18.104.22.168 Specialization of the AIP and Package Descriptions
Two specializations of the AIP are discussed in this subsection, the Archival Information Unit (AIU) and the Archive Information Collection (AIC). Figure 4-19 is a UML diagram illustrating this specialization. Both AIU and AIC are subtypes of the AIP and as such contain constructs to enable both Long Term Preservation and Consumer access. The AIU represents the type used for the preservation function of Content Information that is not broken down into other Archival Information Packages. The AIC organizes a set of AIPs (AIUs and other AICs) along a thematic hierarchy, which can support flexible and efficient access by the Consumer community. Conceptually all the AIPs organized by an AIC are contained in the Content Information of that AIC. The difference between AIUs and AICs is the complexity of their Content Information and their associated Package Descriptions and Packaging Information. This reference model considers the differences in the Content Information and associated Packaging and Description functionality between AIU and AIC to be adequately complex and linked to justify the definition of separate classes.
Figure 4-19: Archival Specialization of the AIP From an Access viewpoint, new subsetting and manipulation capabilities are beginning to blur the distinction between AICs and AIUs. Content objects which used to be viewed as atomic can now be viewed as containing a large variation of contents based on the subsetting parameters chosen. In a more extreme example, the Content Information of an AIU may not exist as a physical entity. The Content Information could consist of several input files (or pointers to the AIPs containing these data files) and an algorithm which uses these files to create the Data Object of interest.
From an information preservation viewpoint, the distinction between AIU and AIC remains clear. An AIU is viewed as having a single Content Information object that is described by exactly one set of PDI. An AIC Content Information is viewed as a collection of other AICs and AIUs, each of which has its own PDI. In addition, the AIC has its own PDI that describes the collection criteria and process.
There are two specializations of the Package Description, the Unit Description and the Collection Description. Figure 4-20 is a UML diagram illustrating this specialization. The difference in these two classes is based on the functionality needed to effectively access the contents of an AIU versus the functionality needed to effectively access AIPs that are contained in an AIC. Figure 4-20: Archival Specialization of the Package
To aid in the understanding of these constructs, the next two subsections of this document will use an example of a company setting up an OAIS of digital versions of movies. This example will focus on the information content of constructs in an AIP. Subsection 4.3 illustrates more of the details of the information transformations and data flows in an OAIS.