Sharad Mehrotra

A transaction model for multidatabase systems

A transaction model for multidatabase system (MDBS) applications in which global subtransactions may be either compensatable or retriable is presented. In this model compensation and retrying are used for recovery purposes. However, since such executions may no longer consist of atomic transactions, a correctness criterion that ensures that transactions see consistent database states is necessary. A commit protocol and a concurrency control scheme that ensures that all generated schedules are correct are also presented. The commit protocol eliminates the problem of blocking, which is characteristics of the standard 2PC protocol. The concurrency control protocol can be used in any MDBS environment irrespective of the concurrency control protocol followed by the local DBMSs in order to ensure serializability.<<ETX>>

The concurrency control problem in multidatabases: characteristics and solutions

A Multidatabase System (MDBS) is a collection of local database management systems, each of which may follow a different concurrency control protocol. This heterogeneity makes the task of ensuring global serializability in an MDBS environment difficult. In this paper, we reduce the problem of ensuring global serializability to the problem of ensuring serializability in a centralized database system. We identify characteristics of the concurrency control problem in an MDBS environment, and additional requirements on concurrency control schemes for ensuring global serializability. We then develop a range of concurrency control schemes that ensure global serializability in an MDBS environment, and at the same time meet the requirements. Finally, we study the tradeoffs between the complexities of the various schemes and the degree of concurrency provided by each of them.

Efficient processing of proximity queries for large databases

Emerging multimedia applications require database systems to provide support for new types of objects and to process queries that may have no parallel in traditional database applications. One such important class of queries are the proximity queries that aims to retrieve objects in the database that are related by a distance metric in a way that is specified by the query. The importance of proximity queries has earlier been realized in developing constructs for visual languages. In this paper, we present algorithms for answering a class of proximity queries-fixed-radius nearest-neighbor queries over point object. Processing proximity queries using existing query processing techniques results in high CPU and I/O costs. We develop new algorithms to answer proximity queries over objects that lie in the one-dimensional space (e.g., words in a document). The algorithms exploit query semantics to reduce the CPU and I/O costs, and hence improve performance. We also show how our algorithms can be generalized to handle d-dimensional objects.<<ETX>>

The Gold Mailer

The Gold Mailer, a system that provides users with an integrated way to send and receive messages using different media, efficiently store and retrieve these messages, and access a variety of sources of other useful information, is described. The mailer solves the problems of information overload, organization of messages and multiple interfaces. By providing good storage and retrieval facilities, it can be used as a powerful information processing engine covering a range of useful office information. The Gold Mailers query language, indexing engine, file organization, data structures, and support of mail message data and multimedia documents are discussed.<<ETX>>

The Gold Text Indexing Engine

The proliferation of electronic communication including computer mail, faxes, voice mail, and net news has led to a variety of disjoint applications and usage paradigms that forces users to deal with multiple different user interfaces and access related information arriving over the different communication media separately. To enable users to cope with the overload of information arriving over heterogeneous communication media, we have developed the Gold document handling system that allows users to access all of these forms of communication at once, or to intermix them. The Gold system provides users with an integrated way to send and recieve messages using different media, efficiently store the messages, retrieve the messages based on their contents, and to access a variety of other sources of useful information. At the center of the Gold document handling system is the Gold Text Indexing Engine (GTIE) that provides a full text index over the documents. The paper describes our implementation of GTIE and the concurrency control protocols to ensure consistency of the index in the presence of concurrent operations.

Interoperability with unstructured data and services

A large percentage of valuable information is not stored in database management systems. Therefore, many applications must deal only only with heterogeneous structured data, but with a wide variety of unstructured and semi-structured data such as electronic mail, documents, files and spreadsheets. In addition, these applications must deal with heterogeneous services such as an electronic library or an airline reservation service. While it is, of course, important to study interoperability among heterogeneous conventional database systems, it is also very important to study interoperation in this broader domain, where application programs must interact with a wide variety of services and must handle many types of structures, unstructured, and semi-structured data.<<ETX>>

Continuous Media Databases

Guest Editorial: Continuous Media Databases A. Zhang, et al. Incremental Scheduling of Mixed Workloads in Multimedia Information Servers G. Nerjes, et al. Threshold-Based Dynamic Replication in Large-Scale Video-on-Demand Systems P.W.K. Lie, et al. Query Processing Techniques for Multimedia Presentations T. Lee, et al. Design of Multi-User Editing Servers for Continuous Media S. Ghandeharizadeh, S.H. Kim. Super-Streaming: A New Object Delivery Paradigm for Continuous Media Servers C. Shahabi, M.H. Alshayeji.