Fedora Repository Views
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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.
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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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It is very simple for a content model to declare itself to be an Entry for a view angle. All it has to do is have a literal relation in the RELS-EXT datastream, by the name "isEntryForViewAngle", in the view namespace (see DomsNameSpacesAndSchemas), to the literal name of the view angle.
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The problem arrives when trying to do this. The View system is designed to ease the computing of a View when knowing the Main Entry object. The reverse is finding the Views, ie. the Main 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, MAINSENTRIES, will have the columns PID, changed and updated. The PID these columns
- entryPid: This is the pid of
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- 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, have just one column, PID, the PID of the object. Each entry in MAINS will have 1..* relations to entries in OBJECTS, and each entry in OBJECTS will have 1..* relations to entries in MAINS.
These lists will be made at one time, by computing the view of all main objects in Doms. All Main objects will have been found, and added to MAIN_OBJECTS. When appropriate changes are made to the objects in DOMS, the object in MAIN_OBJECTS will be marked as changed. Periodically, the View of changed main objects will be computed, and the VIEWS list will be updated, and the objects will be marked as not changed.
We expect to hook the fedora API-M functions directly, so that the updating of the VIEW lists are done without any user input.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
- object Created: The Object was created in DOMS
- object Deleted: The Object was purged from DOMS
- object State Changed: The Object changed state in DOMS
- object Changed: The Object changed in some fashion that does not require the view to be recomputed, such as the content of datastreams.
- object Relations Changed: The Object changed in a fashion that DOES require the view to be recomputed.
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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