Re-use of a web resource depends on the ability to reference it. As explained earlier, referencing issues exist when publishing compound objects to the web. First, there exists no web-parallel to the identifier of the compound object, shown in Figure 1. Second, there is a need to reference a specific resource not just in its own right (i.e., by means of its URI), but in the manner that it appears in the context of a certain compound object.

Referencing the Compound Object as a Whole

The protocol-based URI of the Resource Map identifies an aggregation of resources (components of a compound object) and their boundary-type inter-relationships. While this URI is clearly not the identifier of the compound object itself, it does provide an access point to the Resource Map and its representations that list all the resources of the compound object. For many practical purposes, this protocol-based URI may be a handy mechanism to reference the compound object because of the tight dependency of the visibility of the compound object in web space on the Resource Map (i.e., in ORE terms, a compound object exists in web space if and only if there exists a Resource Map describing it).

We note, however, two subtle points regarding the use of the URI of the Resource Map to reference the compound object. First, doing so is inconsistent with the web architecture and URI guidelines that are explicit in their suggestion that a URI should identify a single resource. Strictly interpreted, then, the use of the URI of the Resource Map to identify both the Resource Map and the compound object that it describes is incorrect. Second, some existing information systems already use dedicated URIs for the identification of compound information objects “as a whole.” For example, many scholarly publishers use DOIs,¹⁶ whereas the Fedora ¹⁷ and aDORe ¹⁸ repositories have adopted identifiers of the info URI scheme.¹⁹ These identifiers are explicitly distinct from the URI of the Resource Map.

These issues suggest that it should be possible to express in the ORE specifications, and therefore in the Resource Map and its representations, an additional URI – the identifier of the compound object itself. Once a URI to identify the compound object ”as a whole” is introduced, it would play the prominent role in the Resource Map of identifying the resource that corresponds with the compound object and that has component parts (resource C in Figure 6).

Referencing Resources in Context

On the web, resources can unambiguously be referenced by means of their URI. As a result, each published component of a compound object, as well as a published Resource Map describing a compound object, can be referenced. As mentioned in the previous section, a compound object “as a whole” can be referenced if it is assigned a dedicated URI. However, as explained earlier by means of the cellular image example, there is often the need in scholarly communication to reference a resource as a component of a specific compound object.

Figure 7 illustrates a scenario in which resource U is used as part of two compound objects. To reveal boundary information regarding the upper compound object, Resource Map X is published; Resource Map Y is published to do the same for the lower compound object. Because U is part of both compound objects, both Resource Maps X and Y reference resource U. In order to accommodate the need to reference U as part of a specific compound object, OAI-ORE proposes to use an identifier pair that consists of the identifier of the resource itself and the identifier of the Resource Map that corresponds with the desired compound object, i.e. (U,X) to reference resource U as part of the upper compound object and (U,Y) to reference it as part of the lower compound object.