On Versioning and Archiving Semantic Web Data

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 (KBs) (a) have not a unique serialization (as it happens with texts) and (b) their versions are usually related by containment (⊆). 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 changebased approach). We report experimental results over synthetic data sets showing that POI offers notable space saving, e.g. compression ratio (i.e. uncompressed/compressed size) ranges between 1,800% and 18,163%, as well as efficiency in various cross version operations. POI is equipped with three version insertion algorithms and could be also exploited in cases where the set of KBs does not fit in main memory. Although the focus of this work is SW data versioning, POI can be considered as a generic indexing scheme for storing set-valued data.