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<MyCatVideo> <is a member of the collection> <GreatCatVideos>

Why are Fedora Digital Object Relationships Important?

The creation of Fedora digital object relationship metadata is the basis for enabling advanced access and management functionality driven from metadata that is managed within the repository. Examples of the uses of relationship metadata include:

• Organize objects into collections to support management, OAI harvesting, and user search/browse
• Define bibliographic relationships among objects such as those defined in Functional Requirements for Bibliographic Records
• Define semantic relationships among resources to record how objects relate to some external taxonomy or set of standards
• Model a network overlay where resources are linked together based on contextual information (for example citation links or collaborative annotations)
• Encode natural hierarchies of objects
• Make cross-collection linkages among objects (for example show that a particular document in one collection can also be considered part another collection)

Where is Digital Object Relationship Metadata Stored?

Object-to-Object relationships are stored as metadata in digital objects within a special Datastream. This Datastream is known by the reserved Datastream identifier of "RELS-EXT" (which stands for "Relationships-External"). Each digital object can have one RELS-EXT Datastream which is used exclusively for asserting digital object relationships.

A RELS-EXT Datastream can be provided as part of a Fedora ingest file. Alternatively, it can be added to an existing digital object via component operations of the Fedora management service interface (i.e., addDatastream). Refer to the FOXML reference example to see an example of the RELS-EXT Datastream in context. Modifications to the RELS-EXT Datastream are made via the Fedora management interface (i.e., modifyDatastream). The RELS-EXT Datastream is encoded as an Inline XML Datastream, meaning that the relationships metadata is expressed directly as XML within the digital object XML file (as opposed the relationship metadata existing in a separate XML file that the digital object points to by reference).

How is Digital Object Relationship Metadata Encoded?

Fedora object-to-object metadata is encoded in XML using the Resource Description Framework (RDF). The relationship metadata must follow a prescribed RDF/XML authoring style where the subject is encoded using <rdf:Description>, the relationship is a property of the subject, and the target object is bound to the relationship property using the rdf:resource attribute. The subject and target of a relationship assertion must be URIs that identify Fedora digital objects. These URIs are based on Fedora object PIDs and conform to the syntax described for the fedora "info" URI scheme. The syntax for asserting relationships in RDF is as follows:

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