Currently efficient knowledge sharing and reuse is rather difficult, as it is hard to find and share ontologies available among the community due to the lack of standards for documenting and annotating ontologies with metadata information. This raises the problem of having many isolated ontologies created by many different parties. Besides the costs of the duplicate efforts, this also hampers interoperability between ontology-based applications. Oyster1 is a Peer-to-Peer application that exploits semantic web techniques in order to provide a solution for exchanging and re-using ontologies. To achieve this, Oyster implements a proposal for a metadata standard, called Ontology Metadata Vocabulary (OMV)2  which is based on discussions and agreements carried out in the EU IST thematic network of excellence Knowledge Web3 as a way to describe ontologies. The decentralised approach provides an ideal solution for users that require a repository to which they have full access and can perform any operation without any consequences to other users. For example, users from academia or industry might use a personal repository for a task dependent investigation, or ontology engineers, might use it during their ontology development process to capture information about different ontology versions. We argue that a decentralized system is the technique of choice, since it allows the maximum of individuality while it still ensures exchange with other users. A centralized approach, on the other hand, allows reflecting long-term community processes in which some ontologies become well accepted for a domain or community and others become less important. However, both approaches could be combined to cover a variety of use cases.
Oyster provides an innovative solution for sharing and re-using knowledge (i.e. ontologies), which is a crucial step to enable Semantic Web. The Oyster system has been implemented as an instance of the Swapster system architecture4. In Oyster, ontologies are used extensively in order to provide its main functions (importing data, formulating queries, routing queries, and processing answers).
Creating and Importing Metadata: Oyster enables users to create metadata about ontologies manually and also to import ontology files in order to automatically extract the ontology metadata available and let the user to fill in the missing values. For the automatic extraction, Oyster supports the OWL5, DAML+OIL6, and RDF-S7 ontology languages. The ontology metadata entries are aligned and formally represented according to two ontologies: (1) the proposal for a metadata standard OMV that describes the properties of the ontology, and (2) a topic hierarchy (i.e. DMOZ8) that describes specific categories of subjects to define the domain of the ontology.
Formulating Queries: Users can search the repository for ontologies by means of simple keyword searches, or more advanced, semantic searches (c.f. the left pane of figure 1). Queries are formulated in terms of these two ontologies. This means that queries can refer to fields like name, acronym, ontology language, etc. or they may refer to topic terms.
Routing Queries: As shown in the upper left pane of figure 1, users may query a single specific peer (e.g. their own computer, or a certain peer because this peer is known as a big provider of information), or they may query a specific set of peers (e.g. all the members of a specific organization), or they may query the entire network of peers (e.g. when the user has no idea where to search). In the latter case, queries are routed automatically through the network depending on the expertise of the peers, describing which topic of the topic hierarchy a peer is knowledgeable about. In order to achieve this expertise based routing, a matching function determines how closely the semantic content of a query matches the expertise of a peer .
Processing results: The results matching a query are presented in a result list (c.f. upper right pane in figure 1). The answer of a query might be very large and may contain many duplicates due to the distributed nature and potentially large size of the Peer to Peer network. Such duplicates might not be exact copies because of the semi structured nature of the metadata, so the ontologies are used again to measure the semantic similarity between different answers and to remove apparent duplicates. Then a merged representation that combines the knowledge from the individual and potentially incomplete items is presented to the user. Details of the particular results are shown in the lower right side of Figure 1. Users can save the results of a query into their local repository for future use. Also, as proposed by OMV, all the specific realizations of an ontology can be grouped by the same ontology conceptualisation to organize the answer.
Oyster provides an infrastructure for storing, sharing and finding ontologies making use of the proposal for a metadata standard OMV. OMV compromises the OMV Core, which captures information relevant to most of the ontology reuse settings and various OMV Extensions that allow ontology developers/users to specify task/application-specific ontology-related information. The OMV elements are classified according to the type and purpose of the contained information (e.g. availability, provenance, applicability, relationship, etc.), and also according to their impact on the prospected reusability of the described ontology content (e.g. required, optional, extensional). Furthermore, the OMV core distinguishes between an ontology conceptualization (OC) and its implementation(s) (OI) in specific representation languages. In concrete, an OC represents the (abstract) core model or idea behind an ontology. It describes the core properties of an ontology, independently of any implementation details. While an OI represents a specific realization of an OC, describing properties of an ontology that are related to the realization/implementation. The distinction between the two concepts provides an efficient mechanism for the carrying out of several ontology management tasks (e.g. the tracking of versions and the evolution flow of an ontology). OMV also models additional classes and properties required to support the reuse of ontologies, especially in the context of the Semantic Web. For a full description of OMV please refer to .
A closely related application is the Onthology9 central repository, which also exploits the OMV. Onthology offers a complementary application to Oyster as both applications have a different usage perspective and are appropriate for different tasks. Similar approaches to our proposed solution can be found, but in general their scope is quite very limited. E.g. the DAML ontology library10 provides a catalog of DAML ontologies that can be browsed by different properties. The FIPA ontology service11 defines an agent wrapper of open knowledge base connectivity. The Semantic Web search engine SWOOGLE12 makes use of particularly metadata which can be extracted automatically. Finally the SchemaWeb Directory13 is a repository for RDF schemas expressed in RDFS, OWL and DAML+OIL.
To conclude, the reuse of existing ontologies within communities is a key issue for sharing knowledge on the Semantic Web. This task, however, is rather difficult because of the heterogeneity, distribution and diverse ownership of the ontologies as well as the lack of sufficient metadata. As we summarized in this paper, our contribution, Oyster, addresses exactly these challenges by implementing a proposed standard for metadata for describing ontologies. Oyster is already being applied in the KnowledgeWeb project which has partners across the European Union. The latest release of Oyster has been downloaded 40 times in 6 weeks from the collaborative development platform Ontoware. It is ranked as the number one in the list of top downloaded projects of Ontoware (642 downloads, including all versions and releases). Currently, there are around 250 ontologies shared in Oyster network. We are in the process of collecting usage statistics. Finally, our future work includes addressing many challenges like the integration of Oyster with central repository, improving group detection at the result presentation, evaluating expertise ranking and performance.
Our thanks to our partners from the EU projects Knowledge Web (FP6-507482) and NeOn (FP6-27595) for their present and future collaboration.
 Haase P. et al. Peer selection in peer-to-peer networks with semantic topologies. In proc of ICNSW'04, Paris, June 2004.
 Hartmann, J., Palma, R.OMV-Ontology Metadata Vocabulary for the semantic web.2005. http://omv.ontoware.org
 Hartmann J. et al. Ontology metadata vocabulary and applications. In proc of SWWS'05, Cyprus, 2005.
1. Available at http://oyster.ontoware.org/
2. More information at http://omv.ontoware.org/