Catálogo de publicaciones - libros

Just as the industrial society of the last century depended on natural resources, today’s society depends on information. A lack of resources in the industrial society hindered development just as a lack of information hinders development in the information society. Consequently, the exchange of information becomes essential for more and more areas of society: Companies announce their products in online marketplaces and exchange electronic orders with their suppliers; in the medical area patient information is exchanged between general practitioners, hospitals and health insurances; public administration receive tax information from employers and offer online services to their citizens. As a reply to this increasing importance of information exchange, new technologies supporting a fast and accurate information exchange are being developed. Prominent examples of such new technologies are so-called Semantic Web and Peer-to-Peer technologies . These technologies address different aspects of the inherit complexity of information exchange. Semantic Web Technologies address the problem of information complexity by providing advanced support for representing and processing information. Peer-to-Peer technologies, on the other hand, address system complexity by allowing flexible and decentralized information storage and processing.

RDF Query Language proposals are numerous. However, the most prominent proposals are query languages that were conceived as first generation tryouts of RDF querying, with little or no RDF-specific implementation and use experience to guide design, and based on an ever changing set of syntactical and semantic specifications. In this chapter, we introduce a set of general requirements for an RDF query language. This set is compiled from discussions between RDF implementors, our own experience and user feedback that we received on our work in Sesame, as well as general principles of query language design. We go on to show how we have compiled these requirements into designing the SeRQL query language, and conclude that SeRQL can be considered a real second generation RDF querying and transformation language.

The Resource Description Framework (RDF) is a step towards the support for integrated and uniform access to information sources. It is designed to standardize the definition and use of metadata descriptions of Web-based resources. It is complemented with RDF Schema (RDFS) that lets developers define an extensible, object-oriented type system for RDF data models. While RDF is targeted towards representing metadata, it can represent the underlying data as well. Together, RDF and RDFS provide a sound basis for the capture of domain knowledge in a form that lends itself for automatic processing. Since the Web is inherently distributed, RDF querying systems should also be able to handle the distribution of multiple, autonomous RDF repositories. A sound approach is needed for querying distributed RDF sources to disclose the information they contain. In this chapter we examine several architectures for querying distributed RDF (meta)data and point out a number of issues that will have to be dealt with in order to provide a viable solution. The discussion traces the development of distributed applications that attempt to exploit the promises of interoperability of the Semantic Web and have to deal with the requirements that arise from that. The chapter focuses on the problems associated with the introduction of RDF (meta)data in the distributed information system arena.

In Peer-to-peer (P2P) systems a very large number of autonomous computing nodes (the peers) pool together their resources and rely on each other for data and services. More and more P2P data management systems rely nowadays on intensional (i.e., schema) information for integrating and querying peer bases. Such information can be easily captured by emerging Semantic Web languages such as RDF/S. In this chapter, we present the SQPeer middleware for processing RQL queries over peer RDF/S bases (materialized or virtual), which are advertised using adequate RVL views. The novelty of SQPeer lies on the interleaved execution of the query routing and planning phases using intensional advertisements of peer bases under the form of RDF/S schema fragments (i.e., views). More precisely, routing is responsible for discovering peer views relevant to a specific query based on appropriate subsumption techniques of RDF/S schema fragments. On the other hand, query planning relies on the obtained data localization information, as well as compile and run-time optimization techniques. The generated plans are then executed in a fully distributed way by the involved peers for obtaining as fast as possible the first results of a query available in peer bases. This can be achieved by initially considering the peer bases that answer the whole query and at each iteration round, to route and evaluate smaller query fragments. The interleaved execution not only favors intra-peer processing, which is less expensive that the inter-peer one, but additionally takes benefit of a parallel execution of the query routing, planning and execution in different peers. Peers can exchange query plans and results, as well as, revisit established plans using appropriate communication channels. We finally demonstrate through examples the execution of two main query processing phases for two different architectural alternatives, namely a hybrid and a structured RDF/S schema-based P2P system.

Static DHT topologies influence important features of DHT systems such as their scalability, communication load balancing properties, routing efficiency and their fault tolerance. While obviously dynamic DHT algorithms which have to approximate these topologies for dynamically changing sets of peers play a very important role for DHT networks, important insights can be gained by clearly focussing on the static DHT topology as well. In this paper we analyze and classify current DHTs in terms of their static topologies based on the Cayley graph group-theoretic model and show that most DHT proposals use Cayley graphs as static DHT topologies, thus taking advantage of several important Cayley graph properties such as symmetry, decomposability and optimal fault tolerance. Using these insights, Cayley DHT design can directly leverage algebraic design methods to generate high-performance DHTs adopting Cayley graph based static DHT topologies, extended with suitable dynamic DHT algorithms.

In Peer-to-Peer networks, finding the appropriate answer for an information request, such as the answer to a query for RDF(S) data, depends on selecting the right peer in the network. We here investigate how social metaphors can be exploited effectively and efficiently to solve this task. To this end, we define a method for query routing, REMINDIN’, that lets (i) peers observe which queries are successfully answered by other peers, (ii) memorizes this observation, and, (iii) subsequently uses this information in order to select peers to forward requests to. REMINDIN’ has been implemented for the SWAP Peer-to-Peer platform as well as for a simulation environment. We have used the simulation environment in order to investigate how successful variations of REMINDIN’ are and how they compare to baseline strategies in terms of number of messages forwarded in the network and statements appropriately retrieved.

Peer-to-Peer systems have proven to be an effective way of sharing data. Finding the data in an efficient and robust manner still is a challenging problem. We propose a model in which peers advertise their expertise in the Peer-to-Peer network. The knowledge about the expertise of other peers forms a semantic overlay network (SON). Based on the semantic similarity between the subject of a query and the expertise of other peers, a peer can select appropriate peers to forward queries to, instead of broadcasting the query or sending it to a random set of peers. We evaluate the model in a bibliographic scenario, where peers share bibliographic descriptions of publications among each other. In simulation experiments complemented with a real-world field experiment we show how expertise based peer selection improves the performance of a Peer-to-Peer system with respect to precision, recall and the number of messages.

The Bibster system is an application of the use of semantics in Peer-to-Peer systems, which is aimed at researchers that share bibliographic metadata. In this paper we describe the design and implementation of recommender functionality in the Bibster system which allows personalized access to the bibliographic metadata available in the Peer-to-Peer network. These functions are based on a semantic user profile which is created from content and usage information as well as a similarity function. Furthermore, these functions make use of the semantic topology of the Peer-to-Peer system.

Publish/subscribe systems are an alternative to query-based systems in cases where the same information is asked for over and over, and where clients want to get updated answers for the same query over a period of time. Recent publish/subscribe systems such as P2P-DIET have introduced this paradigm in the P2P context. In this chapter we build on the experience gained with P2P-DIET and the Edutella super-peer infrastructure and present a semantic publish/subscribe system supporting metadata and a query language based on RDF. We define formally the basic concepts of our system and present detailed protocols for its operation.

A large amount of data, organized in heterogeneous schemas, is now available through the web. The problem of matching such schemas in order to allow global access to information has been recognized as one of the main challenges of the Semantic Web. In this paper, we propose a method for discovering mappings across schemas based on a new approach. This shifts the problem of schema matching from the process of computing linguistic or structural similarities between schema elements (what most other proposed approaches do) to the problem of deducing relations between sets of logical formulas representing the meaning of schema elements. We then show how to apply the approach to an interesting family of schemas, namely hierarchical classifications.