On storage policies for Semantic Web repositories that support versioning

This paper concerns versioning services over Semantic Web (SW) repositories. We propose a novel storage index (based on partial orders), called POI, that exploits the fact that RDF Knowledge Bases (KB) have not a unique serialization (as it happens with texts). POI can be used for storing several (version-related or not) SW KBs. We discuss the benefits and drawbacks of this approach in terms of storage space and efficiency both analytically and experimentally in comparison with the existing approaches (including the change-based approach). For the latter case we report experimental results over synthetic data sets. POI offers notable space saving as well as efficiency in various cross version operations. It is equipped with an efficient version insertion algorithm and could be also exploited in cases where the set of KBs does not fit in main memory.