## 4 Hierarchically Distributed Mobile Metadata (HDMM) as an Architectural Style
Users of today's web browsers may not be familiar with the engineering of the hidden infrastructure system that enables them to navigate to any web site around the world. But it is the IRIS-DNS infrastructure system, which is responsible for registering domain names and mapping them to numerical IP addresses, that makes it possible for the user to browse the web in such an effortless manner almost always without ever typing, seeing, or even being aware of the existence of the numerical IP addresses. Moreover, from the user's perspective, what is most important now is that the speed of this conversion from domain name to IP address occurs so rapidly that the user does not experience it as a hindrance or delay in browsing. Even if the particular web page itself downloads and displays slowly, usually at least the web site address is found quickly. And that happens because the small amount of metadata (domain name and IP address) moves so quickly across the internet even if the larger amount of data (web page text and media) does not. Because of this important point, the phrase _Hierarchically Distributed Mobile Metadata_ and acronym _HDMM_ was introduced (9 May 2009 at www.portaldoors.org) as a name for this architectural style that characterizes both IRIS-DNS and PORTAL-DOORS. The term _mobile metadata_ emphasizes the principle that the metadata moves throughout the distributed network of nodes which may include both stationary nodes such as wired rackmount servers and mobile nodes such as wireless handheld devices. When considering the latter case, the movement of the _mobile metadata_ and the movement of the _mobile node_ must be understood as different kinds of mobility. More generally, _mobile metadata_ must be distinguished from _mobile software_ and from _mobile systems_. Further, in the acronym HDMM, the MM serves as a mnemonic not only for _Mobile Metadata_ but also for _Metadata about Metadata_ (see Section 5.2), while the D recalls not only _Distributed_ referring to location but also _Dynamic_ referring to content. In other words, the metadata may both move to distributed and redistributed locations throughout the network, and also change frequently or intermittently with dynamically updated content. These HDMM principles do require hierarchical control and distribution of metadata records, but do not require hierarchical identification of resources. Whereas IRIS-DNS does employ a hierarchical identification scheme with top-level domain names, domain names, and sub-domain names, PORTAL-DOORS does not require any such hierarchical naming scheme. In fact, PORTAL-DOORS allows complete freedom with an identification scheme for which globally unique labels are simply required to be URIs. These URIs may or may not be hierarchical, and they may or may not be resolvable URLs, as long as they are URIs. However, both IRIS-DNS and PORTAL-DOORS systems do employ hierarchical control and distribution of metadata records. Thus, for the purposes of defining an architectural style applicable to both IRIS-DNS and PORTAL-DOORS, the interpretation and use of the term _hierarchical_ pertains to the distribution of metadata records but not to the identification of resources (see Table 1). Whereas IRIS-DNS implements the HDMM architectural style for the original web, PORTAL-DOORS extends and implements this style for the semantic web and grid. Table 1 summarizes some of the similarities and differences between PORTAL-DOORS and IRIS-DNS from the perspective of considering both as distributed online database systems with entity registering and attribute publishing implemented with the HDMM architectural style (see HDMM Principles 1-5).