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1. The subject must be encoded as an <rdf:Description> element, with an "rdf:about" attribute containing the URI of the digital object in which the RELS-EXT Datastream resides. Thus, relationships are asserted about this object only. Relationship directionality is from this object to other objects.
2. The relationship assertions must be RDF properties associated with the <rdf:Description>. Relationship assertions can be properties defined in the default Fedora relationship ontology, or properties from other namespaces.
3. Prior to 2.1, the objects of relationships were restricted to other Fedora digital object URIs. This has since been relaxed so that a relationship property may reference any URI or literal, with the following exception: a relationship may not be self-referential, rdf:resource attribute must not point to the URI of the digital object that is the subject of the relationship.
4. There must be only one <rdf:Description> in the RELS-EXT Datastream. One description can have as many relationship property assertions as necessary.
5. There must be no nesting of assertions. Specifically, there cannot be an <rdf:Description> within an <rdf:Description>. In terms of XML "depth," the RDF root is considered at the depth of zero. The must be one <rdf:Description> element that must exist at the depth of one. The relationship assertions are RDF properties of the <rdf:Description> that exist at a depth of two.
6. Assertions of properties from certain namespaces for forbidden in RELS-EXT. There must NOT be any assertion of properties from the Dublin Core namespace or from the FOXML namespace. This is because these assertions exist elsewhere in Fedora objects and may conflict if asserted in two places. The RELS-EXT Datastream is intended to be dedicated to solely object-to-object relationships and not used to make general descriptive assertions about objects.

Resource Index - RDF-based Indexing for Digital Objects

Yes! The Fedora repository service automatically indexes the RELS-EXT Datastreams for all objects as part of the RDF-based Resource Index.

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By combining these very different views, CMA has the potential to provide a way to build an interoperable repository for integrated information access in our organizations and to provide durable access to our intellectual works. As we introduce CMA concepts, we will discuss the rationale behind the design decisions. This is only the first generation of the CMA and, like the rest of Fedora, we expect it to evolve. An understanding of design decisions behind this "first-generation" CMA is a key element for community participation in future generations of CMA development. Most important is an understanding of three significant and interrelated developments in software engineering: (1) object-oriented programming, (2) design patterns, and (3) model-driven architectures. It is beyond the scope of this document to discuss any of these developments in detail but we will make reference in this document to aspects of them which inform the design of the CMA.

Content Model Architecture Overview

The Content Model Architecture (CMA) describes an integrated structure for persisting and delivering the essential characteristics of digital objects in Fedora. In this section we will describe the key elements of the architecture, how they relate, and the manner in which they function. The original motivation for the CMA was to provide a looser binding for Disseminators, an element of the Fedora architecture used to stream a representation to a client. However, the CMA as described in this document has encompassed a far greater role in the Fedora architecture, in many ways forming the over-arching conceptual framework for future development of the Fedora Repository.

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While the CMA does not force you to use a specific content modeling language, Fedora 3.0 contains a reference implementation that enables the Fedora Repository to operate much as it did in prior versions. The following sections describe CMA in more detail and provide instructions on how to use the reference content modeling language so you can create your own CMA compatible objects immediately. Over time Fedora Commons will support the development of one or more content modeling languages as part of solution bundles that may be used by the community with minimum effort.

Specializing Digital Objects

One of the basic elements of the Fedora architecture is the Fedora digital object. Every digital object stored in a Fedora repository is some variation of the same basic Fedora digital object model. In Fedora, digital objects containing data (Data object) utilize an XML element called the Datastream to describe the raw content (a bitstream or external content). In Fedora 2 and prior versions, digital objects containing data may also have contained Disseminators. The Disseminator is a metadata construct used by Fedora to describe how a client can access content within the digital object or remotely referenced by the digital object. If you only needed to access the raw content, default functionality was provided by the Fedora Repository which did not require that a Disseminator be explicitly added to the digital object. Unfortunately, the older design meant that the Disseminator was repeated in every Data object.

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