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Versions

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V 0.1 of the NEEO document. Changes with respect to version 2.3.1 of "MPEG21 DIDL Document Specifications for repositories" by Maurice Vanderfeesten (SURF)

1. Relaxed the requirement that the top Item identifier must be persistent.
2. More strict formulation for Item identifiers not being the same as the OAI identifier and the DIDL document identifier.
3. Replaced dip:objectType by rdf:type.
4. The use of simple Dublin Core is not mandatory and is not even recommended.
5. MODS is the recommended metadata scheme.
6. Left out all references to 'work' or 'expression'.
7. Item elements stand for Digital Items (they are Digital Item Declarations). Type statements (using rdf:type) type the Digital Items.
8. The use of identifiers (URIs) for Digital Items is mandatory.
9. Added new semantics: publishedVersion|authorVersion, embargo, description, access rights.

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1. One or more object files is changed in zero, one or more object files
2. Removed example; has to be replaced by a new example.

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1. Only minor changes

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1. Some minor editorial changes
2. 3.3.2 - Object File Item
   • Addition of deposit date as a <dcterms:issued>
   • Vocabulary for <dcterms:accessRights>

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1. 1. 1. Less strict remarks on the location of the namespace declarations.
      2. The number of Descriptors is unlimited.
      3. Type of a Digital Item is expressed as an URI being the value of the rdf:resource attribute of a rdf:type element.
      4. Type statements are also allowed at the top level.
      5. Instead of using not yet registered terms from info:eu-repo/semantics/, the terms from the Eprints Access Rights Vocabulary must be used for expressing the accessibility of an object
      6. For the date of deposit of an object in a repository dcterms:dateSubmitted is to be used (was dcterms:issued)

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This document describes the use of DIDL in the context of institutional repositories. The DIDL Document Specification was originally developed within the DARE programme of SURF as a solution for:

• the harvesting of the digital resources (PDFs etc.) from the local repositories for ingest into the E-Depot system of the Royal Library for long term preservation
• the harvesting of the digital resources (PDFs etc.) from the local repositories by a service provider (e.g. a search portal that indexes the full text of documents)
• the representation of complex documents such as doctoral theses that consist of multiple digital resource files
• the confusing use of dc:identifier; dc:identifier can be used for different types of objects; the identifier itself doesn't indicate what type of object is identified; in the context of repositories dc:identifier can point to a jump-off page (JOP) and/or to object files.

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What's new

V 0.1 of the NEEO document. Changes with respect to version 2.3.1 of "MPEG21 DIDL Document Specifications for repositories" by Maurice Vanderfeesten (SURF)

1. Relaxed the requirement that the top Item identifier must be persistent.
2. More strict formulation for Item identifiers not being the same as the OAI identifier and the DIDL document identifier.
3. Replaced dip:objectType by rdf:type.
4. The use of simple Dublin Core is not mandatory and is not even recommended.
5. MODS is the recommended metadata scheme.
6. Left out all references to 'work' or 'expression'.
7. Item elements stand for Digital Items (they are Digital Item Declarations). Type statements (using rdf:type) type the Digital Items.
8. The use of identifiers (URIs) for Digital Items is mandatory.
9. Added new semantics: publishedVersion|authorVersion, embargo, description, access rights.

V 0.2 of the NEEO document

1. One or more object files is changed in zero, one or more object files
2. Removed example; has to be replaced by a new example.

V 03 of the NEEO document

1. Only minor changes

V 04 of the NEEO document

1. Some minor editorial changes
2. 3.3.2 - Object File Item
   • Addition of deposit date as a <dcterms:issued>
   • Vocabulary for <dcterms:accessRights>

V 3.0

1. 1. 1. Less strict remarks on the location of the namespace declarations.
      2. The number of Descriptors is unlimited.
      3. Type of a Digital Item is expressed as an URI being the value of the rdf:resource attribute of a rdf:type element.
      4. Type statements are also allowed at the top level.
      5. Instead of using not yet registered terms from info:eu-repo/semantics/, the terms from the Eprints Access Rights Vocabulary must be used for expressing the accessibility of an object
      6. For the date of deposit of an object in a repository dcterms:dateSubmitted is to be used (was dcterms:issued)

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Table of Contents

1 Introduction
1.1 Compound and Digital Objects as Digital Items
2 OAI Response with a DIDL document
3 DIDL as a wrapper
3.1 Item Descriptors
3.1.1 Item 'Identifier' Statement
3.1.2 Item 'modified' Statement
3.1.3 Item 'type' Statement
3.2 The Top Item as a Compound Object
3.3 Second level Items
3.3.1 Metadata Item
3.3.2 Object File Item
3.3.3 Jump-off-page / Human Start page Item
3.4 Example of full OAI-PMH record with a MPEG-21 DIDL document

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Introduction

This document describes the use of DIDL in the context of institutional repositories. The DIDL Document Specification was originally developed within the DARE programme of SURF as a solution for:

• the harvesting of the digital resources (PDFs etc.) from the local repositories for ingest into the E-Depot system of the Royal Library for long term preservation
• the harvesting of the digital resources (PDFs etc.) from the local repositories by a service provider (e.g. a search portal that indexes the full text of documents)
• the representation of complex documents such as doctoral theses that consist of multiple digital resource files
• the confusing use of dc:identifier; dc:identifier can be used for different types of objects; the identifier itself doesn't indicate what type of object is identified; in the context of repositories dc:identifier can point to a jump-off page (JOP) and/or to object files.

DIDL has been in use by the DARE community since the summer of 2006. One of the results is that the content of all Dutch repositories are now part of the E-Depot of the Royal Library, the national library of The Netherlands.

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Compound and Digital Objects as Digital Items

The digital objects that populate institutional repositories can be seen as compound objects that consist of parts or components that are also digital objects. In the DIDL model the basic entity is a Digital Item. The compound objects and their objects play the role of Digital Items in the model that underlies DIDL. In a DIDL document the Item elements represent the Digital Items. The top Item element that is situated directly below the DIDL root element is used for the compound object. The Item elements that are the children of the top Item element represent the objects that are part of the compound object. The objects that are part of a compound object can themselves be a compound object. When a part object is also a compound object, then its parts are not described in the same DIDL document, but a separate DIDL document is used to describe this compound object with its parts. This means that in this application profile there are only two levels of Digital Items within a DIDL document. Although DIDL allows for a hierarchy of Digital Items, this profile restricts the hierarchy to two levels: the level of the top Digital Item, the compound object and the level of the Digital Items that are parts of the top Digital Item. This version of the application profile doesn't give (yet) guidelines for the case of a compound object that is part of another compound object.
\\
This profile distinguishes three types of digital objects: descriptive metadata, object files and jump-off pages. This list is extensible; other types can be added.
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The figure below is a schematic representation of a DIDL document of a compound object that consists of one or more descriptive metadata records, zero or more object files and zero or one jump-off page. Metadata that apply to the metadata records, object files and jump-off pages can be placed in Descriptor elements within the respective Item elements. In the figure the most used Descriptors are shown. The list of Descriptor elements in an Item is extensible. 
\\
A digital object can have one or more representations. A representation is the thing that can be displayed on a computer screen or that can be printed. A representation MUST have a medium type (mimetype). In DIDL, representations are handled by the Resource element. A Resource is contained in a Component element which in its turn is a child of the Item element. There are two ways of including a representation in a Resource element. The first way is *by-value{*}: the representation as such is included as content of the Resource element. This is the usual way that metadata records formatted in XML are included. The second way is *by-ref{*}: the Resource element stays empty, but the representation is referred to by an URL that is the value of the ref-attribute of the Resource element. Normally, the URL will point to a file in the repository.
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Each Digital Item MUST have an identifier with the exception of jump-off pages for which the identifier is optional. This identifier MUST be an URI. The URI of a Digital Item should be different from the URLs of its representations. The identifiers of the Digital Items must be persistent. The URLs of the representations and the medium types can change, while the identifier of the Digital Item stays the same. This allows, e.g., for replacing a file that can only be processed by an old-fashioned word processor by a version with the same content that can be read at all contemporary desk tops. Or a file can be moved to another location; the identifier of the Digital Items stays the same indicating that it is still the same file. If the policy of a repository is to preserve the different representations of a Digital Item then the repository is advised to treat the representations as separate Digital Items, each with its own persistent identifier. So it is possible that in one repository the PDF, the Word and the HTML versions of a publication are combined into one Digital Item, while in another repository they are treated as separate Digital Items. Another use of Digital Item identifiers is to relate Digital Items to each other.
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DIDL\[1\]Item\[1\]Descriptor/Identifier
(Persistent Identifier)  Component/Resource
(URL of representation of this Item)Item\[1..8\] (of type metadata)Item\[0..8\] (of type object file)
Item\[0..1\] (of type jump-off page)
  Component/Resource
(representation by value (XML) or by refrerence (URL))Descriptor/Identifier
(Persistent Identifier)Descriptor/type  Component/Resource
(representation by ref. (URL))  Component/Resource
(representation by ref. (URL))Descriptor/dateModified (optional)Descriptor/Identifier
(Persistent Identifier)Descriptor/typeDescriptor/dateModified (optional)Descriptor/Identifier (optional)Descriptor/typeDescriptor/dateModified (optional)
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OAI Response with a DIDL document

The DIDL document is part of an OAI-PMH response. The DIDL document will be returned within an OAI-record when using didl as value of the metadataPrefix verb. This enables the repository to generate this particular didl format that is described in the document below.
Within the OAI XML structure, the DIDL resides within the metadata element. See below:

<OAI-PMH ...> ... <request ... metadataPrefix="didl"> ... <record> <header>...</header> <metadata> <didl:DIDL xmlns:didl="urn:mpeg:mpeg21:2002:02-DIDL-NS" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dcterms="http://purl.org/dc/terms/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dii="urn:mpeg:mpeg21:2002:01-DII-NS" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" urn:mpeg:mpeg21:2002:02-DIDL-NS http://standards.iso.org/ittf/PubliclyAvailableStandards/MPEG-21_schema_files/did/didl.xsdImage Added urn:mpeg:mpeg21:2002:01-DII-NShttp://standards.iso.org/ittf/PubliclyAvailableStandards/MPEG-21_schema_files/dii/dii.xsd"> ... </didl:DIDL> </metadata> <about>...</about> </record> ... </OAI-PMH>

Remarks:

1. Don't forget the DIDL tag in the OAI-PMH response.
   1. Make a declaration of the didl, dii, dc and dcterms namespaces here, in the DIDL tag.
2. The about element is optional in OAI-PMH

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DIDL as a wrapper

The DIDL document is a document with one top-level Item element. The Item contains several child Item elements. These child Item elements describe three different types: descriptive metadata, object files and jump-off pages. Between brackets the cardinality of the XML elements are shown:

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Item Descriptors

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DIDL\[1\]Item\[1\]Item\[1..8\] (of type metadata)Item\[0..8\] (of type object files)
Item\[0..1\] (of type jump-off page)
Descriptor\[2.. 8\]Descriptor\[2.. 8\]Descriptor\[2.. 8\]Descriptor\[1.. 8\]
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Item Descriptors provide information about the Digital Item. A Descriptor contains a Statement with information about the Item. For each "statement" a new Descriptor is used.
The top level *Item* element *MUST* contain _two_ *Descriptor* elements. One *Descriptor* element for the (Persistent) Identifier and one *Descriptor* element for the modification date. 

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The digital objects that populate institutional repositories can be seen as compound objects that consist of parts or components that are also digital objects. In the DIDL model the basic entity is a Digital Item. The compound objects and their objects play the role of Digital Items in the model that underlies DIDL. In a DIDL document the Item elements represent the Digital Items. The top Item element that is situated directly below the DIDL root element is used for the compound object. The Item elements that are the children of the top Item element represent the objects that are part of the compound object. The objects that are part of a compound object can themselves be a compound object. When a part object is also a compound object, then its parts are not described in the same DIDL document, but a separate DIDL document is used to describe this compound object with its parts. This means that in this application profile there are only two levels of Digital Items within a DIDL document. Although DIDL allows for a hierarchy of Digital Items, this profile restricts the hierarchy to two levels: the level of the top Digital Item, the compound object and the level of the Digital Items that are parts of the top Digital Item. This version of the application profile doesn't give (yet) guidelines for the case of a compound object that is part of another compound object.
This profile distinguishes three types of digital objects: descriptive metadata, object files and jump-off pages. This list is extensible; other types can be added.
The figure below is a schematic representation of a DIDL document of a compound object that consists of one or more descriptive metadata records, zero or more object files and zero or one jump-off page. Metadata that apply to the metadata records, object files and jump-off pages can be placed in Descriptor elements within the respective Item elements. In the figure the most used Descriptors are shown. The list of Descriptor elements in an Item is extensible.
A digital object can have one or more representations. A representation is the thing that can be displayed on a computer screen or that can be printed. A representation MUST have a medium type (mimetype). In DIDL, representations are handled by the Resource element. A Resource is contained in a Component element which in its turn is a child of the Item element. There are two ways of including a representation in a Resource element. The first way is by-value: the representation as such is included as content of the Resource element. This is the usual way that metadata records formatted in XML are included. The second way is by-ref: the Resource element stays empty, but the representation is referred to by an URL that is the value of the ref-attribute of the Resource element. Normally, the URL will point to a file in the repository.
Each Digital Item MUST have an identifier with the exception of jump-off pages for which the identifier is optional. This identifier MUST be an URI. The URI of a Digital Item should be different from the URLs of its representations. The identifiers of the Digital Items must be persistent. The URLs of the representations and the medium types can change, while the identifier of the Digital Item stays the same. This allows, e.g., for replacing a file that can only be processed by an old-fashioned word processor by a version with the same content that can be read at all contemporary desk tops. Or a file can be moved to another location; the identifier of the Digital Items stays the same indicating that it is still the same file. If the policy of a repository is to preserve the different representations of a Digital Item then the repository is advised to treat the representations as separate Digital Items, each with its own persistent identifier. So it is possible that in one repository the PDF, the Word and the HTML versions of a publication are combined into one Digital Item, while in another repository they are treated as separate Digital Items. Another use of Digital Item identifiers is to relate Digital Items to each other.
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The DIDL document is part of an OAI-PMH response. The DIDL document will be returned within an OAI-record when using didl as value of the metadataPrefix verb. This enables the repository to generate this particular didl format that is described in the document below.
Within the OAI XML structure, the DIDL resides within the metadata element. See below:

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Remarks:

1. Don't forget the DIDL tag in the OAI-PMH response.
   1. Make a declaration of the didl, dii, dc and dcterms namespaces here, in the DIDL tag.
2. The about element is optional in OAI-PMH

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The DIDL document is a document with one top-level Item element. The Item contains several child Item elements. These child Item elements describe three different types: descriptive metadata, object files and jump-off pages. Between brackets the cardinality of the XML elements are shown:

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Item Descriptors provide information about the Digital Item. A Descriptor contains a Statement with information about the Item. For each "statement" a new Descriptor is used.
The top level Item element MUST contain two Descriptor elements. One Descriptor element for the (Persistent) Identifier and one Descriptor element for the modification date.

1. Modifications MUST be made visible by changing the modification date. When there are no modifications the modification date can be left out from the second level Items.
2. Changes of the modification date in child Item elements MUST be propagated to the parent Item element.
3. When a Descriptor element for modification date is used also a Descriptor element with an identifier MUST be used (they go in pairs). Rationale: In order to compare similar harvested Item elements wrt modification date, an identifier must be added.
4. For the second level Item elements:
   1. the "type" Descriptor element MUST always be used
   2. the "identifier" Descriptor element MUST be used in the metadata and objectfile Descriptor elements. This is optional for the jump-off page Descriptor element
   3. the "modification date" Descriptor element MAY be used in all of the second level Item elements.

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1. In this example the root Item has a Persistent Identifier.
2. The identifier MUST be an URI. Some repositories can make use of registered URIs, e.g., in the Netherlands the repositories can make use of National Bibliography Numbers. If a repository doesn't have access to registered URIs, UUIDs or TAGs can be used. More information about UUIDs can be found at http://en.wikipedia

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   .org/wiki/UUID and for TAGs the best starting point is http://www.taguri.org/. TAGs for the repository of Tilburg University could be constructed in the following way. All TAGs start with 'tag:uvt.nl,<year>:<record id>'. <year> is the year that the compound object entered the repository. The URIs for MODS records can look like 'tag:uvt.nl,<year>:<record id>/mods' and the URIs for object files can look like 'tag:uvt.nl,<year>:<record id>/<file id>'.
3. The identifiers of the root Item and the second level Item elements SHOULD NOT be equal to the OAI identifier or the DIDL document identifier.
4. Identifiers should not change. Different identifier implies a different object.
5. The namespace for dii SHOULD be declared in the DIDL tag.

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1. Declare the dcterms namespace in the DIDL tag.
2. The format of the date is Zulu-time; which means that it can be sorted as text.
3. There can be only one Statement element in a Descriptor element, which means that dii:identifier and dcterms:modified reside in separate Descriptor elements.

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Item 'type' Statement

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In Maurice Vanderfeesten (2008), "MPEG21 DIDL Document Specification for repositories", version 2.3.1, the dip:ObjectType is used. Here, this is replaced by rdf:type as more appropriate. For compatibility with Driver and SURFshare both Descriptors can be used. In "MPEG21 DIDL Application Profile for NEEO Repositories" the URI is placed as a literal in the content of the rdf:type element. This is not in line with the use of rdf. Service providers should be aware of these different versions of expressing the type of a Digital Item.

The third descriptor contains the Digital Item type. This type is mainly used the second level Item elements, however, it is also possible to type the top Digital Item.

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The DIDL document contains at least one metadata Item element. This metadata can be in different formats, simple Dublin Core, qualified Dublin Core, MODS, MARC21, etc. The metadata can be included by-value or can be pointed to by-reference. one of the metadata Item elements MUST contain MODS, and the MODS record MUST be included by-value.

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<didl:Item>

<didl:Descriptor> <didl:Statement mimeType="application/xml"> <rdf:type rdf:resource="info:eu-repo/semantics/descriptiveMetadata" /> </didl:Statement> </didl:Descriptor>

<didl:Descriptor> <!-- This metadata instance has its own ID number --> <didl:Statement mimeType="application/xml"> <dii:Identifier>urn:nbn:nl:ui:13-74836724783</dii:Identifier> </didl:Statement> </didl:Descriptor>

<didl:Descriptor> <!-- This record has its own Modification date --> <didl:Statement mimeType="application/xml"> <dcterms:modified>2006-12-20T10:29:12Z</dcterms:modified> </didl:Statement> </didl:Descriptor>

<didl:Component>

<didl:Resource mimeType="application/xml"> <!-- the MODS data --> <mods:mods xmlns:mods="http://www.loc.gov/mods/v3" xsi:schemaLocation= "http://www.loc.gov/mods/v3 * http://www.loc.gov/standards/mods/v3/mods-3-3.xsdImage Added "{*}> <mods:titleInfo>...</mods:titleInfo> <mods:name>...</mods:name> <mods:typeOfResource> ... </mods:typeOfResource> ... </mods:mods> </didl:Resource>

</didl:Component>

</didl:Item>

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• Information about the accessibility of an object file should go in the dcterms:accessRights element. The vocabulary Following SWAP: http://www.ukoln.ac.uk/repositories/digirep/index/Eprints_AccessRights_Vocabulary_Encoding_Scheme^{Image Added} to be used for its content is as follows :
  • http://purl.org/eprint/accessRights/ClosedAccess^{Image Added}
  • http://purl.org/eprint/accessRights/RestrictedAccess^{Image Added}
  • http://purl.org/eprint/accessRights/OpenAccess^{Image Added}

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The third ObjectType Item element contains a link to the jump-off page or intermediate page. This is done in the same way as for the Object Item element. This Item element is optional. There should not be more than one Item of this type. The identifier element and modified elements are optional.

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Example of full OAI-PMH record with a MPEG-21 DIDL document

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===== TO DO =====