On Storing Voluminous RDF Descriptions: The Case of Web Portal Catalogs

The academic, corporate and industrial interest in information systems such as corporate memories, vertical aggregators, infomediaries, etc. enabling to select, classify and access, in a useful and meaningful way, various information resources (e.g., sites, documents, data) for diverse target audiences has been increasing over the last few years. These systems, termed Community Web Portals, rely on a Catalog holding descriptions, i.e., metadata, about the available resources on corporate intranets or the Web [6]. In order to e ectively disseminate community knowledge, Portal Catalogs assimilate and organize information in a multitude of ways, which are far more exible and powerful than those provided by standard (object, relational) or XML databases [3, 1]. Recent Web standards such as the W3C Resource Description Framework (RDF) [8, 5] are proved to be more suitable for creating and exchanging resource descriptions between Community Webs. In this paper, we address storage issues of voluminous RDF metadata using an object-relational DBMS (ORDBMS) and the catalog of the Open Directory Portal exported in RDF as test metadata. RDF [8] provides i) a Standard Representation Language for metadata based on directed labeled graphs in which nodes are called resources (or literals) and edges are called properties, and ii) an XML syntax for expressing metadata in a form that is both humanly readable and machine understandable. The most distinctive RDF feature is its ability to support superimposed descriptions for the same Web resources, enabling content syndication and hence, automated processing in a variety of application areas (e.g., administration, recommendation, content rating, intellectual property rights, site maps, push channels, etc.). To interpret these descriptions within or across communities, RDF allows for the de nition of schemas [5] i.e., vocabularies of labels for graph nodes (i.e., classes) and edges (i.e., properties) that can be used to describe and query RDF description bases. Many content providers (e.g., ABCNews, CNN, Time Inc.), Web Portals (e.g., Open Directory, CNET, XMLTree), browsers (e.g., Netscape 6.0, W3C Amaya), as well as, emerging application standards for Web data and services syndication (e.g., the RDF Site Summary [4], Dublin Core [15], or the Web Service Description Language [16]) have already adopted RDF. In a nutshell, the growing number of Web resources and the proliferation of description services lead nowadays to large volumes of RDF metadata (e.g., the Open Directory Portal of Netscape comprises over 170M of Subject Topics and 700M of indexed URIs). Storing Web data, such as RDF schema and resource descriptions, in an ORDBMS is not a novel issue. However, existing database representations and benchmarks [7, 13, 12, 14] need to be reassessed in our setting, since: a) RDF has a di erent data model featuring labels on both nodes and edges, as well as, taxonomies of labels (see Section 2), and, hence, demands a di erent querying functionality than in semistructured or XML databases [1]; b) our test metadata contain a signi cantly larger volume of schema information than that used in previous testbeds (i.e., XML elements, attributes). In this paper, we use PostgreSQL to compare the performance of two well-known database representations (generic versus speci c), adapted to the peculiarities of RDF. More precisely, the schema-speci c representation outperforms the generic representation both in the required storage volume and in query execution time. The performance of the two representations has been evaluated with a series of tests involving queries on schema, data or a combination thereof.