Common URN Schema

Introduction

All the resources used in cultural heritage — text documents, database tables, museum records, media files — need to be uniquely identifiable.

Identification and Reference of Resources

There are a couple of different schemes in use, some of them open, some proprietary (see the report on persistent identifiers). In ICRIM, we prefer Uniform Resource Names (URN), since they are open for all uses and easily extensible. They can also be written using UTF-8 encoding, and thus internationalized, using the escape mechanism proposed for the Internationalized Resource Identifier (IRI).

Unlike the more common Uniform Resource Identifier (URI), URNs do not point to physical locations of resources, which might change over time. Instead they provide persistent and unique names, including the namespaces, which can be used for unlimited abstract referencing without knowing the physical location of the resource at any given time. References between museum records, text documents (articles), inventories, image descriptions, from document to document or from document to external data etcetera can thus easily be accomplished by just naming the resource.

The full syntax of an URN is the following:

urn:namespaceIdentifier:namespaceSpecificString

For CollectioOrg URN, icrim has been chosen as Namespace Identifier, making the schema like this:

urn:icrim:[memberCode]:[collectionCode]:[objectcode](:[partCode])

memberCode: The member code is from 00001 to FFFFF in hexadecimal system. This leaves room for over a million participants. An example would be urn:icrim:00001 for the Capitoline Museum.
collectionCode: The collection code is defined freely by the museum. An example would be urn:icrim:00001:scu for the sculpture collection. Common codes are scu (for a sculpture), img (for images), epi (for inscriptions) &c. but there are no restrictions.
objectCode: The object code, too, is defined freely by the museum. An example would be urn:icrim:00001:scu:01234. Once an URN has been assigned, it cannot be changed anymore: the URN is effectively frozen.
partCode: The part code is optional: it might denote part of an object (for example, the separated head of a statue). An example would be urn:icrim:00001:scu:01234:01.

Thus we gain the following advantages:

Resources can reference other resources. For example, a description of a series of restoration interventions can reference the objects cited by their URN – without caring about their physical location,
References can be made even to external resources, whose physical location (URL) isn't controlled,
Resources which have lost original their location (through closure of a museum, repatriation of a work of art etc.) can be transferred and stored on any other domain without invalidating the URN.
A Back-Link capability becomes possible, i.e., getting a list of all the peripherical resources which link back to a given primary resource. This allows contextualization and is enormously helpfull, for example, when dealing with stylistic comparisons.
Resources which do not yet have a physical equivalent (for example, because the digitalizing work is still in progress) can nevertheless be referenced – and fully used as soon as it is physically available,
In its extreme, resources do not even have to be physical resources, since they might point to persons or abstract concepts like Caracci, Baroque Art and so on.

De-Referencing URNs

Since URNs are unique identifiers for resources, but do not locate them physically, it is necessary to de-reference the URNs to URLs.

In most cases, URNs can be de-referenced by the host system itself, because the URN points to a resource residing inside the same database. A simple query like http://[member-domain]/search?urn=urn:icrim:00001:scu:01234 should return the requested resource – the Query effectively then becoming the URI.

In those cases where the URN points to a resource with a foreign member code – and thus outside the home domain (urn:icrim:005E:epi:123456) –, the URN has to be de-referenced using a look-up table, available on the Internet server of ICRIM and in local copies, which lists the member codes and assigns the relevant domain names and their corresponding servers.

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Representing Relationships between Documents

Expressing relationships between items are basic requirements for any serious museum documentation. Not only is it necessary to express qualifying relationships between items inside the same museum, but also to items in other museums.

The possibility to declare relationships like part-of, related to, similar-to, equal-to, model-for, copy-of, and reproduction-of is part of the Infoset of each CollectioML document.

Since the CollectioOrg systems is using a database-architecture, a Back-Link capability is possible: i.e., getting a list of all the documents (records, texts, image descriptions etcetera) which link back to a given document. This allows to view the context of a document in the many ways it has been cited in other documents or referenced in other records and is enormously helpfull, for example, when dealing with stylistic comparisons.

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