Margaret H. Dunham

Using semantic caching to manage location dependent data in mobile computing

Location-dependent applications are becoming very popular in mobile environments. To improve system performance and facilitate disconnection, caching is crucial to such applications. In this paper, a semantic caching scheme is used to access location dependent data in mobile computing. We first develop a mobility model to represent the moving behaviors of mobile users and formally define location dependent queries. We then investigate query processing and cache management strategies. The performance of the semantic caching scheme and its replacement strategy FAR is evaluated through a simulation study. Our results show that semantic caching is more flexible and effective for use in LDD applications than page caching, whose performance is quite sensitive to the database physical organization. We also notice that the semantic cache replacement strategy FAR, which utilizes the semantic locality in terms of locations, performs robustly under different kinds of workloads.

A mobile transaction model that captures both the data and movement behavior

Unlike distributed transactions, mobile transactions do not originate and end at the same site. The implication of the movement of such transactions is that classical atomicity, concurrency and recovery solutions must be revisited to capture the movement behavior. As an effort in this direction, we define a model of mobile transactions by building on the concepts of split transactions and global transactions in a multidatabase environment. Our view of mobile transactions, called Kangaroo Transactions, incorporates the property that transactions in a mobile computing system hop from one base station to another as the mobile unit moves through cells. Our model is the first to capture this movement behavior as well as the data behavior which reflects the access to data located in databases throughout the static network. The mobile behavior is dynamic and is realized in our model via the use of split operations. The data access behavior is captured by using the idea of global and local transactions in a multidatabase system.

Semantic caching and query processing

Semantic caching is very attractive for use in distributed systems due to the reduced network traffic and the improved response time. It is particularly efficient for a mobile computing environment, where the bandwidth of wireless links is a major performance bottleneck. Previous work either does not provide a formal semantic caching model, or lacks efficient query processing strategies. This paper extends the existing research in three ways: formal definitions associated with semantic caching are presented, query processing strategies are investigated and, finally, the performance of the semantic cache model is examined through a detailed simulation study.

Location dependent query processing

The advances in wireless and mobile computing allow a mobile user to perform a wide range of aplications once limited to non-mobile hard wired computing environments. As the geographical position of a mobile user is becoming more trackable, users need to pull data which are related to their location, perhaps seeking information about unfamiliar places or local lifestyle data. In these requests, a location attribute has to be identified in order to provide more efficient access to location dependent data, whose value is determined by the location to which it is related. Local yellow pages, local events, and weather information are some of the examples of these data. In this paper, we give a formalization of location relatedness in queries. We differentiate location dependence and location awareness and provide thorough examples to support our approach.

Mobile computing and databases: anything new?

Recent advances in hardware technologies, such as portable computers and wireless communication networks, have led to the emergence of mobile computing systems. No one challenges the idea that mobile computing offers many opportunities for research within the Computer Science area. However, one could ask are there really any new database problems introduced when a mobile computing environment is assumed. We feel that the status of data management in mobile computing is similar to that of distributed data management versus centralized data management in the late 60s. Namely that many of the issues are the same, but the solutions are different. This analogy has been raised by others [1, 12]. We use this as the basis by which we answer the above question. We concentrate on discussing the differences between data management solutions in a mobile computing environment and those in a distributed database environment. The purpose of this paper is twofold: to spawn further interest in mobile computing research and to convince the skeptics that there are new research topics in mobile computing worthy of further examination.

Mining association rules: anti-skew algorithms

Mining association rules among items in a large database has been recognized as one of the most important data mining problems. All proposed approaches for this problem require scanning the entire database at least or almost twice in the worst case. We propose several techniques which overcome the problem of data skew in the basket data. These techniques reduce the maximum number of scans to less than 2, and in most cases find all association rules in about 1 scan. Our algorithms employ prior knowledge collected during the mining process and/or via sampling, to further reduce the number of candidate itemsets and identify false candidate itemsets at an earlier stage.

Efficient mining of traversal patterns

Abstract A new problem of mining traversal patterns from Web access logs is introduced. The traversal patterns are defined to keep duplicates as well as consecutive ordering in the sessions. Then an efficient algorithm is proposed. The algorithm is online, which allows the user to see the incremental results with respect to the scanned part of the database. The algorithm also adapts to large databases through dynamic compressions and effective pruning. Finally the algorithm is evaluated through experiments with real Web logs.

Common subexpression processing in multiple-query processing

The efficiency of common subexpression identification is critical to the performance of multiple-query processing. In this paper, we develop a multigraph for representing and facilitating the processing of multiple queries. In addition to the traditional multiple-query processing approaches in exploiting common subexpressions for identical and subsumption cases, the proposed multigraph processing also covers the overlap case. A performance study shows the viability of this technique when compared to an earlier multigraph approach.

Indexing valid time databases via B/sup +/-trees

We present an approach, named MAP21, which uses standard B/sup +/-trees to provide efficient indexing of valid time ranges. The MAP21 approach is based on mapping one dimensional ranges to one dimensional points where the lexicographical order among the ranges is preserved. The proposed approach may employ more than one tree, each indexing a disjoint subset of the indexed data. When compared to the Time Index and the B/sup +/-tree we show that MAP21s performance is comparable to or better than those, depending on the type of query. In terms of storage, MAP21s structure was less than 10 percent larger than the B/sup +/-trees and much smaller than the Time Indexs. The main contribution of this paper though, is to show that standard B/sup +/-trees, available in virtually any DBMS, can be used to provide an efficient temporal index.

Impact of mobility on transaction management

We examine the impact of mobility on three different options to managing mobile transactions. Using an analytic performance study we show that no one management approach is always the best and that the performance is impacted by the location management strategy used. The primary objective of this work is to illustrate that the underlying implementation strategies do indeed impact mobile transaction processing and should be considered when proposing mobile transaction processing techniques.