Pathways core: a data model for cross-repository services

As part of the NSF-funded Pathways project, we have created an interoperable data model to facilitate object re-use and a broad spectrum of cross-repository services. The resulting Pathways Core data model is designed to be lightweight to implement, and to be widely applicable as a shared profile or as an overlay on data models currently used in repository systems and applications. We consider the data models underlying the Fedora, Dspace and aDORe repository systems, and a number of XML-based formats used for the representation of compound objects, including MPEG-21 DIDL, METS, and IMS/CP.At the heart of the Pathways Core data model (Fig. 1) are the entity and datastream elements. entity elements model the abstract aspects of digital objects and align with works and expressions in FRBR [1]. An entity can model anything from a digital object to a collection of digital objects (other entities), to a node created merely to express abstract properties. Core properties of entities are hasIdentifier, hasProviderInfo, hasLineage, and hasProvider-Persistence. If a repository attaches providerInfo to an entity, it provides a handle to access the entity from the repository, supporting its use and re-use. Persistence of this handle may be indicated with providerPersistence. The hasLineage property is used to indicate the entity (or entities) from which the entity to which the hasLineage is attached was derived. Other properties, such as hasSemantic, that convey the intellectual genre of the entity (i.e. journal article), can be added. datastream elements model the concrete aspects of a digital object; these align with items in FRBR, and can be thought of as aspects at the level of bitstreams. An entity may have any number of datastreams. Two properties of datastream have been defined as part of the Pathways Core: hasLocation conveys a URI that can be resolved to yield a bitstream; and hasFormat conveys the digital format of the bitstream. If a datastream has multiple hasLocation properties, resolution of the conveyed URIs yields bit-equivalent bitstreamsThe Pathways Core data model can be serialized in a variety of ways, and, an RDF serialization as well as a profile of MPEG-21 DIDL have been created as reference implementations. We have also conducted the following experiment to illustrate the power of the Pathways Core. A number of heterogeneous repositories implemented an OpenURL-based obtain interface from which, given the providerInfo of an entity, an RDF serialization of the entity compliant with the Pathways Core could be retrievedUsing this interface, an overlay journal can collect serializations of some entities (scholarly papers) from the different collaborating repositories, and assemble those into a new issue of the journal. The overlay journal then itself implemented the same obtain interface, and as a result, an RDF serialization of the entire journal, an issue, and an article could be extracted. This interface could then, for example, be used by a preservation repository to collect content from the overlay journal for ingest and mirroring. This experiment illustrates how cross-repository services and workflows can be facilitated through support of an interoperable data model (the Pathways Core) and an interoperable service interface (the OpenURL-based obtain interface)