## 5 Architectural Design of the PORTAL-DOORS System
### Core Design  
Figure 2 displays a diagram depicting the structure of data records at PORTAL registries and DOORS directories. Figure 3 displays a server network diagram for root, primary, and secondary DOORS directories interacting with root, primary, and secondary PORTAL registries with examples from problem-oriented specialty domains in biomedical informatics [39]. Details of the PORTAL-DOORS architecture have been previously elaborated in the publications [1, 11, 45] with formal definitions and specifications of the core model for database record fields and web server functions. Some important characteristics of the design include the following principles:  
* A distributed network of registries and directories for resource metadata oriented by problem domain or specialist community rather than by technology format of the resource. * A hierarchical system enabling local independence of communities while simultaneously maintaining global interoperability and compatibility for communication between and search amongst different specialty communities. * A hybridized architecture with both XML Schemas and terminologies serving the original web and also RDF triples and OWL ontologies serving the semantic web to bridge and transition from the original web to the semantic web. * Pervasively distributed and shared infrastructure permitting use of any micro-format, terminology or ontology to promote democratization and evolutionary adoption (_i.e._, survival of the fittest, not necessarily the first). * Hierarchical authorities (root, primary, secondary, forwarding, caching) and globally unique identifiers to prevent namespace conflicts when identifying resources while maintaining autonomy of local communities with control over local policies. * Designed to accomodate any resource -- whether abstract or concrete, offline or online, semantic or non-semantic -- with either non-semantic descriptions using tags referencing terminologies or semantic descriptions using RDF triples referencing ontologies. * Supported with cross-references to other systems whether legacy or contemporaneous. The PORTAL-DOORS System is not another attempt once again to create a so-called 'one stop shop' that claims to be the 'one and only' destination for 'all shopping needs'. In fact, the general philosophy of HDMM systems turns that notion upside down and argues that centralized single site 'one stop shops' also constitute single points of failure and/or single points of biased view, and thus cannot and will not solve all problems. Instead, there should be a multiplicity and diversity of registries and directories. Anybody who desires should be able to construct and maintain their own specialized registry or directory capable of exchanging the interoperable mobile metadata that becomes highly distributed, redistributed, and cached everywhere for speed and efficiency of search and location. Maintaining the interoperability of all registries and directories to communicate with each other transparently within the same PORTAL-DOORS infrastructure system facilitates achieving the goal of efficient search. Figure 3: PORTAL-DOORS System Server Network: PDS server networks with interacting clouds of NEXUS registrars, PORTAL registries, and DOORS directories. NEXUS servers may expose either the NEXUS registrar service for the separate design or the integrated set of NEXUS registrar, PORTAL registry, and DOORS directory services for the combined design. These resource metadata server networks for PORTAL registering of labels and tags and DOORS publishing of locations and descriptions are analogous to domain metadata server networks for IRIS registering of names and DNS publishing of addresses. Primary PORTAL registries may be established by an organization or person who maintains any local policies governing registration of resources at that particular primary PORTAL registry. Examples shown here (GeneScene, BrainWatch, ManRay) implement policies with a problem-oriented focus on their respective specialty domains.