Fedora Repository Views

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But the angle one views the repository might also affect the number of entries seen. The above, recursive approach will always lead to one entry per data object. The remedy for this is to mark some classes as Entries for a certain view angle. This means that to compute the records for a given view angle, the view of all objects of a class that is an Entry should be computed. This is the view of the repository.

Fedora Implementation 

This section describes how the above could be implemented in Fedora. 

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Set<Object> visitedObjects;
 
List<Object> CalculateView(Object o) {
	List<Objects> view = new List<Objects>();
 
	if (visitedObjects.contain(o){
		return view;
	}


	visitedObjects.add(o);
 	if (o.isDeleted){
		return view;
	}
	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;
}

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• When finding the annotated relations for a given view angle for a data object, get the list from each of the content models, merge it to one list and remove the duplicates. These are the view relations of this object. Do the same with the inverse relations.
• If a content model marks an object as an Entry for a given view angle, the object is an entry. It does not matter if it has other content models that does not mark it as an entry. An object can of course be an entry for several view angles.   

Update Tracking

This document describes how the backend services should use to view to detect changes in records.

Changing an object and marking the view as updated

Whenever one of the components of a View is changed, the whole View count as updated. As such, any services that subscribe to the View in any way need to be notified. If there is a search index for the Views, and one is updated, its state in the index must be recomputed.

The problem arrives when trying to do this. The View system is designed to ease the computing of a View when knowing the Entry object. The reverse is finding the Views, ie. the Entry objects, that have this data object in their View. Rather than encoding this information in the model, we chose to keep an external record of all the views.

The external record will be SQL based, or something similar. It will have two tables.

The first table, ENTRIES, will have these columns

• entryPid: This is the pid of the entry object
• viewAngle: This is the viewangle
• state: This is the fedora object state
• dateForChange: This is the timestamp when this row was created

EntryPid, viewAngle and State will form an unique key.

To explain the reasoning: Each Entry Object can be an entry object for multiple viewAngles. If the object state is changed, the old entry should remain. As such, each entry object can result in many rows.

The second table, OBJECTS, will have these columns

• objectPid: The pid of this object
• entryPid: The pid of the entry object that includes this object
• viewAngle: The name of the view angle by which the entry object includes this object

There are a fixed number of operations that can be done on objects in doms. For each of these, this is what should be done on the index as a result

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Object Created can be implemented as Object State Changed, followed by Object Relations Changed.

There are several situations where the list will be used

• A data object is modified: (ModifyDatastreanBy*) Look up the PID of the object, find the main object(s), and mark them as modified.
• A non-main data object is added: Do nothing
• A main data object is added: (ingest/AddRelationship->Contentmodel) Register the PID as a main object, and mark it as changed.
• A relation is added/deleted in an data object: (AddRelationship/PurgeRelationship) Look up the PIDs of both the object and the subject of the relation, and mark both's main objects as changed.
• A relation is modified in a data object:(API?) Look up the PIDs of all the object, and the new and old subject, and find the main objects of all three, and mark these as changed.

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