Ee-Peng Lim
Ying Lu
Abstract
The main purpose of a digital library is to facilitate users easy access to enormous amount of globally networked information. Typically, this information includes preexisting public library catalog data, digitized document collections, and other databases. In this article, we describe the distributed query system of a digital library prototype system known as HARP. In the HARP project, we have designed and implemented a distributed query processor and its query front-end to support integrated queries to preexisting public library catalogs and structured databases. This article describes our experiences in the design of an extended Sequel (SQL) query language known as HarpSQL. It also presents the design and implementation of the distributed query system. Our experience in distributed query processor and user interface design and development will be highlighted. We believe that our prototyping effort will provide useful lessons to the development of a complete digital library infrastructure.
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Index Terms
- Harp: a distributed query system for legacy public libraries and structured databases
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Reviews
Svetlana Segarceanu
Harp is a prototype distributed query system for a digital library. This work addresses the query-processing problem involving both legacy library catalogues and structured data in SQL databases. The design of the system takes into account the model of legacy public library catalogues, specifically the representation of the bibliographic records in machine-readable cataloguing (MARC) format; the query capabilities of the remote access protocol Z39.50; and the integration of different types of data by extended predicates. The introduction presents the main objectives in digital library research and the research issues considered here. Section 2 relates this work to other research in the field. Section 3 describes the HarpSQL query language for writing integrated queries to bibliographic and SQL databases. Its features include foreign SQL and bibliographic tables, the MARCString data type, and the definition of the Extract and Contain predicates to suit different selection criteria on the MARCString attributes and perform the join operation on BIB tables and SQL tables. Section 4 introduces the distributed query processing architecture, which consists of two types of processes: query managers that associate the query with a specific graph, and query agents that process the subqueries of the graph. Based on this, section 5 presents the query processing strategy. Section 6 covers implementation issues for the distributed query processor. The query manager and agents are implemented as separate processes, and the query manager and the agent processes communicate using message queues. The HarpSQL server is realized by extending the POSTGRES database system. Section 7 gives the design and implementation of a graphical query formulation tool. Possible extensions to HarpSQL and conclusions are given in the last two sections. This work integrates the research efforts to ease remote, simultaneous access to legacy library collections and other archival data on the Internet. It focuses on distributed query processing, a less explored area of this field. The paper presents interesting and worthwhile research in multidatabase query processing.
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