The Natural Keys pattern encourages the creation of URIs from existing non-global identifiers. While this provides a way to begin identifying a resource so that we can describe it in RDF, it does not address the issue of how to publish these existing identifiers. Nor does it address situations where natural keys change over time, e.g. the move from ISBN-10 to ISBN-13 in the publishing world.

Create a custom property, as a sub-property of the dc:identifier property for relating the existing literal key value with the resource.

The NASA dataset in dataincubator uses Patterned URIs based on the NSSDC international designator, but includes these as literal values associated with each spacecraft using a custom property.

While hackable URIs are a useful short-cut they don't address all common circumstances. For example different departments within an organization may have different non-global identifiers for a resource; or the process and format for those identifiers may change over time. The ability to algorithmically derive a URI is useful but limiting in a global sense as knowledge of the algorithm has to be published separately to the data.

By publishing the original "raw" identifier as a literal property of the resource we allow systems to look-up the URI for the associated resource using a simple SPARQL query. If multiple identifiers have been created for a resource, or additional identifiers assigned over time, then these can be added as additional repeated properties.

For systems that may need to bridge between the Linked Data and non-Linked Data views of the world, e.g. integrating with legacy applications and databases that do not store the URI, then the ability to find the identifier for the resource provides a useful integration step.