Update Tracking

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

Motivation

Given that we want to work on Views of data, we want to be able to monitor when an object View has changed. We say that an object View has changed if any of the objects in the View have changed. We use the term Record to denote the set of objects in a view.

States in Fedora get special treatment for Records. A Record is considered Active if the entry object in the record is Active. A Record is considered Deleted if the entry object is Deleted. And of course, a Record is Inactive if the entry object is inactive. A Record cannot formally be in several states at once, but it is more useful to consider the three states as three branches of the Record. Even if the Record is inactive, you can get the latest active version of the Record.

We want to be able to return all Records in a given Collection for a given State that have been modified after a given Time. To do this, we maintain a database of Views that is updated on all changes of an object.

Finding Changed Records

To find changed records we will ask for a set of entry objects with the following criteria

We will then get a list of records, sorted by the timestamps with these fields

. For each entry pid, we can construct the Record by the method CalculateView, as it looked at the given timestamp.

Maintaining state

Whenever one of the objects in a Record is changed, the whole Record counts as updated. As such, any services that subscribe to the Repository in any way need to be notified. If there is a search index for the Records, 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 Record when knowing the Entry object. The reverse is finding the Records, 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 a database. It will have two tables.

Database Schema

 

The first table, RECORDS, will have these columns

EntryPid, viewAngle and Collection will form a key. 

The second table, OBJECTS, will have these columns

The two tables will have a many-to-many relation, linking objectPid to (EntryPid,ViewAngle,Collection)

Changing an object and marking the view as updated

Basically, we need three kinds of operations to handle updates:

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

  1. Object Created: The Object was created in DOMS
    Fedora operations: 
    - ingest
    Action:

      modifyState(Inactive)
      reconnectObjects()
      updateTimestamps()

     

  2. Object Deleted: The Object was purged from DOMS
    Fedora operations:
    - purgeObject
    Action:

      modifyState(Deleted)
      updateTimestamps()
      if content model
    	for all objects of this class
    		reconnectObjects()
    		updateTimestamp()

     

  3. Object State Changed: The Object changed state in DOMS
    Fedora operations:
    - modifyObject
    Action:

      modifyState(state)
      updateTimestamp()

     

  4. Datastream Changed: The Object datastreams changed. Handled differently depending on whether this is the relations datastream
    Fedora operations:
    addDatastream
    - modifyDatastreamByReference
    - modifyDatastreamByValue
    purgeDatastream
    - setDatastreamState
    - setDatastreamVersionable
    updateTimestamp
    Action:

      if RELS-EXT
        reconnectObjects(this)
      fi
      updateTimestamp(this)
      if VIEW and Content Model
    	for all objects of this class
    		reconnectObjects(object)
    		updateTimestamp(object)
      fi 
    
  5. Object Relations Changed: The Object changed in a fashion that DOES require the view to be recomputed.
    Fedora operations:
    - addRelationship
    - purgeRelationship
    Action:

      reconnectObjects(this)
      updateTimestamp(this)
      if this is a content model
    	  for all objects of this class
    		reconnectObjects(object of this class)
    		updateTimestamp(object of this class)


Each of these operations will be elaborated below

modifyState

Param new state (one of Active, Inactive or Deleted)

if new state is not Deleted

If the new state is Deleted

Update Timestamps

 

select Records from OBJECTS where Deleted Timestamp <= Inactive Timestamp

for each

Reconnect Objects

An object's relations changed or an object was deleted. This could change which objects are in which entry's views. 

Get the view Information about this object (Which viewAngles is this object entry for)

get the Collection information about this object (which collections is it in)

Create Records in RECORDS corresponding to all these view angles and collections (if they do not exist already) with the Inactive Timestamp set

For each Record in RECORDS with this entry pid and not in this set of view angles or not in this set of collections

for each Record this object is part of (query OBJECTS with objectPid = this pull)

 

There are a number of cases, which are better to discuss now

  1. A view relation is added, meaning that some other object will now be included in the view. We recalculate the view and update OBJECTS, so this will be noticed.
  2. A non-view relation is added, meaning that the view will not change We recalculate the view and update OBJECTS, but this will be a no-change.
  3. A view relation is removed, meaning that the view will now contain one object less. We recalculate the view and update OBJECTS, so this will be noticed.
  4. A non-view relation is added, pointing to an object in another view, which have this relation as an inverseViewRelation, meaning that this object will now be part of that other view. This will not currently be noticed, which is a problem
  5. A content model relation is added. If this content model makes the object an Entry object, this will be noticed. If this object was already part of a view, the view will be recalculated, and thus the change will be noticed. If the object was not part of a view and the change did not make it an entry, it should not be noticed.
  6. A collection relation is added. If this object is or becomes an entry, this will be noticed. Otherwise, it should not be noticed. 

Implementation considerations

 


Purge of content models are meaningful. Mark as deleted does not matter, as no content model states matter

A content model table, to quickly check if you are a content model and what you are entry for could speed things up a lot