## 6 Implementation and Application of the PORTAL-DOORS System
### General Usage Scenarios for the PORTAL-DOORS System  
PORTAL-DOORS has been designed to be as flexible as possible with both backward and forward compatibility from Web 1.0 to Web 3.0. Given the partition with lexical non-semantic services on the PORTAL side and semantic services (with use of the RDF/OWL/SPARQL stack) on the DOORS side, and also the partition with both _required_ and _permitted_ elements for each of PORTAL and DOORS, there are many possible scenarios for usage of the entire PORTAL-DOORS System. Some examples include:  
* Minimal use of required elements for both PORTAL registries and DOORS directories: This scenario essentially reduces use of the system to an alternative equivalent to the use of PURLs [50] (and other similar services). However, it does so _without_ requiring use of a pre-determined URL identifier root like purl.oclc.org and instead allowing use of any identification scheme as long as it is a URI or IRI.
* Maximal use of permitted elements for PORTAL registries but minimal use of required elements for DOORS directories: This scenario enables exploiting the full metadata management facilities of the PORTAL non-semantic services (which include provisions for tags, micro-formats, cross-references, etc) without any obligation to use the DOORS semantic services (that necessitate use of the RDF/OWL/SPARQL stack of technologies and tools). This scenario enables resource agents to publish metadata now in non-semantic formats and defer until later any possible transition to semantic formats which would then be facilitated by the prior staging in the non-semantic formats.
* Minimal use of required elements for PORTAL registries but maximal use of permitted elements for DOORS directories: This scenario serves those situations where there is no barrier to transition the metadata from original web formats to semantic web formats, and the resource owner and agent do not wish to maintain the metadata in both semantic and non-semantic formats. This scenario requires that the resource agent registering and publishing the metadata already has access to established ontologies that can be referenced by semantic tools for describing the resource.
* Maximal use of permitted elements for both PORTAL registries and DOORS directories: This usage scenario provides the significant benefit of exposing as much metadata as possible to as many clients as possible including both older non-semantic as well as newer semantic tools and applications.  
Enabling these usage scenarios constitutes an important goal for the PORTAL-DOORS Project which also includes the following tasks:  
* Complete development of a specification model for the PORTAL-DOORS System as the interoperable informatics infrastructure using the Hierarchically Distributed Mobile Metadata (HDMM) architectural style for a distributed network of registries and directories.
* Complete implementation of a reference model with XML Schemas for the interoperable communication interface standards and with RESTful web services for the transport protocol.
* Build open source software clients and servers for multiple platforms, operating systems and programming languages according to the detailed roadmap (see Sec. 6.5) for continuing development of the previously published designs and prototypes.  
The PORTAL-DOORS Project for development of the PORTAL-DOORS System thus serves to build the necessary foundation and core infrastructure for an information-seeking support system [51] upon which higher-level applications can be constructed.