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We must have a unique, unambiguous method to identify the class. For this purpose, in the CMA we have defined the "Content Model object," a digital object that both represents the notion of the class and can contain the content model. We use the identifier of the content model object (or CModel) as the class identifier. Following the rules of Fedora identifiers, the identifier of the CModel object can be encoded within a URI. We will describe the rationale for this decision in a later section but this approach provides two immediate benefits: (1) it provides a scheme which works within the Fedora architecture with minimal impact, and (2) it is compatible with the Web architecture, RDF and OWL. We can even build functionality using just the knowledge of the identifier without creating a content model. Having a uniform method for identifying a digital object's class maximizes interoperability.

The CMA does not require that digital objects explicitly conform to its architecture or explicitly declare any of its metadata elements beyond providing well-formed Fedora digital objects - unless you want to use the advanced features provided by the Fedora repository. The CMA uses a "descriptive" approach where the Fedora Repository will issue a run-time error for any operation it cannot perform. In most cases, you should still be able to disseminate bitstreams exactly as they are stored. CMA's dissemination approach is more consistent with the Web architecture, and provides a better balance between durable access to content and future innovations.

The minimum requirement to participate in the CMA is for a digital object to assert a relation to record its class' identity. Digital objects that do not explicitly identify their class are assumed to belong to a system-defined "Basic Content Model" which has a repository-defined reserved identifier. In CMA, you should use a digital object (see CModel below) as a way to register a "class" in a repository federation.

The remaining functionality is enabled by creating the content model and storing it within Fedora digital objects like any other content. This permits applications or the repository itself to disseminate the content model, interpret it and use it to provide functionality for the set of objects it describes. Content designers are free to develop content models (or even content modeling languages). Content Models and Content Model objects are designed to be shared. Digital objects having the same content model automatically form an interoperable information community. If the whole Content Model object is shared, keeping the object's identifier the same across multiple repositories, the community can easily extend across multiple organizations.

There are two basic approaches to using content models. First, the content and content models can be disseminated to applications able to interpret them to deliver the essential characteristics of the content. Disseminating the content model to external services can be also be used when creating new digital objects for ingest, validation, transformation and replication.

Alternately, the Fedora Repository can be used as a content mediation layer which interprets the content model to disseminate the content correctly. To accomplish this, the Fedora Repository must have access to code compatible with the Content Model. The compatible code may be added to the Fedora Repository using its plug-in architecture in combination with service mechanisms. While all the tools needed to plug in your own Content Model mediation are not complete in Fedora 3.0, this feature of the CMA will permit serving several generations of digital objects in the same repository reducing the need to update objects whenever there are new releases of Fedora. This will increase the durability of collections and enable longer periods between migrations of Fedora digital objects to new formats.

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.

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Specializing Digital Objects

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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.

In Fedora 2 and prior versions, three specializations of the Fedora digital object were used: the Data object, the Behavior Definition (BDef) and the Behavior Mechanism (BMech). The BDef contained a description of the access interface which, in turn, was implemented by the BMech. In combination, the Fedora digital object model provides a uniquely flexible way to persist and access digital content. However, these features were not as simple to use as they could be. In particular, repeating the Disseminator in every Data object made changes to the access interface very difficult since every object had to be changed.

In the CMA, the Fedora digital object remains the model for all digital objects just as it was in prior versions of Fedora. However, for the CMA we define four specialized variations of the Fedora digital object (see Table 1).

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Table 1: Fundamental Fedora Object Types

Each of these specialized digital objects will be described in much greater detail below. The BDef and BMech have long been recognized as having reserved functions in Fedora
and we often called them "control" objects because they contain data used to control object-specific functionality. CMA adds a new control object, the CModel. However, some of the data needed for object-specific functionality was located in the Data object (often just labeled generically as the digital object) in prior Fedora versions. In particular, Disseminators were defined in the Data object. The CMA eliminates the Disseminator and redistributes control data in a more logical and reusable fashion among the "control" objects. Care should be taken in equating the names of object types in the Fedora 3.0 from the prior (Release 2 and earlier) Fedora architecture because there are some similarities to roles played by these objects in both the old and new architecture. However, the CMA is a major redesign so care should be taken not to automatically equate the roles of the control objects in the old and new implementations. In pre-release reviews with community members, we discovered that there were concerns about the potential of confusion so we have used a new naming scheme for Fedora 3.0. Also, care should be taken not to equate the digital object as a container and any data (or metadata) it contains. Since the deployment, functional , persistence and information views of the Fedora architecture tend to intermingle more than in most architectures, it is easy to accidently conflate characteristics of the architectural elements across views. We will try to make the distinctions clear where needed. Table 2 lists the supported relationships between fundamental object types in the CMA.

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