A technical infrastructure for storing, querying and managing RDF data is a key element in the current semantic web development. Systems like Jena, Sesame or the ICS-FORTH RDF Suite are widely used for building semantic web applications. Currently, none of these systems supports the integrated querying of distributed RDF repositories. We consider this a major shortcoming since the semantic web is distributed by nature. In this paper we present an architecture for querying distributed RDF repositories by extending the existing Sesame system. We discuss the implications of our architecture and propose an index structure as well as algorithms for query processing and optimization in such a distributed context.  Categories and Subject Descriptors The need for handling multiple sources of knowledge and information is quite obvious in the context of semantic web applications. First of all we have the duality of schema and information content where multiple information sources can adhere to the same schema. Further, the re-use, extension and combination of multiple schema files is considered to be common practice on the semantic web [7] . Despite the inherently distributed nature of the semantic web, most current RDF infrastructures (for example [4] ) store information locally as a single knowledge repository, i.e., RDF models from remote sources are replicated locally and merged into a single model. Distribution is virtually retained through the use of namespaces to distinguish between different models. We argue that many interesting applications on the semantic web would benefit from or even require an RDF infrastructure that supports real distribution of information sources that can be accessed from a single point. Beyond Copyright is held by the author/owner(s). May 17-22, 2004, New York, New York, USA. ACM 1-58113-844-X/04/0005. the argument of conceptual adequacy, there are a number of technical reasons for real distribution in the spirit of distributed databases: The commonly used approach of using a local copy of a remote source suffers from the problem of changing information. Directly using the remote source frees us from the need of managing change as we are always working with the original. Keeping different sources separate from each other provides us with a greater flexibility concerning the addition and removal of sources. In the distributed setting, we only have to adjust the corresponding system parameters. In many cases, it will even be unavoidable to adopt a distributed architecture, for example in scenarios in which the data is not owned by the person querying it. In this case, it will often not be permitted to copy the data. More and more information providers, however, create interfaces that can be used to query the information. The same holds for cases where the information sources are too large to just create a single model containing all the information, but they still can be queried using a special interface (Musicbrainz is an example of this case). Further, we might want to include sources that are not available in RDF, but that can be wrapped to produce query results in RDF format. A typical example is the use of a free-text index as one source of information. Sometimes there is not even a fixed model that could be stored in RDF, because the result of a query is only calculated at runtime (Google, for instance, provides a programming interface that could be wrapped into an RDF source). In all these scenarios, we are forced to access external information sources from an RDF infrastructure without being able to create a local copy of the information we want to query. On the semantic web, we almost always want to combine such external sources with each other and with additional schema knowledge. This confirms the need to consider an RDF infrastructure that deals with information sources that are actually distributed across different locations. In this paper, we address the problem of integrated access to distributed RDF repositories from a practical point of view. In particular, starting from a real-life use case where we are considering a number of distributed sources that contain research results in the form of publications, we take the existing RDF storage and retrieval system Sesame and describe how the architecture and the query processing methods of the system have to be extended in order to move to a distributed setting. 631
Stuckenschmidt et al. REF proposed an index structure for distributed RDF repositories based on schema paths (property chains) rather than on statistical summaries of the graph-structure of the data.
Index structures and algorithms for querying distributed RDF repositories
WWW '04
Computer Science