of dynamically trading consistency for availability using a continuous consistency model. In this model, applications specify a maximum deviation from strong consistency on a per-replica basis. In this paper, we: i) evaluate availability of a prototype replication system running across the Internet as a function of consistency level, consistency protocol, and failure characteristics, ii) demonstrate that simple optimizations to existing consistency protocols result in significant availability improvements (more than an order of magnitude in some scenarios), iii) use our experience with these optimizations to prove tight upper bounds on the availability of services, and iv) show that maximizing availability typically entails remaining as close to strong consistency as possible during times of good connectivity, resulting in a communication versus availability trade-off.
30353816
SybilGuard: defending against sybil attacks via social networks
7603120
Defragmenting DHT-based Distributed File Systems
Existing DHT-based file systems use consistent hashing to assign file blocks to random machines. As a result, a user task accessing an entire file or multiple files needs to retrieve blocks from many different machines. This paper demonstrates that significant availability and performance gains can be achieved if instead, users are able to retrieve all the data needed for a given task from only a few DHT nodes. We explore the design and implications of such a "defragmented" DHT-based distributed file system, called D2, that also maintains important DHT properties like storage load balance. We show using real-world file system traces that a simple key encoding scheme is sufficient to maintain good defragmentation for most user tasks. Using both simulation and an actual 1,000 node deployment, we show that D2 increases availability by over an order of magnitude and improves user-perceived latency by 30- 100% compared to a traditional design.
2044757
Consistent and automatic replica regeneration
Reducing management costs and improving the availability of large-scale distributed systems require automatic replica regeneration, that is, creating new replicas in response to replica failures. A major challenge to regeneration is maintaining consistency when the replica group changes. Doing so is particularly difficult across the wide area where failure detection is complicated by network congestion and node overload.In this context, this article presents Om, the first read/write peer-to-peer, wide-area storage system that achieves high availability and manageability through online automatic regeneration while still preserving consistency guarantees. We achieve these properties through the following techniques. First, by utilizing the limited view divergence property in today's Internet and by adopting the witness model, Om is able to regenerate from any single replica, rather than requiring a majority quorum, at the cost of a small (10−6 in our experiments) probability of violating consistency during each regeneration. As a result, Om can deliver high availability with a small number of replicas, while traditional designs would significantly increase the number of replicas. Next, we distinguish failure-free reconfigurations from failure-induced ones, enabling common reconfigurations to proceed with a single round of communication. Finally, we use a lease graph among the replicas and a two-phase write protocol to optimize for reads, so that reads in Om can be processed by any single replica. Experiments on PlanetLab show that consistent regeneration in Om completes in approximately 20 seconds.
30916182
Reducing the rollback on mobile computing environments
1171693
A direct LDA algorithm for high-dimensional data - with application to face recognition
However, for a task with very high dimensional data such as images, the traditional LDA algorithm encounters several difficulties. Consider face recognition for example. A low-definition face image of size 64 by 64 implies a feature space of 64 × 64 = 4096 dimensions, and therefore scatter matrices of size 4096 × 4096 = 16M. First, it is computationally challenging to handle big matrices (such as computing eigenvalues). Second, those matrices are almost always singular, as the number of training images needs to be at least 16M for them to be non-degenerate. Due to these difficulties, it is commonly believed that a direct LDA solution for such high-dimensional data is infeasible. Thus, ironically, before LDA can be used to reduce dimensionality, another procedure has to be first applied for dimensionality reduction. In face recognition, many techniques have been proposed (For a good review, see [2]). Among them, the most notable is a two-stage PCA+LDA approach [4,1]:
2884857
A genetic approach to planning in heterogeneous computing environments
Planning is an artificial intelligence problem with a wide range of real-world applications. Genetic algorithms, neural networks, and simulated annealing are heuristic search methods often used to solve complex optimization problems. In this paper, we propose a genetic approach to planning in the context of workflow management and process coordination on a heterogenous grid. We report results for two planning problems, the Towers of Hanoi and the Sliding-tile puzzle.
22691758
A neural network for 500 word vocabulary word spotting using non-uniform units
We introduce acoustic sub-word units to neural networks for speaker-independent continuous speech recognition. The functions of segmenting input and detecting words are implemented with networks of simple structures. The non-uniform unit which we introduce in this research can model phoneme variations caused by co-articulation spread over several phonemes and between words. These units can be segmented by the network according to stationary and transition parts of speech without iteration or without considering all possible position shifts. A word lexicon can be trained by the network, which can effectively memorize all transcription variations in the training utterances of words. The results of speaker-independent word spotting of 520 words with TIMIT data are described.
120152362
Graph matching using spectral embedding and alignment
This paper describes how graph-spectral methods can be used to transform the node correspondence problem into one of point-set alignment. We commence by using the ISOMAP algorithm to embed the nodes of a graph in a low-dimensional Euclidean space. With the nodes in the graph transformed to points in a metric space, we can recast the problem of graph matching into that of aligning the points. Here, we use a variant of the Scott and Longuet-Higgins algorithm to find point correspondences. We experiment with the resulting algorithm on a number of real-world problems.