TableOfContents

1. Extensions for Fedora

The DOMS data model have been make use of the Fedora extension:

ImageLink(http://merkur/viewvc/trunk/docs/datamodel/fig/DOMSBaseCollection.png?root=doms&view=co,alt=DOMS base collection,width=1024)

The DOMS datamodel describes how the Type system underlying DOMS is realised in Fedora 3. The figure above will serve as a guide through the following sections.

1.1. DOMS Content Models

Fedora provides a repository for digital objects. All objects in the repository can, in principle, be unique, but Fedora provides a way of specifying that an object has a given type. Unfortunately, the type-definitions in Fedora, called Content Models, are rather simplistic by default. We use them as the basis of our type system, with certain enhancements.

For our purposes, there are two kinds of digital objects in Fedora

The Content Model object, as used in DOMS, describes the compulsary and legal content of an object of this type. It contains the information nessesary to verify if the given object is indeed of this type.

A data object can specify the Content Model describing its contents, via a fedora-model:hasModel relation, and in DOMS we require it to be present. A data object will be said to "subcribe" to a Content Model. Content Model inheritance, as specified in FedoraOntology, will be used.

1.1.1. Inheritance

The special Content Model object "ContentModel_DOMS" is the root object. All Content Models must have an "doms:extendsModel" relation to this object, possibly through a number of other Content Models. The complete description of a data object is defined as the set of the descriptions in the Content Model specified with "hasModel" and all Content Models that can be reached from this, by following "extendsModel" relations.

A Content Model can "extend" more than one other Content Model. There is no overriding of Content Models, a subscribing object must be valid in regards to all the Content Models in the inheritance tree.

1.1.2. Schema Objects

Many of the schemas used in DOMS need to be referenced many times. To avoid duplication, we have made objects containing only schemas, suscribing to the Content Model "doms:ContentModel_Schema". The describing datastream in a schemabinding might contain the schema directly, or it might contain the URL to the datastream that does. Either way, it should be invisible to programs accessing the datastream through the API.

1.1.3. VIEW

A view is a way of combining objects in the DOMS into a domain-relevant group. It is a way of seeing a number of objects as related - as a whole; information that can be useful for the GUI-generator when generating GUI-windows.

Those views that we imagine as being suitable for a screen or window in the GUI, are called main views. Each main view contains an object that the main view is centered around. We call this the main object, and the ID of this view is the ID of the main object. Views of other objects are simply called views. The main object is the object that represents the main view - other objects in that view are related to the main object and would presumably be relevant to edit in the GUI at the same time. For a CD modelled in DOMS, for example, a CD object would be the main object, and objects for tracks, cover, lyrics and so on would constitute the rest of the main view.

We imagine that results appearing in searches in the GUI will all be main views. In fact every view that will be the basis for a screen/window will be a main view.

A view for an object O is represented by a Datastream VIEW on the Content Model object for O. This Datastream also mark the object as Main, if this is the case. Please note that the view is defined on Content Model level, so the same rules are used to generate the view for all objects using that Content Model. When creating totally new objects in the GUI, they should subscribe to main view content models from the current collection.

The datastream will just contain a list of relation names and reverse relation names. Following these relations will give you the view.

Definitions:

In addition, we suggest to augment the 1-step approach with the idea of "includes". What this means is that when object O has a view defined by following relations from O once, and an object P is in the view of O, then the view of P will be included in the view of O.

View datastream contain xml of the form 

<?xml version="1.0" encoding="UTF-8"?>
<view:views  xmlns:view="http://doms.statsbiblioteket.dk/types/views/0/1/#">
  <view:view name="GUI" mainobject="true">
    <view:relations>
      <doms:hasFile xmlns:doms="http://doms.statsbiblioteket.dk/relations/default/0/1/#"/>
    </view:relations>
    <view:inverse-relations>
      <doms:isPartOfCollection xmlns:doms="http://doms.statsbiblioteket.dk/relations/default/0/1/#"/>
    </view:inverse-relations>
    <view:datastreams>
      <view:datastream>DC</view:datastream>
    </view:datastreams>
  </view:view>
</view:views>

As can be seen, it describes all relations to be followed outwards, both directly and reverse. When including the object, only the named datastreams from the datastreams tag should be used. There can be several views, with different views in an object. The GUI should use the view with the name "GUI".

1.1.3.1. Inheritance rules

Views are inherited when Content Models extends each other. Keep three seperate lists, one for datastreams, one for relations and one for inverse relations. Just concatenate the entries from all parent content models to these lists, and remove duplicates. Then use these three lists to generate the list of objects in the view.

1.2. Predefined Content Models

Shorthand:

1.2.1. ContentModel_DOMS

ContentModel_DOMS is the root of the content model inheritance tree. All content models derive from this model. As all objects in DOMS must have a content model, all objects must adhere to the restrictions defined in this content model.

The following variables are used:

Requirements for objects described by ContentModel_DOMS

1.2.2. ContentModel_File

Extends ContentModel_DOMS

In DOMS, we have found it beneficial to separate the abstract concept of "Image" or "Audio" from the concrete implementations such as "jpeg" and "mp3". The metadata about the image will be relevant no matter the manifestation of the image, and as such should not reside along with the technical metadata about the manifestation. To support this separation, we have introduced the concept of File objects.

A File object is an object, that contain a link to the file (in Bitstorage), and the technical metadata about this file. Only File objects are allowed to reference a file in Bitstorage. File objects must all have a Content Model that extends ContentModel_File.

The following variables are used:

Requirements for objects described by ContentModel_File

The characterisation datastream could look like this. 

<?xml version="1.0" encoding="UTF-8"?>
<char:characterisation xsi:schemaLocation="http://doms.statsbiblioteket.dk/types/characterisation/0/1/# http://developer.statsbiblioteket.dk/DOMS/types/characterisation/0/1/characterisation/characterisation-0-1.xsd"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:char="http://developer.statsbiblioteket.dk/DOMS/types/characterisation/0/1/#"
  xmlns:jhove="">
  <char:characterisationRun>
    <char:tool>JHove</char:tool>
    <char:output>
      <jhove:...>
      </jhove:...>
    </char:output>
  </char:characterisationRun>
</char:characterisation>

1.2.3. ContentModel_ImagePreservationFile

Extends ContentModel_File

Requirements for objects described by ContentModel_ImagePreservationFile

1.2.4. ContentModel_TextPreservationFile

Extends ContentModel_File

Requirements for objects described by ContentModel_TextPreservationFile

1.2.5. ContentModel_VideoPreservationFile

Extends ContentModel_File

Requirements for objects described by ContentModel_VideoPreservationFile

1.2.6. ContentModel_AudioPreservationFile

Extends ContentModel_File

Requirements for objects described by ContentModel_AudioPreservationFile

1.2.7. ContentModel_Image

Extends ContentModel_DOMS

The following variables are used:

Requirements for objects described by ContentModel_Image

1.2.8. ContentModel_Audio

Extends ContentModel_DOMS

The following variables are used:

Requirements for objects described by ContentModel_Audio

1.2.9. ContentModel_Video

Extends ContentModel_DOMS

The following variables are used:

Requirements for objects described by ContentModel_Video

1.2.10. ContentModel_Text

Extends ContentModel_DOMS

The following variables are used:

Requirements for objects described by ContentModel_Text

1.2.11. ContentModel_License

Extends ContentModel_DOMS

Requirements for objects described by ContentModel_License

1.2.12. ContentModel_Schema

Extends ContentModel_DOMS

Requirements for objects described by ContentModel_Schema

1.2.13. ContentModel_Collection

Extends ContentModel_DOMS

Requirements for objects described by ContentModel_Collection: None

2. Collections

The DOMS system will be a system that models several collections of digital objects. Each object belongs to one or more collections. This is represented by having one or more "isPartOfCollection" relations to the parent collections. This goes for a collection object as well - it belongs to another collection. One collection has special status though: the "doms:Root_Collection" does not belong to any other collection, and is thus the bottom element for the "isPartOfCollection" relation. Every other collection has a "isPartOfCollection" relation to "doms:Root_Collection".

In addition, DOMS contains another special collection - the "doms:DOMS_Base_Collection". This collection provides objects such as content models and licenses that are utilized by (and mandatory for) the other collections in the DOMS. This collection is meant to be ingested as the first collection in the DOMS.

2.1. Technical Metadata

A file object should contain technical metadata. In this context it refers

In addition, it must have a relation "hasOriginal" if it was migrated from another file that exists in DOMS.

3. Licenses

Licenses, in DOMS, have, as their only purpose, to restrict who can view what material. They are only a concern for people using the material in DOMS, not users working with the GUI, or otherwise administrating the contents.

Licenses are implemented by using the Fedora XACML engine. When a user authenticates with the Fedora server (or with a server passing authentication tokens to the DOMS), he gets a number of attributes. Each of these attributes name one license that the user can access material under.

Each data object in DOMS has a POLICY datastream. This datastream is just an URL, referring to a License object's LICENCE datastream. This datastream is an XACML stream, that evaluates if the user posses the attribute that specify that the user can use this License. If yes, the user is granted access to the original object, otherwise he is denied.

4. Audit Trail

Each user that will use the GUI will need to login. They will authenticate with some external server, probably the SB LDAP server. The access control is not really a concern for the DOMS system. As such, all GUI users will have equal and full access to the DOMS repository.

Audit trails, however, are a concern. Each change to a datastream in a data object will, per default Fedora behaivour, create a new version of this datastream, marked with the creation time and the username. For this reason the Fedora operations PurgeObject and PurgeDatastream are blocked, as they destroy the audit trail. Real deletion of information is not possible, but both objects and datastreams can be marked as "deleted", again per standard Fedora behaviour. Any tools working with or on DOMS should respect this flag. The GUI should only concern itself with the most recent version of a datastream.

= Working with the Datamodel or How to work without transactions=

The STATE datastream, present in all data objects are the key. To emulate a transaction, follow these steps:

  1. Ingest any new objects, with STATE=draft
  2. change the STATE of all involved objects with STATE=published to STATE=intermediate
  3. Perform all changes that must be performed
  4. Change the objects back to STATE=published. This will provoke a validation of the objects by the DOMS system. If the objects failed the validation, the STATE will not change

DOMS only try to enforce a structure on objects with STATE=published. By marking objects as intermediate, the system will disregard them, without losing the relations to them.