RDF and RDF Schema are two W3C standards aimed at enriching the Web with machine-processable semantic data.

Real-scale Semantic Web applications, such as Web Portals and E-Marketplaces, require the management of voluminous metadata repositories containing descriptive information (i.e., metadata) about the available Web resources and services. Better knowledge about the meaning, usage, accessibility or quality of these resources and services will considerably facilitate the automated processing of both Web content and services. In this context, the Resource Description Framework (RDF) enables the creation and exchange of metadata as any other Web data. Although large volumes of RDF descriptions are already appearing (e.g., as exported Portal catalogs or service descriptions), sufficiently expressive declarative languages for querying both RDF descriptions and schemas are still missing. In this paper, we propose RQL, a new RDF query language, relying on a formal graph model that permits the interpretation of superimposed resource descriptions. RQL is an OQL-inspired adaptation of XML query languages to the peculiarities of RDF but, foremost, is an extension of this functionality for uniformly querying both descriptions and schemas. We illustrate the syntax, semantics and core functionality of RQL bymeans of a set of benchmark queries and report on the performance of RSSDB, our persistent RDF Store, for storing and querying voluminous RDF descriptions.

Matchmaking is an important aspect of e-commerce interactions. Advanced matchmaking services require rich and flexible metadata that are not supported by currently available industry standard frameworks for e-commerce such as UDDI and ebXML. The semantic web initiative at W3C is gaining momentum and generating technologies and tools that might help bridge the gap between the current standard solutions and the requirement for advanced matchmaking services. In this paper we examine the problem of matchmaking, highlighting the features that a matchmaking service should exhibit and deriving requirements on metadata for description of services from a matchmaking point of view. We then assess a couple of standard frameworks for e-commerce against these requirements. Finally, we report on our experience of developing a semantic web based matchmaking prototype. In particular, we present our views on usefulness, adequacy, maturity and tool support of semantic web related technologies such as RDF and DAML. Keywords. Semantic Web, E-Commerce, Matchmaking Services, Automated Negotiation, Electronic Marketplaces, Ontology

Storing and querying Resource Description Framework (RDF) data is one of the basic tasks within any Semantic Web application. A number of storage systems provide assistance for this task. However, current RDF database systems do not use optimized indexes, which results in a poor performance behavior for querying RDF. In this paper we describe optimized index structures for RDF, show how to process and evaluate queries based on the index structure, describe a lightweight adaptable implementation in Java, and provide a performance comparison with existing RDF databases.

RDF and RDF Schema provide the first W3C standard to enrich the Web with machine-processable semantic data. However, to be able to use this semantic data, a scalable, persistent RDF store and a powerful query engine using an expressive query language are needed. Sesame is an extensible architecture implementing both of these. Sesame can be based on arbitrary repositories, ranging from traditional Data Base Management Systems, to dedicated RDF triple stores. Sesame also implements a query engine for RQL, the most powerful RDF/RDF Schema query language to date. 1

Despite the intense interest towards realizing the Semantic Web vision, most existing RDF data management schemes are constrained in terms of efficiency and scalability. Still, the growing popularity of the RDF format arguably calls for an effort to offset these drawbacks. Viewed from a relationaldatabase perspective, these constraints are derived from the very nature of the RDF data model, which is based on a triple format. Recent research has attempted to address these constraints using a vertical-partitioning approach, in which separate two-column tables are constructed for each property. However, as we show, this approach suffers from similar scalability drawbacks on queries that are not bound by RDF property value. In this paper, we propose an RDF storage scheme that uses the triple nature of RDF as an asset. This scheme enhances the vertical partitioning idea and takes it to its logical conclusion. RDF data is indexed in six possible ways, one for each possible ordering of the three RDF elements. Each instance of an RDF element is associated with two vectors; each such vector gathers elements of one of the other types, along with lists of the third-type resources attached to each vector element. Hence, a sextupleindexing scheme emerges. This format allows for quick and scalable general-purpose query processing; it confers significant advantages (up to five orders of magnitude) compared to previous approaches for RDF data management, at the price of a worst-case five-fold increase in index space. We experimentally document the advantages of our approach on real-world and synthetic data sets with practical queries.

This paper presents the notion of Semantic Associations as complex relationships between resource entities. These relationships capture both a connectivity of entities as well as similarity of entities based on a specific notion of similarity called ρ-isomorphism. It formalizes these notions for the RDF data model, by introducing a notion of a Property Sequence as a type. In the context of a graph model such as that for RDF, Semantic Associations amount to specific certain graph signatures. Specifically, they refer to sequences (i.e. directed paths) here called Property Sequences, between entities, networks of Property Sequences (i.e. undirected paths), or subgraphs of ρ-isomorphic Property Sequences. The ability to query about the existence of such relationships is fundamental to tasks in analytical domains such as national security and business intelligence, where tasks often focus on finding complex yet meaningful and obscured relationships between entities. However, support for such queries is lacking in contemporary query systems, including those for RDF. This paper discusses how querying for Semantic Associations might be enabled on the Semantic Web, through the use of an operator ρ. It also discusses two approaches for processing ρqueries on available persistent RDF stores and memory resident RDF data graphs, thereby building on current RDF query languages.

This paper focuses on the optimization of the navigation through voluminous subsumption hierarchies of topics employed by Portal Catalogs like Netscape Open Directory (ODP). Weadvocate for the use of labeling schemes for modeling these hierarchies in order to efficiently answer queries such as subsumption check, descendants, ancestors or nearest common ancestor, which usually require costly transitive closure computations. We first give a qualitative comparison of three main families of schemes, namely bit vector, prefix and interval based schemes. We then show that two labeling schemes are good candidates for an efficient implementation of label querying using standard relational DBMS, namely, the Dewey Prefix scheme[6] and an Interval scheme byAgrawal, Borgida and Jagadish[1] We compare their storage and query evaluation performance for the 16 ODP hierarchies using the PostgreSQL engine.

The growing use of ontologies in applications creates the need for an infrastructure that allows developers to more easily combine different software modules like ontology stores, editors, or inference engines towards comprehensive ontology-based solutions. We call such an infrastructure Ontology Software Environment. The papers discusses requirements and design issues of such an Ontology Software Environment. In particular, we present this discussion in light of the ontology and (meta)data standards that exist in the Semantic Web and present our corresponding implementation, the KAON SERVER.

The Semantic Web is based on XML and RDF as its fundamental standards for exchanging and storing information on the World Wide Web. Event-condition-action (ECA) rules are a natural candidate for supporting reactive functionality on XML or RDF repositories. In this paper we describe a language for ECA rules on XML and a prototype implementation of this language. We also discuss some preliminary ideas regarding a language for ECA rules operating on a graph/triple representation of RDF, and we describe the architecture of a distributed deployment of such RDF ECA rules.