Fedora defines a generic digital object model that can be used to persist and deliver the essential characteristics for many kinds of digital content including documents, images, electronic books, multi-media learning objects, datasets, metadata and many others. This digital object model is a fundamental building block of the Content Model Architecture and all other Fedora-provided functionality.

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The Fedora Digital Object Model

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• Datastream Identifier: an identifier for the datastream that is unique within the digital object (but not necessarily globally unique)
• State: the Datastream's state: Active, Inactive, or Deleted
• Created Date: the date/time that the Datastream was created (assigned by the repository service)
• Modified Date: the date/time that the Datastream was modified (assigned by the repository service)
• Versionable: an indicator (true/false) as to whether the repository service should version the Datastream (by default the repository versions all Datastreams)
• Label: a descriptive label for the Datastream
• MIME Type: the MIME type of the Datastream (required)
• Format Identifier: an optional format identifier for the Datastream such as emerging schemes like PRONOM and the Global Digital Format Registry (GDRF)
• Alternate Identifiers: one or more alternate identifiers for the Datastream (such identifiers could be local identifiers or global identifiers such as Handles or DOI)
• Checksum: an integrity stamp for the Datastream content which can be calculated using one of many standard algorithms (MD5, SHA-1, etc.)
• Bytestream Content: the content (as a stream resource) represented or encapsulated by the Datastream (such as a document, digital image, video, metadata record)

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  Control Group: the approach used by the Datastream to represent or encapsulate the content as one of four types or control groups:
  • Internal XML Content - the content is stored as XML in-line within the digital object XML file
  • Managed Content - the content is stored in the repository and the digital object XML maintains an internal identifier that can be used to retrieve the content from storage
  • Externally Referenced Content - the content is stored outside the repository and the digital object XML maintains a URL that can be dereferenced by the repository to retrieve the content from a remote location. While the datastream content is stored outside of the Fedora repository, at runtime, when an access request for this type of datastream is made, the Fedora repository will use this URL to get the content from its remote location, and the Fedora repository will mediate access to the content. This means that behind the scenes, Fedora will grab the content and stream in out the the client requesting the content as if it were served up directly by Fedora. This is a good way to create digital objects that point to distributed content, but still have the repository in charge of serving it up.
  • Redirect Referenced Content - the content is stored outside the repository and the digital object XML maintains a URL that is used to redirect the client when an access request is made. The content is not streamed through the repository. This is beneficial when you want a digital object to have a Datastream that is stored and served by some external service, and you want the repository to get out of the way when it comes time to serve the content up. A good example is when you want a Datastream to be content that is stored and served by a streaming media server. In such a case, you would want to pass control to the media server to actually stream the content to a client (e.g., video streaming), rather than have Fedora in the middle re-streaming the content out.

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