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Object Types within the NDR

Information in the NSDL Data Repository is organized into Fedora ( digital objects – content containers that be dynamically viewed with associated web services. Digital objects may contain one or more data streams of locally or remotely stored content, and may have one or more associated web services to expose views of the object.

The object types in the NDR are described below.


Resource objects contain data streams for locally or remotely stored content. Every resource object is a handle that may be used to refer to it. Resource objects may also contain data streams for locally or remotely stored historic versions of the content.

Note: The NSDL ingest process scans incoming metadata items for Dublin Core identifier values, and generates a Resource Object for every distinct value found.


Metadata objects contain information (usually bibliographic) about a particular NDR Resource object. Metadata objects contain one or more data streams to support all the representations (formats) of the metadata received and/or all the metadata formats made available by the NDR. Metadata objects are roughly analogous to “items” in the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) – see In fact, when the NSDL harvests metadata from an OAI data provider, each OAI item represented in the harvest corresponds to a Metadata object in the NDR.

Metadata objects must be associated with at least one NDR Resource object and must be associated with an NDR MetadataProvider object.

A Metadata object is often associated with more than one Resource Object.


An Agent object represents an institution, individual, group, or other entity providing information to the NDR. Agent objects indicate the source of information such as metadata, resources, or relationships. Agent objects have data describing the agent as well as a ServiceDescription for each function they perform, such as aggregating resources, providing metadata that describes resources, or identifying relationships among resources.


MetadataProvider objects are used to administer metadata harvested from a single source, such as an OAI data provider. Each grouping of metadata provision, such as a particular OAI set to be harvested from an OAI server, requires a unique MetadataProvider object. For example, if the NDR harvests the “moose” set from the OAI server at http://oai.baseurl for the NSDL-moose aggregation of resources, and harvests the “caribou” set from the OAI server at http://oai.baseurl for the NSDL-caribou aggregation of resources, then two MetadataProvider objects are needed, one for the “moose” harvest and one for the “caribou” harvest.

Every MetadataProvider object must be associated with a single MetadataProvider Service Description in an Agent object -- metadata about a resource must be attributable to the Agent that provided the metadata. Note that a single Agent object can have Service Descriptions for many MetadataProvider objects, just as many separate harvests may occur for a single OAI data provider.


Aggregator objects represent a grouping of resources.

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