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Datastreams represent the digital content that is the essence of the digital object (e.g., digital images, encoded texts, audio recordings). All forms of metadata, except system metadata, are also treated as content, and are therefore represented as Datastreams in a digital object. All Datastreams have the potential to be disseminated from a digital object. A Datastream can reference any type of content, and that content can be stored either locally or remotely to the repository system.

Datastreams

A Datastream is the element of a Fedora digital object that represents a content item. A Fedora digital object can have one or more Datastreams. Each Datastream records useful attributes about the content it represents such as the MIME-type (for Web compatibility) and, optionally, the URI identifying the content's format (from a format registry). The content represented by a Datastream is treated as an opaque bit stream; it is up to the user to determine how to interpret the content (i.e. data or metadata). The content can either be stored internally in the Fedora repository, or stored remotely (in which case Fedora holds a pointer to the content in the form of a URL). The Fedora digital object model also supports versioning of Datastream content (see the Fedora Versioning Guide for more information).

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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 state of 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. Examples of emerging schemes are 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 which can be calculate using one of many standard algorithms (MD5, SHA-1, etc.)
• Bytestream Content: the "stuff" of the Datastream is about (such as a document, digital image, video, metadata record)
• Control Group: pertaining the the bytestream content, a new Datastream can be defined as one of four types, or control groups, as follows:
  • Internal XML Metadata - In this case, the Datastream will be stored as XML that is actually stored inline within the digital object XML file. The user may enter text directly into the editing window or data may imported from a file by clicking Import and selecting or browsing to the location of the XML metadata file.
  • Managed Content - In this case, the Datastream content will be stored in the Fedora repository and the digital object XML file will store an internal identifier to that Datastream. To get content, click Import and select or browse to the file location of the import file. Once import is complete, you will see the imported file in a preview box on the screen.
  • External Referenced Content - In this case, the Datastream content will be stored outside of the Fedora repository, and the digital object will store a URL to that Datastream. The Datastream is "by reference" since it is not actually stored inside the Fedora repository. 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. To create this type of Datastream, specify the URL for the Datastream content in the Location URL text box.
  • Redirect Referenced Content - In this case, the Datastream content is also stored outside the repository and the digital object points to its URL ("by-reference"). However, unlike the External Referenced Content scenario, the Redirect scenario signals the repository to redirect to the URL when access requests are made for this Datastream. This means that the Datastream will not be streamed through the Fedora repository when it is served up. This is beneficial when you want a digital object to have a Datastream that is stored and served up 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. To create a Redirect Datastream, specify the URL for the content in the Location text box.

Digital Object Model — An Access Perspective

Below is an alternative view of a Fedora digital object that shows the object from an access perspective. The digital object contains Datastreams and a set of object properties (simplified for depiction) as described above. A set of access points are defined for the object using the methods described below. Each access point is capable of disseminating a "representation" of the digital object. A representation may be considered a defined expression of part or all of the essential characteristics of the content. In many cases, direct dissemination of a bit stream is the only required access method; in most repository products this is only supported access method. However, Fedora also supports disseminating virtual representations based on the choices of content modelers and presenters using a full range of information and processing resources. The diagram shows all the access points defined for our example object.

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