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== Technical issues ==
=== Using HTTP URIs to identify abstract resources ===
Using URLs, and singularly HTTP URIs, to identify abstract resources, such as classes, properties or other kind of concepts, is a frequent practice, for example in RDFS or OWL [[Ontology (computer science) | ontologies]]. Since such URIs are associated with the HTTP protocol, the question arised of which kind of representation, if any, should be get for such resources through this protocol, typically using a Web browser, and if the syntax of the URI itself could help to differentiate "abstract" resources from "information" resources. The URI specifications such as RFC 3986 let to the protocol specification the task of defining actions performed on the resources and they don't provide any answer to this question. It had been suggested that http URIs identifying a resource in the original sense, file, document or any kind of so-called information resource, should be "slash" URIs, in other words should not contain fragment identifiers, whereas URIs used to identify concepts or abstract resources should be "hash" URIs using [[fragment identifier | fragment identifiers]]. For example <nowiki>''http://www.sillywidgets.org/catalogue/widgets.html''</nowiki> would both identify and locate a web page (maybe providing some human-readable description of the widgets selled by Silly Widgets, Inc.) whereas <nowiki>''http://www.widgets.org/ontology#Widget''</nowiki> would identify the abstract concept or class "Widget" in this company ontology, and would not necessarily retrieve any physical resource through http protocol. But it has been answered that such a distinction is impossible to enforce in practice, and famous standard vocabularies provide counter-examples widely used. For example the Dublin Core concepts such as "title", "publisher", "creator" are identified by "slash" URIs like ''http://purl.org/dc/elements/1.1/title''.
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