In this paper, we propose a communication-efficient top-k continuous query processing method on distributed local nodes where data are horizontally partitioned. A designated coordinator server takes the role of issuing queries from users to local nodes and delivering the results to users. The final results are requested via a top-k subscription which lets local nodes know which data and updates need to be returned to users. Our proposed method makes use of the active previously posed queries to identify a small set of needed top-k subscriptions. In addition, with the pre-indexed nodes' skylines, the number of local nodes to be subscribed can be significantly reduced. As a result, only a small number of subscriptions are informed to a small number of local nodes resulting in lower communication overhead. Furthermore, according to dynamic data updates, we also propose a method that prevents nodes from reporting needless updates and also maintenance procedures to preserve the consistency. The results of experiments that measure the volume of transferred data show that our proposed method significantly outperforms the previously proposed methods.

As Object-Oriented Database Systems (OODBS) play an increasingly important role in advanced database systems, OODBS performance becomes a significant issue. It is well known that there is a strong relationship between performance and the concurrency control algorithms employed by the Database Management System (DBMS). Class Granularity Locking (CGL) and Class Hierarchy Granularity Locking (CHGL) are proposed as the concurrency control algorithms for OODBS to minimize the locking overhead. However, their basic characteristics, including the licking overhead and concurrency, have not been extensively investigated and it is not known which one is most appropriate for the general case. In this paper, we construct a simulation model for OODBS and carry out several performance evaluation studies on these two Class-Hierarchy Locking protocols and the Non Class-Hierarchy Locking (NCL) protocol. The NCL protocal is a variation of the conventional two phase locking protocol being applied to OODBS data structures.

Recently, through high speed computer networks, multifarious information such as text, moving and still images, video, voice and control data is available. There is a natural demand to store such multimedia data in databases to facilitate their reuse in a wide variety of applications. Therefore, important research issues pertain to the investigation of appropriate database systems in multimedia computer network environments. In this paper, we first discuss the required technologies for multimedia information systems. Then we look at many multimedia information services through computer networks, and consider the importance of storing and effectively reusing such available multimedia data. To facilitate developing databases for use in these environments, we discuss the evolution of the notion of database systems. Finally, we demonstrate, as practical examples of such database systems, two prototype systems that we are currently implementing, i.e., the campus wide news on demand system and the ASN. 1 database system.

Many network protocols for routing messages have been proposed for mobile computing environments. In this paper, we consider the query processing strategy which operates over these network protocols. To begin with, we introduce five fundamental location update methods based on ideas extracted from the representative network protocols. They are the single broadcast notification (SBN), the double broadcast notification (WBN), the single default notification (SDN), the double default notification (WDN), and the no notification (NN). As a network protocol, each method is strong in performance in some system enrivonment, but weak in others. In practical situations, where various kinds of applications are used for various purposes, however, it is required to use a single method. We therefore propose an adaptive query processing strategy where these five location update methods can be dynamically selected. Moreover, we analyze the performance of this adaptive query processing strategy via the Markov chain. We also use the statistical approach to estimate the traffic of individual hosts. Finally, we show the efficiency of our proposed strategy over a wide area of system environments.

Mobile databases will play an important role in mobile computing environment, to provide data storing and data retrieval functionalities which are needed by most applications. In mobile computing environment, the wireless communication poses some problems, which require us to minimize its use. Replication is a database technique that is commonly used to fulfill the requirement in minimizing network usage. In this paper, we propose two replica allocation strategies, called primary-copy tracking replica allocation (PTRA) and user majority replica allocation (UMRA), which are better suited to the mobile computing environment. Their proposals are intended to cope with cost performance issues in data replication due to user mobility in mobile computing environment. To investigate their effectiveness, we provide access cost analysis and comparison on these strategies and the static replica allocation (SRA) strategy. We show that our proposed strategies outperform the SRA strategy when user mobility (inter-cell movement) is relatively low as compared with data access rate.

In recent years, the rapid advancements of wireless communication technology and computer down-sizing technology have enabled users to utilize computing resources anywhere in the computer network. New applications constructed on the mobile database system are becoming popular. However, the current database systems do not provide special facilities for specific update operations in a mobile computing environment. Moreover, due to the lack of a common data handling method and a mutual communication mechanism, varieties in implementations may cause applications to be incompatible with each other. In this paper, we take up the issue of data handling, in a mobile computing environment, and propose an active mobile database system (AMDS) to solve this issue. First, we review the difficulties of dynamic update of databases in a mobile computing environment, and provide a basic concept of AMDS as a solution for these difficulties. In order to construct an AMDS, we focus on asynchronous events such as the appearance and disappearance of a mobile computer in a wireless communication cell. Then we provide a facility to specify the behavior of each system in Event-Condition-Action(ECA) rules in the same way as normal active database systems. Moreover, we show the architecture and the design of our implementation of AMDS. And, finally AMDS can be easily implemented as a common database infrastructure and work well on heterogeneous systems through indoor experiments.

Recently, mobile computing has received much attention from database community. Sharing information among mobile users is one of the most challenging issues in mobile computing due to user mobility. Replication is a promising technique to this issue. However, adopting replication into mobile computing is a non-trivial task, since we are still facing other problems such as the lack in disk capacity and wireless network bandwidth used by mobile users. We have proposed a dynamic replica allocation strategy called User Majority Replica Allocation (UMRA) that is well suited to the modern architecture of mobile computing environment while avoiding such problems mentioned above. In this paper, we propose two relocation decision policies for UMRA and we provide a cost analysis for them. We also provide a cost analysis for another replica allocation strategy called Static Replica Allocation (SRA) for a comparison purpose.

With the advancement of technologies in wireless communication and computer down-sizing, it becomes possible to access a global network using handy terminals which are equipped with wireless communication facilities. In such a mobile computing environment, data management is one of the primary objectives of effective computer uses. However, since conventional distributed data management technologies assume that servers and clients are fixed at certain locations in a network, they do not provide any tools to construct advanced applications which make full use of dynamically changing information in such an environment. In this paper, in order to incorporate data distributed over mobile computing environment, we propose a dynamic data integration mechanism called MobiView which is an enhanced view mechanism of a conventional database system. In MobiView, we introduce four methods for database indication without using conventional host name or its local name: host-specified, cell-specified, location-dependent, and MobiView-oriented, through which we discuss how to handle both mobile database servers and mobile database clients.