Surrogate Items

=Concept for Surrogate Items=

Representation
The concept of a surrogate item allows the re-use of the existing scientific content in an eSciDoc repository across contexts. This is achieved by adding an implicit relation referring the original item. The relation is expressed by an additional item property holding the reference to the original item resource.

The representation of a surrogate item consists of the components, content-streams and the metadata records which are inherited from the original item and the properties, resources, and relations of the surrogate object itself. Optionally, the surrogate item may be enriched with additional metadata records. Due to issues with the different views on the original item depending on access rights, a surrogate object may only point to a released object (see below for details).

Referencing the Original Item
The optional property “origin” in the properties-section of the item representation contains a reference to the original item, which can be either a fixed or floating reference. The original item itself should be a regular item, not a surrogate item. A fixed reference contains a version number suffix in the item ID and points to the specified released version of the original item that the surrogate item represents. A fixed reference to a non-released version is not allowed. A floating reference does not contain a version number suffix in the item ID and points to the latest released version that the surrogate item represents. One has to be aware, if a new release of the original item is created, the surrogate item representation changes while using a floating reference. The inherited parts of the original item are explicitly tagged in the surrogate item representation by the attribute “inherited”, set to the value “true”. The XML schema of Item is therefore extended to allow for this additional attribute in the elements md-record, content-stream and component. It has a default value “false”, which avoids the necessity to change existing items in the repository. The representation of regular items is not affected by this schema change. Metadata If a surrogate item has no own metadata records, all metadata records of the original item are inherited and therefore all elements “md-record” in the surrogate item representation contain the attribute “inherited” with the value “true”. Consequently, the implicit DC-metadata record generated from the mandatory metadata record “escidoc” is also inherited from the original item.

If a surrogate item has own metadata records, they appear in the surrogate item representation. As a result, the surrogate item representation may include metadata records from the original item and metadata records from the surrogate item. As metadata records are identified by their name, this behavior may lead to conflicts with identical names in both the original and the surrogate item. The conflicts are automatically resolved by an overlay mechanism. Metadata records of the surrogate item hide the metadata record of the original item. Thus, the user can decide whether to complement the metadata of the original item with own metadata records or to locally replace them. It is always clear whether a metadata record is a local (to the surrogate item) or inherited original metadata record by checking the “inherited” attribute of the element. If a metadata record owned by the surrogate item, which is hiding a metadata record of the original item with the same name, gets deleted, the original metadata record will re-appear, clearly indicated by the “inherited” attribute. This rule also applies to the mandatory metadata record: if a surrogate item has its own mandatory metadata record, this record will hide the original mandatory metadata record and thus appear in the surrogate item representation (without the attribute “inherited”). Accordingly, a DC- metadata record is generated from it according to the XSLT prescribed by a Content Model for the surrogate item (which might be different from the XSLT of the original item!).

Components, Content-streams, Resources, Relations and Properties
All components and content streams of a surrogate item are always inherited from the original item and therefore always have the attribute “inherited” with a value “true” in the surrogate item representation.

A surrogate item provides its own virtual resources, relations and properties, not the ones of the original item. Therefore, the “resources”, “relations” and “properties” elements never have the attribute “inherited”.

Behavior of Surrogate Items
User can execute all CRUD operations and perform state changes on surrogate item resources using the standard ItemHandler API methods, just as with regular items.

All elements of the surrogate item representation tagged with the attribute inherited=“true”and inherited elements “components” and “content-streams” are always ignored on create and update operations. Thus because properties and resources cannot be modified, only metadata records without the attribute “inherited” and content relations will be stored during create and update (content relations never have the attribute “inherited”, as the surrogate item representation doesn’t include the content relations of the original item).

The lifecycles of a surrogate item and of the original item are independent of each other. The version history of a surrogate item therefore refers to its own history, not the one of the original item.

A user cannot create a surrogate item pointing to a withdrawn item. But a representation of an existing surrogate item is not affected, if its original item is withdrawn, unless user privileges to access the original item are changed due to a status change.

Users can filter and search for surrogate items based on the surrogate item representation, which is a combination of surrogate item and the original item.

Impact on behaviour of the method ItemHandler.release
The business logic of the method ItemHandler.release checks whether the item it is working on is referenced by a floating reference by any surrogate items. In that case the representations of these surrogate items are updated in the internal cache and the search index.

Access rights
To access a surrogate item representation a user has to have both: access rights on a surrogate item and read access rights on the referenced version of the original item. If a user has privileges to access a surrogate item, but no privileges to access a referenced version of the original item, it can however access the sub resources “properties”, “relations”, “md-records”, “resources” owned by a surrogate item. An attempt to access the whole surrogate item representation in this case results with an AuthorizationException containing a message with a fail reason and the information about a possibility to access sub resources.

The method create of the ItemHandler throws an Athorization Exception, if a creator of a surrogate item has no privileges on a referenced version of the original item. If the creator lost the privileges to access the original item in the meantime, an attempt to update the surrogate item via the update method of the ItemHandler throws the AuthorizationException containing a message, which informs about a possibility to update the data owned by the surrogate item via update methods on the sub resources.

Reason to reference a released version of the original item
The requirement that the referenced version of the original item has to be released avoids the confusion arising from situations with floating references to the original item. In the case of a floating reference either the latest release or latest version of the original item will be delivered with a surrogate item representation by the business logic depending on the user access rights on the original item. If the creator of a surrogate item has unrestricted read access privileges on the original, the content of the metadata records owned by the surrogate item can be based on the content of the latest version of the origin. A user who has restricted access on the original item will get the surrogate item representation of the latest release of the original item. He can be confused because the own metadata of the surrogate item does not match the inherited content.

=Discussion=

Advantages

 * The DB Cache will contain the surrogate item properties and all metadata records the user chose to expose in the surrogate item representation. This allows for filter operations within the context of the surrogate objects with metadata from the original object.
 * The Search Service retrieves the surrogate item representation and the content from the original object, thus allowing for mixed queries of both surrogate and original metadata records combined with the original full-text within the context of the surrogate items.

Disadvantages

 * Merging parts of the original item with the surrogate item may lead to confusion
 * If access rights disallow the access to the original item after a surrogate item has been created, it may lead to limitations in the access to the surrogate item

Alternatives

 * See Surrogate_items for FACES
 * Use proxy objects which just have a content relation pointing to the original item (no merging of contents)