Fedora Repository Views

DOMS employs an overall atomistic data model. Atomistic data models are much more flexible than traditional compound data models, but they have one big (and largely unmet) challenge. When working with data objects you will frequently need to operate on a number of objects as if they were a common whole. The easiest usecase for this is the public dissemination of data. If the data that should go into one Dissemination Information Package is distributed over several objects, the system needs to understand this. Search indexing is another usecase. Search services tend to use a flat index, each record contain all it's metadata.

The solution to this is the concept of repository views.

Theoretical basis

A repository contain data. This data can be separated into a number of records. A record does not nessesarily correspond to a data object, but is some atomic, selfcontained entry. As they are atomic, they cannot reasonably be further broken down. As they are selfcontained, they are only weakly linked to other entries. A repository view is the mapping from the repository data into these records.

What constitues atomic selfcontained entries are dependent on the reason for accessing the repository. A search engine harvester might want to see one kind of records, while an export function might want another. We call such reasons "view angles". The mapping of data into records is dependent on the view angle.

Fedora Views

Fedora is a repository not just of data, but of digital objects. So, the view mapping should be from a number of objects into a record of some format. I assume A to be a data object.

A reasonable requirement is that for an object to be in the view of A, it must be related somehow to A. Thus, A is connected through some chain of relations, to every other object in it's view.

The second requirement, and this is very fundamental, is that A does not know it is being viewed. A is just a data object. It cannot be expected to keep up with new ways of accessing the repository, and new ways to view the data. So, A must not store any information that pertain solely to this or any other view angle. The relations of A should only be structural, in regards to the data it contains.

So, finding the view of A seems an impossible task, but it is not. For while the second requirement forbids A from knowing about the view angle, the class of A could. In Fedora, the classes of data objects are represented by content models. So, the content model(s) of A could know about this and other view angles of A. But a content model cannot say anything about A specifically, it can only describe the entire class of objects like A. So what it can do it annotate the relations of A. It could say "For this class of objects and this view angle, these structural relations denote references to other objects that are in the view."

This naturally lends itself to a recursive approach. The view of A is A plus the view of any object related to A through such an annotated relation. But this leads to the problem of separation. ...

PIDs angle

The easiest situation (angle) is when mapping to a list of PIDs. Each record is a list of the PIDs of the data object it consist of. This is the situation I will handle first.

Each view is centered around some data object, and contain a number of other objects. So, if A is a data object, view(A) = A.pid + other pids. The view is identified by the object it centered around, i.e. the PID of the data object.

All the other objects in the View are related to the center object by some chain of relations. Therein lies a crucial feature of this View system; Rather than having special relations between data objects to other objects in their view, some of the structural relations are annotated to be view relations.

Or rather, we list the relations that should be followed to find the objects in the view, rather than define view-relations. Actually, we annotate both relations to and from a given object as view relations.

The view nessesary for a proper public dissemination of the objects might not be the same as what is required for a useful GUI access, through. So, it is desirably to define multiple views on the same objects. Each named view has its own set of annotated relations to follow. In no way do they interact, and we can therefore have radically different ways of viewing the same data.

But the views of certain classes of object will tend to more useful than others. Each content model can declare itself to be a main content model for any named view. The exact semantic meaning of being a main view is defined by the systems using this view.

Named views so far:

• "GUI": The GUI view specify which objects should be opened as a combined whole, and which should be regarded as external to this whole. A whole opened in the GUI will always be centered around a object having a content model declaring itself to be main content model for the GUI view.

The main view declaration

It is very simple for a content model to declare itself to be a main object for a named view. All it has to do is have a literal relation in the RELS-EXT datastream, by the name "isMainForNamedView", in the view namespace (see DomsNameSpacesAndSchemas), to the literal name of the view.

Add this relation to any content models that should describe main views for the GUI view.

<view:isMainForNamedView xmlns:view="http://doms.statsbiblioteket.dk/types/view/0/2/#">GUI</view:isMainForNamedView>

The VIEW datastream

Now we come to another crucial feature of this view system; Views are defined on the content model level. The content model describes how the view, from this class of objects, should be generated. Everything is defined in the classes of objects, never in the actual data objects. As such, it is easy to change and add views on a class-wide basis.

To facilitate this, the "VIEW" datastream in content models have been designated as Reserved and Required. The "VIEW" datastream is, basicaly, a sequence of named views, each with their designated relations.

Anchor(ViewSchema) The schema for the VIEW datastream is as follows:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            targetNamespace="http://doms.statsbiblioteket.dk/types/view/0/2/#"
            xmlns="http://doms.statsbiblioteket.dk/types/view/0/2/#"
            elementFormDefault="qualified"
            attributeFormDefault="unqualified">

    <xsd:element name="views" type="viewsType"/>

    <xsd:complexType name="viewsType">
        <xsd:sequence>
            <xsd:element name="view" type="viewType" minOccurs="0" maxOccurs="unbounded"/>
        </xsd:sequence>
    </xsd:complexType>

    <xsd:complexType name="viewType">
        <xsd:sequence>
            <xsd:element name="relations" type="relationsType" minOccurs="0" maxOccurs="1"/>
            <xsd:element name="inverse-relations" type="inverse-relationsType" minOccurs="0" maxOccurs="1"/>
        </xsd:sequence>
        <xsd:attribute name="name" type="xsd:string" use="required"/>
    </xsd:complexType>

    <xsd:complexType name="relationsType">
        <xsd:sequence>
            <xsd:any namespace="##any" processContents="skip" maxOccurs="unbounded"/>
        </xsd:sequence>
    </xsd:complexType>

    <xsd:complexType name="inverse-relationsType">
        <xsd:sequence>
            <xsd:any namespace="##any" processContents="skip" maxOccurs="unbounded"/>
        </xsd:sequence>
    </xsd:complexType>

</xsd:schema>

Multilevel Views

Of course, it is very preferable to be able to have deeply nested views. Achiving this is easy. Above we defined the annotated relations as marking which objects should belong to the view. What we really meant was which object-views should be included in the view.

The formal definition of the semantic meaning of the relations in the "VIEW" datastream is therefore: Each data object has a view, encompassing the object and the views of other directly related data objects. So, if the VIEW datastream in a object was

<view:views  xmlns:view="http://doms.statsbiblioteket.dk/types/views/0/1/#">
  <view:view name="GUI">
    <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:view>
</view:views>

then the View of this object encompass the object itself, and the "GUI" View of any objects that the object has a "doms:hasFile" relation to and any object that has a "doms:isPartOfCollection" relation to this object.

The procedure to calculate the total view of a object is detailed in this bit of pseudo code. It basicly performs a depthfirst search of the objects. The order of the objects in the View does not carry any sort of meaning, and will be random.

Set<Object> visitedObjects;

List<Object> CalculateView(Object o) {
   List<Objects> view = new List<Objects>();

   if (visitedObjects.contain(o){
      return view;
   }

   visitedObjects.add(o);
   ContentModel c = o.getContentModel();
   List<Relation> view-rels = c.getViewRelations();
   List<Relation> object-rels = o.getRelations();

   for (Relation r : object-rels){
     if (view-rels.contain(r)){
       view.addAll(CalculateView(r.getObject());
     }
   }

   List<Relation> view-invrels = c.getInverseViewRelations();
   List<Relation> object-invrels = o.getInverseRelations();
   for (Relation r : object-invrels){
     if (view-invrels.contain(r)){
       view.addAll(CalculateView(r.getSubject());
     }
   }

   return view;
}

Datastream View

The described view system can designate exactly which objects are part of a view. But it is not always enough to know just the objects. For the GUI, it is nessesary to know exactly which datastreams should be presented, and how. For this purpose we have designed an DS-COMPOSITE extension, which follows the system laid down in FedoraTypeChecking.

Anchor(DSCompositeGUISchema)

<xsd:schema
        targetNamespace="http://doms.statsbiblioteket.dk/types/dscompositeschema/guirepresentation/0/1/#"
        xmlns="http://doms.statsbiblioteket.dk/types/dscompositeschema/guirepresentation/0/1/#"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        elementFormDefault="qualified"
        attributeFormDefault="unqualified">

    <xsd:element name="guirepresentation">
        <xsd:complexType>

            <xsd:attribute name="presentAs" use="required">
                <xsd:simpleType>
                    <xsd:restriction base="xsd:string">
                        <xsd:enumeration value="importable"/>
                        <xsd:enumeration value="editable"/>
                        <xsd:enumeration value="uploadable"/>
                        <xsd:enumeration value="readonly"/>
                        <xsd:enumeration value="invisible"/>
                    </xsd:restriction>
                </xsd:simpleType>
            </xsd:attribute>

        </xsd:complexType>

    </xsd:element>

</xsd:schema>

The semantic meaning of the five types are really decided by the GUI, but the approximate meaning is as follows

• importable: The content is inline xml, and should be the result of an import function. Once written, the datastream count as "readonly"
• editable: The contents is inline xml, and should be parsed according to their schema, and presented in the GUI.
• uploadable: The contents is a link to a file in bitstorage. If the datastream does not exist, the GUI should present a way to upload a file. Otherwise the link to Bitstorage should appear, readonly.
• readonly: The contents is inline xml, generated by some other means. The user should be able to read the contents in the GUI, but not change them. The GUI might hide the contents by default, but they must be accessible.
• invisible: The GUI should totally disregard this datastream, and behave as if it is not there. This is the default, if no guirepresentation is defined for a datastream.

So, an example of a datastream entry in DS-COMPOSITE would now be:

<dsTypeModel ID="DC">
    <form MIME="text/xml"/>
    <extensions name="SCHEMA">
        <schema:schema type="xsd" datastream="DC_SCHEMA" object="doms:DublinCore_Schema"/>
    </extensions>
    <extensions name="GUI">
        <gui:guirepresentation presentAs="editable"/>
    </extensions>
</dsTypeModel>

Content Model Inheritance and Views

DOMS employ inheritance for content models, as detailed in FedoraOntology. This interferes with the View system.

As you cannot mark something as NOT being in the view, there are few potential conflicts. For a data object, just take the list of view relations from each of its content models and their ancestors and concatenate and remove duplicates. This is the view relations for this object. Same with the inverse view relations.

Previously we required that you could only mark (as view relations) relations that had been defined in the same content model. This will now be problematic. Rather, the rule now is: In the VIEW datastream, you can only mention relations that are defined in this content model or one of its parents. The inverse relations can still be freely mentioned.

The inheritance rules for datastream views is the same as for datastream definitions. So, like the schema extension, where it is only the last schema that takes effect, it is also only the last guirepresentation that should be considered by the gui. The different extentions do not interfere with each other, so the SCHEMA extension could be defined at the top of the inheritance tree, but the GUI extension near the bottom.

Main views are inherited, as any object that has a content model also has every supertype of this content model. So, they will be objects of a content model that mark them as main view objects.