## 2 Related Work  
### Metadata Management Systems  
Prior work on metadata management systems includes diverse examples from the peer reviewed literature [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29], patent literature [30; 31; 32; 33], and published technical reports [34; 35]. A doctoral dissertation written in 1998 by Dolin [36] remains especially noteworthy for its discussion of different kinds of hierarchical metadata for multilevel information systems. Dolin built a distributed system called Pharos [36] for locating heterogeneous information sources. Pharos employed concept-based classification trees with three different kinds of information hierarchies: topical, geographical and temporal. However, not all multilevel analyses are restricted to the task of locating resources [37]. Moreover, not all multilevel models are required to be hierarchical [38]. Thus, any review of metadata management systems must compare and contrast the different definitions and interpretations of the terms _hierarchical_ and _multilevel_ as used or implied by the corresponding features of the various metadata management systems.  
These distinctions can best be revealed by posing questions. Does the multilevel or hierarchical aspect refer to the entity itself (_i.e._, the data, information, source, or resource) or to the metadata about the entity? Regardless of whether the target object is the entity or the metadata, does the multilevel or hierarchical aspect refer to the identification, location, or both identification and location of the target object? Is the fundamental purpose of the metadata management system to classify objects, store and retrieve them, search and discover them, or some other purpose? How similar or dissimilar are the objects with respect to the criteria deemed most relevant and significant to the metadata management system?  
As examples, metadata management systems for storage and retrieval of files in computer file systems [17; 23; 33] operate under a very different set of requirements and constraints when compared with metadata management systems for discovery of services or resources on the web [15; 34; 35]. Moreover, even the most successful and well-known metadata management system for the web employs at least two different kinds of multilevel or hierarchical features. In particular, the IRIS-DNS System maintains one hierarchical scheme for the identification of resources, and simultaneously, another independent hierarchical scheme for the distribution of the metadata records about the resources. For identification of resources, IRIS-DNS uses a hierarchical naming scheme with top-level domain names, domain names,sub-domain names, etc. For distribution of metadata records, IRIS-DNS uses a hierarchical request-forwarding and response-caching scheme with root servers, primary servers, secondary servers, etc.  
As evident from this brief review and discussion, there are many kinds of multilevel hierarchical metadata management systems. However, in all cases found so far in the published literature, the definition and interpretation of multilevel or hierarchical can be considered to be a form of resource identification, resource location, resource content classification, or metadata record distribution. Nevertheless, in the course of work on the software development necessary to move from a prescriptive architecture to a descriptive architecture for PDS, a new and different form of multilevel metadata organization and management has been introduced which offers important practical advantages as explained in Section 5.2.