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1. Fedora Digital Object or FDO – This is the basic unit for information aggregation in Fedora. At a minimum an a FDO has:
   
   
   1. A Persistent Identifier or PID – The PID provides the key by which the FDO is accessed from the repository.
      
      
   2. Dublin Core – It provides a basic description of the FDO.
      
      
2. Datastream – A component of an a FDO that represents a data source. An A FDO may have just the basic Dublin Core Datastream, or any number of additional Datastreams. Each Datastream can be any MIME-typed data or metadata, and can either be content managed locally in the Fedora repository or by some external data source (and referenced by a URL). When you create a new Datastream in an a FDO, you assign it to one of four types, or control groups, depending on the nature of the data that it represents.
   
   
   1. Managed Content (M): Datastream content is stored and managed within the Fedora repository's persistent storage. The content can be any MIME type including XML.
      
      
   2. Inline XML (X): A special case of M, restricted to well-formed XML. In this case, the Datastream content is stored as part of the XML structure of the FDO itself and is thus included when the it is exported (e.g., for archival purposes).
      
      
   3. Externally Referenced (E): Datastream content is external to the Fedora repository and is referenced by a URL that is recorded within the FDO. The content can be any MIME type including XML.
      
      
   4. Redirected Content (R): Like E, but Datastream content is delivered to the client without any mediation by Fedora; i.e., via an HTTP redirect. You should use this Datastream type when the external content is a web page with relative links or it is streaming audio or video. The content can be any MIME type including XML.

Decisions about what to include in an a FDO and how to configure its Datastreams are basic modeling choices as you develop your repository. The examples in this tutorial demonstrate some common models that you may find useful as you develop your application.

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It is often useful to provide access to a digital document in several formats. For example an ePrints server might provide HTML for those who wish to render the document in a browser, PDF for those who wish to view the document with author-determined formatting, and TeX for those who wish to access and use the document source. This example demonstrates how to construct an a FDO where each Datastream corresponds to an available format. More advanced techniques, demonstrated later in this tutorial, make it possible to achieve the same results by generating formats dynamically from a single base format. But for now, we'll stick to simple static aggregation.

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1. You will notice that the Control Group of the DC Datastream is Internal XML Metadata. As explained earlier, Fedora has a number of control group types, of which this is one. This type is appropriate for metadata that is represented in XML – Dublin Core metadata being one example. An A FDO can have multiple metadata Datastreams, for example MARC, LOM, Dublin Core, and others.
   
2. You can directly edit the Dublin Core metadata – e.g., add new Dublin Core fields – by selecting the Edit button and modifying the contents of the text pane. When you press Save Changes..., Fedora will check that the Datastream is well-formed XML.

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The examples described so far demonstrate the basic content aggregation features of Fedora. As mentioned already, the power of Fedora lies in its ability to associate the data in an a FDO with Web services to produce dynamic disseminations. Some examples of this capability are as follows:

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