## 1 Introduction
PDS has also been conceived to operate as a bridge between the original web and the semantic web by enabling use of the system in diverse scenarios. These scenarios range from the simplest case with the minimal set of features required by the original web to the most sophisticated case with the maximal set of features permitted by the semantic web (see Section 6.3). Thus, PDS directly addresses the problem of transition barriers. Moreover, PDS has been devised to bootstrap itself in a manner in which both the infrastructure system and its content are distributed physically and virtually in terms of both the content and control of content. If fully implemented and operated as an internet-scale pervasively distributed infrastructure, this distributed design of the PORTAL-DOORS System itself would prevent the possibility of search engine consolidation in a manner entirely analogous to the success of the IRIS-DNS System in preventing the consolidation of internet domain name registries or directories. However, a system's _descriptive architecture_ as realized when implemented may deviate from the system's _prescriptive architecture_ as intended when designed [8, page 59]. This discrepancy results from a process called _architectural degradation_ which comprises the related phenomena of _architectural drift_ and _architectural erosion_[8, page 61]. These phenomena are distinguished by the discrepancies that arise in the degradation process. Thus, in the process of erosion, the fundamental design decisions of the prescriptive architecture are violated in the descriptive architecture, whereas in the process of drift, these decisions are not violated [8, page 64]. For any system that envisions an internet-scale pervasively distributed infrastructure, the tasks of examining, documenting and reporting the descriptive architecture at various stages of implementation and deployment in comparison with the prescriptive architecture should remain a critical step in the overall process of iterative software development. These analyses enable asking important questions about the presence of any architectural degradation. More importantly, they enable a reasoned deliberation about whether the design decisions of the original prescriptive architecture should be maintained or whether any innovative deviations in the new descriptive architecture should be pursued. Beyond the original published design [1] that serves as the abstract blueprint for PDS (_i.e._, the prescriptive architecture), some concrete interface schemas together with basic ontologies have been drafted for prototype registries in fields relevant to biomedical computing and radiological informatics. These draft prototypes include a formal semantic definition of pharmacogenomic molecular imaging which provides a use case that demonstrates search across multiple specialty domains [9]. However, such XML-based models represent only a piece of the puzzle. A full reference implementation (_i.e._, one that results in a more complete descriptive architecture) requires many other software components, especially back-end database servers and front-end clients for the PORTAL registries and DOORS directories. In order to gain practical experience testing development of alternative implementations, it is necessary to begin working with real data stored in actual database servers. Therefore, the roadmap for PDS development shifted from the revision of XML schemas and OWL ontologies to the construction of a prototype relational database model. This database model has been purposefully chosen to be initially a traditional relational model rather than an RDF-based triple store. This decision was made not only because of the guiding principle that PDS must be capable of operating as a hybrid and a bridge but also because of the pervasive availability of relational databases in comparison with newer kinds of databases.