7547507
Efficient Numerical Error Bounding for Replicated Network Services
The goal of this work is to support replicated network services that accept updates to numerical records from multiple wide-area locations. Given the high overhead of maintaining strong consistency, many replicated services can tolerate divergence of their shared data, as long as the numerical error is bounded. Target distributed services include replicated stock quotes services, online auctions, distributed sensor systems, wide-area resource accounting and load balancing for replicated servers. We present two algorithms to e ciently bound absolute error using only local information. Split-Weight AE separately bounds increases and decreases, while Compound-Weight AE bounds them together. The two algorithms can be combined to provide good performance and low space overhead. Our Inductive RE algorithm transforms relative error to absolute error solely based on local knowledge, taking advantage of the fact that the divergence was properly bounded prior to each invocation of the algorithm. We also discuss two optimizations that reduce the space and computational overheads in the algorithms. This work is supported in part by the National Science Foundation (EIA-99772879). Vahdat is also supported by an NSF CAREER award (CCR-9984328). Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the VLDB copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Very Large Data Base Endowment. To copy otherwise, or to republish, requires a fee and/or special permission from the Endowment. Proceedings of the 26th VLDB Conference, Cairo, Egypt, 2000.
26402360
Signed quorum systems
With n servers that independently fail with probability of p < 0.5, it is well known that the majority quorum system achieves the best availability among all quorum systems. However, even this optimal construction requires (n+1)/2 functioning servers out of n. Furthermore, the number of probes needed to acquire a quorum is also lower bounded by (n+1)/2.Motivated by the need for a highly available and low probe complexity quorum system in the Internet, this paper proposes signed quorum systems (SQS) that can be available as long as any O(1) servers are available, and simultaneously have O(1) probe complexity. SQS provides probabilistic intersection guarantees and exploits the property of independent mismatches in today’s Internet. Such key property has been validated previously under multiple Internet measurement traces. This paper then extensively studies the availability, probe complexity, and load of SQS, derives lower bounds for all three metrics, and constructs matching upper bounds. We show that in addition to the qualitatively superior availability and probe complexity, SQS also decouples availability from load and probe complexity, so that optimal availability can be achieved under most probe complexity and load values.
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.
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.
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.
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.
9764475
Towards a multimodal meeting record
Face-to-face meetings usually encompass several modalities including speech, gesture, handwriting, and person identification. Recognition and integration of each of these modalities is important to create an accurate record of a meeting. However, each of these modalities presents recognition difficulties. Speech recognition must be speaker and domain independent, have low word error rates, and be close to real time to be useful. Gesture and handwriting recognition must be writer independent and support a wide variety of writing styles. Person identification has difficulty with segmentation in a crowded room. Furthermore, in order to produce the record automatically, we have to solve the assignment problem (who is saying what), which involves people identification and speech recognition. This paper examines a multimodal meeting room system under development at Carnegie Mellon University that enables us to track, capture and integrate the important aspects of a meeting from people identification to meeting transcription. Once a multimedia meeting record is created, it can be archived for later retrieval.
15030041
Adaptive reduction parallelization techniques
In this paper, we propose to adapt parallelizing transformations, more specifically, reduction parallelizations, to the actual reference pattern executed by a loop, i.e., to the particular input data and dynamic phase of a program. More precisely we will show how, after validating a reduction at run-time (when this is not possible at compile time) we can dynamically characterize its reference pattern and choose the most appropriate method for parallelizing it. For this purpose, we develop a library of parallel reduction algorithms, including both previously known and novel schemes, which includes algorithms specialized for different classes of access behavior. In particular, each algorithm in our library has identified strengths related to specific reference pattern characteristics, which are matched, at run-time, with measured characteristics of the actual reference pattern. The matching of algorithm to reference pattern is performed using a decision-tree based selection scheme. The contribution of this work consists in new optimizations for reduction parallelization and in the introduction of a new approach to the optimization of irregular applications: Characteristic based customization.
16750086
Supporting transactional cache consistency in mobile database systems
In a mobile computing environment, caching of frequently accessed data has been shown to be a useful technique for reducing contention on the narrow bandwidth of the wireless channels. However, the traditional client/server strategies for supporting transactional cache consistency that require extensive communications between a client and a server are not appropriate in a mobile computing environment. In this paper, we propose a new protocol, called OCC-UTS (Optimisitic Concurrency Control with Update TimeStamp), to support transactional cache consistency in a wireless mobile computing environment by utilizing the broadcast-based solutions for the problem of invalidating caches. The consistency check on accessed data and the commitment protocol are implemented in a truly distributed fashion as an integral part of cache invalidation process, with most burden of consistency check being downloaded to mobile clients. Also, our esperiments based on an analytical model substantiate the basic idea and study the performance characteristics. Experimental results show that OCC-UTS protocol without local cache outperforms other competitor protocol, and the more frequent a mobile client accesses data items the more efficient OCC-UTS protocol with local cache is. With respect to disconnection, a mobile client aborts its transaction if it disconnects more than the invalidation broadcast window size. Thus, the tolerance to disconnection is improved if the invalidation broadcast window size is extended.
2395247
Combining Configurational and Statistical Approaches in Image Retrieval
We develop a framework for combining configurational and statistical approaches in image retrieval. While configurations have semantic description power, the explicit representation of an image by a set of configurations lacks the vector space structure from which the statistical feature-based representations have benefitted. That makes concept learning and prediction harder. Our framework treats configurations analogously to words occurring in a document. It combines a configuration-based approach with statistical approaches to take advantage of both the semantic description power of the former, and the simple vector-space structure of the latter.
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]: