## 4 Hierarchically Distributed Mobile Metadata (HDMM) as an Architectural Style  
According to Taylor _et al._[8, page 73], \"an architectural style is a named collection of architectural design decisions that (1) are applicable in a... context, (2) constrain architectural design decisions [for] a system within that context, and (3) elicit beneficial qualities in each resulting system.\" Architectural styles are distinguished from architectural patterns in scope, abstraction and relationship. For example, with regard to scope, \"an architectural style applies to a development _context..._ while an architectural pattern applies to a specific design _problem_\" [8, page 73]. Moreover, architectural patterns are larger in scale than design patterns. Thus, in software engineering, there is a increasing progression of scale and scope from design pattern to architectural pattern to architectural style. The REPresentational State Transfer (REST) architectural style [46] serves as an important example of an architectural style for network-based applications on the web. Other styles, such as peer-to-peer, have been named and described for distributed and networked architectures [8]. However, not all styles of distributed and networked architectures have been appropriately characterized and publicized with an identifying name and detailed description of the principles that constitute the essential distinguishing aspects of the style. In particular, the architectural style that characterizes both IRIS-DNS and PORTAL-DOORS has just recently been elaborated with an explicit name and description [45] even though the PORTAL-DOORS System for the semantic web was purposefully designed by the author [1] by analyzing and emulating the architectural principles and paradigm of the IRIS-DNS System for the original web. Here, the current Section 4 presents the formal name and description for the architectural style shared by both IRIS-DNS and PORTAL-DOORS as pervasive registry-directory networks with Hierarchically Distributed Mobile Metadata (HDMM). Then below, the subsequent Sections 5 and 6 further discuss the architectural design and implementation of PORTAL-DOORS within the context of this HDMM architectural style. IRIS registries [47] and DNS directories [48] provide the model for the architectural style that inspired the design of PORTAL registries and DOORS directories [1]. The most essential characteristics of this HDMM architectural style can be summarized by the following principles:  
1. **Distributed infrastructure:** Pervasively distributed and shared infrastructure, content, and control of content including distributed and shared control over both the contribution and distribution of the content defined as the mobile metadata records. 2. **Hierarchical authorities:** A hierarchy of both authoritative and non-authoritative servers (root, primary, secondary, forwarding and caching) enabling global interoperable communication and exchange of the mobile metadata records while permitting independent administrative control of local policies governing the publication and distribution of the metadata records. 3. **Mobile metadata:** A focus on moving the mobile metadata for _who what where_ as fast as possible with pervasive distribution and redistribution from servers in response to requests from clients that access non-authoritative local forwarding and caching servers updated regularly by the authoritative servers. 4. **Separated concerns:** A separation of concerns with registries for identifying resources and directories for locating resources that have been globally uniquely identified in the registries. 5. **Unrestricted identification:** A relative freedom of choice in the selection of identifiers with purposeful absence of any requirement to use the _same_ root name or label for _all_ identifiers, thus enabling essentially unrestricted choice of naming or labeling schemes for identification and thereby avoiding monopolistic control by any single organization.