Wolfgang Nejdl

A scalable and ontology-based P2P infrastructure for Semantic Web Services

Semantic Web Services are a promising combination of Semantic Web and Web service technology, aiming at providing means of automatically executing, discovering and composing semantically marked-up Web services. We envision peer-to-peer networks which allow for carrying out searches in real-time on permanently reconfiguring networks to be an ideal infrastructure for deploying a network of Semantic Web Service providers. However, P2P networks evolving in an unorganized manner suffer from serious scalability problems, limiting the number of nodes in the network, creating network overload and pushing search times to unacceptable limits. We address these problems by imposing a deterministic shape on P2P networks: We propose a graph topology which allows for very efficient broadcast and search, and we provide an efficient topology construction and maintenance algorithm which, crucial to symmetric peer-to-peer networks, does neither require a central server nor super nodes in the network. We show how our scheme can be made even more efficient by using a globally known ontology to determine the organization of peers in the graph topology, allowing for efficient concept-based search.

HyperCuP: hypercubes, ontologies, and efficient search on peer-to-peer networks

Peer-to-peer networks are envisioned to be deployed for a wide range of applications. However, P2P networks evolving in an unorganized manner suffer from serious scalability problems, limiting the number of nodes in the network, creating network overload and pushing search times to unacceptable limits. We address these problems by imposing a deterministic shape on P2P networks: We propose a graph topology which allows for very efficient broadcast and search, and we describe a broadcast algorithm that exploits the topology to reach all nodes in the network with the minimum number of messages possible. We provide an efficient topology construction and maintenance algorithm which, crucial to symmetric peer-to-peer networks, does neither require a central server nor super nodes in the network. Nodes can join and leave the self-organizing network at any time, and the network is resilient against failure. Moreover, we show how our scheme can be made even more efficient by using a global ontology to determine the organization of peers in the graph topology, allowing for efficient concept-based search.

OAI-P2P: a peer-to-peer network for open archives

OAI is designed with a low-barrier technology approach, thus allowing institutions to provide content metadata with little effort. On the other hand, search capabilities are very limited on OAI data providers, and have to be provided by separate service providers. We propose that data providers form a peer-to-peer network which supports distributed search over all connected metadata repositories. Such an approach is already implemented for learning content metadata (project Edutella). We describe how this technology could be reused in the OAI context. This would allow OAI repositories to provide distributed search capabilities and effortless integration of new archives within a peer-to-peer network with little additional implementation effort.

Semantic Web and Peer-to-Peer Technologies for Distributed Learning Repositories

Metadata for the World Wide Web is important, but metadata for Peer-to-Peer (P2P) networks is absolutely crucial. In this paper we discuss the open source project Edutella and related projects, which combine semantic web and peer-topeer technologies in order to make distributed learning repositories possible and useful. We describe the main services of the Edutella network infrastructure and its architecture based on the exchange of RDF metadata, starting with the query service as one of the core services of Edutella. We discuss the Edutella Common Data Model (ECDM) as basis for the Edutella query exchange language (RDFQEL-i) implementing distributed queries over the Edutella network, and discuss as one example an O-Telos-Peer with native Datalog query and inference capabilities. We then sketch a new P2P routing topology which minimizes broadcast traffic and distance between the peers in such a network, as well as a modification language needed for supporting distributed update and annotation. A short section discusses the changing role of adaptation in these distributed repositories (as open corpus hypermedia instead of the closed ones investigated in most research projects so far). Finally, we discuss an application of the Edutella network for digital libraries (OAI-P2P) and the use of Edutella in the ELENA project which aims to create smart spaces for learning.

Integrating schema-specific native XML repositories into a RDF-based e-learning P2P network

As its name implies, a native XML repository supports storage and management of XML in the original hierarchical form rather than in some other representations. In this paper we present our approach for integrating native XML repositories into Edutella, a RDF-based E-learning P2P network, through mapping native XML database schemas onto the Edutella Common Data Model (ECDM) and further translating ECDMs internal query language Datalog into XPath, the local query language of native XML repositories. Due to the considerable incomparability between the ECDM and the XML data model, a generic integration approach for schema-agnostic native XML repositories is found to be unrealistic. Thus our investigations are focused on three schema-specific native XML repositories respectively based on the DCMES, LOM/IMS, and SCORM XML binding data schema. Since these three metadata sets are the most popularly applied learning resource metadata specifications in E-Learning, our integration approach satisfactorily addresses the current usage of Edutella in E-Learning despite that a generic integration approach for schema-agnostic native XML repositories has not been implemented.

Using an O-Telos peer to provide reasoning capabilities in an RDF-based P2P-environment

The open source project Edutella is a Peer-to-Peer (P2P) network for learning materials. It builds upon meta data standards defined for the WWW and aims to provide a RDF-based meta data infrastructure for P2P applications. In this paper we discuss a prototype provider peer for the Edutella network, which provides advanced reasoning peers for RDF data. This prototype peer provides storage and querying services for which it uses the ConceptBase database with its powerful reasoning mechanism as storage and query facility. ConceptBase implements the meta data language O-Telos while Edutella uses RDF(S). Based on previous work in this area we present a mechanism to translate the property-centered meta modeling language RDF to the object-centered meta modeling language O-Telos. Thus we are able to provide a Datalog based reasoning facility for the P2P Edutella network. As the Edutella network also uses the query exchange language RDF-QEL we also describe in detail the peer’s query service. It translates RDF-QEL queries to O-Telos queries and the respective O-Telos answers to RDF-QEL answers.The open source project Edutella is an RDF-based Peer-to-Peer infrastructure for digital resources, focusing on learning materials. Building upon the established meta-data standard RDF(S) appropriate for the description of distributed resources in the World Wide Web WWW, it provides a general RDF-based meta-data infrastructure for P2P applications. In this paper, after a short introduction to the Edutella infrastructure, we describe how an Edutella peer based on the conceptual modeling language O-Telos, using the ConceptBase meta-database, can provide storage and reasoning capabilities on RDF meta-data in the P2P network. To provide this feature, we exploit the similarity of the O-Telos data model to the RDF(S) data model, and show how RDF(S) meta-data can easily be stored in the ConceptBase database. Finally, we describe how the Datalog-based Edutella query exchange language RDF-QEL can be naturally expressed by O-Telos query classes.