| Abstract |
The World Wide Web was initially created in order that the information that is found in it to be consumed by people, and despite the fact that all that exist in it can be written by machines, none of that data is machine-comprehensible. It is very difficult everything that exists in the World Wide Web to be automated, and because of the volume of information that is contained in it, it is impossible for them to be managed manually. A solution, very widely widespread, is the use of metadata for the description of data that is contained in the World Wide Web. Metadata is data about data. Specifically, for the case of World Wide Web metadata is data describing web resources. The Resource Description Framework (RDF) is a foundation for processing metadata. RDF provides interoperability between the applications that exchange machine-understandable information on the Web. A category of applications, in which we have exchange of information in the World Wide Web, are the Multi Agent systems. Multi Agent systems have the ability to communicate with other remoted systems and to execute operations autonomously without being necessary the continuing communication with the user. For example, such systems could manage the enormous volume of information of World Wide Web. However, basic condition in order for all the above to happen is the agents to be able to communicate with each other. The simple transmission of binary digits is not enough for an effective communication between the agents. The agents need to communicate in a higher level and to represent complex concepts, at this point RDF is useful. RDF allows the creation of classes of hierarchies and can be used for the representation of ontologies, consequently it can be used for the description of information and the negotiations between the agents. Our work proposes an enrichment of RDF/S in order to be used in Multi Agents systems, in changes detection systems of repository information and more generally to give the ability to RDF to store information that has been said, independently from the fact that their truth is certain or not. RDF/Ss enrichment lies in the support of the reification mechanism. Reification gives the ability of the transformation of a statement in an object with a unique identifier, so that assertions can be made for this statement. For example, the statement Picasso painted Guernica can be considered as a new object and in this statement new attributes can be ascribed, like who and when formulated it. The example above constitutes a reification sample in the data level. However, in the present work is also being studied the use of reification in schema level, in order the storage of information on the RDF Schema Versioning to be possible. Additionally, in this work a hypergraph model, which replaces the simple directed graph that is used for the representation of schema and metadata in the RDF/S, is presented. Another issue, which is examined is the storage of reified statements in metadata repositories. Finally, the enrichment of metadata query language, RQL, with the required elements so that queries of reified statements can be supported, is proposed
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