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255 lines
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<html xmlns="http://www.w3.org/1999/xhtml"
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xmlns:foaf='http://xmlns.com/foaf/0.1/'
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xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
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xmlns:dc='http://purl.org/dc/elements/1.1/'
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xmlns:ical='http://www.w3.org/2002/12/cal/ical#'
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xmlns:vcard='http://www.w3.org/2001/vcard-rdf/3.0#'
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xmlns:talk='http://www.w3.org/2004/08/Presentations.owl#'
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>
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<head>
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<!--
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<rdf:RDF
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xmlns:foaf='http://xmlns.com/foaf/0.1/'
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xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
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xmlns:dc='http://purl.org/dc/elements/1.1/'
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xmlns:ical='http://www.w3.org/2002/12/cal/ical#'
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xmlns:vcard='http://www.w3.org/2001/vcard-rdf/3.0#'
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xmlns:talk='http://www.w3.org/2004/08/Presentations.owl#'
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>
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<rdf:Description rdf:about="">
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<dc:title>Layers of Abstractions: Net, Web, Graph</dc:title>
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<dc:date>2007-10-23</dc:date>
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</rdf:Description>
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<foaf:Person rdf:about="http://www.w3.org/People/Berners-Lee/card#i">
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<foaf:name>Tim Berners-Lee</foaf:name>
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<foaf:homepage rdf:resource="http://www.w3.org/People/Berners-Lee/"/>
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<foaf:made rdf:resource=""/>
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</foaf:Person>
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</rdf:RDF>
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-->
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<title>Abstractions in Web architecture - Design Issues</title>
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<link rel="Stylesheet" href="di.css" type="text/css" />
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<meta http-equiv="Content-Type" content="text/html" />
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</head>
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<body>
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<address>
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<span rel="foaf:maker" resource="http://www.w3.org/People/Berners-Lee/card#i" typeof="foaf:Person">
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<a property="foaf:homepage" href="http://www.w3.org/People/Berners-Lee/">
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<span property="foaf:name">Tim Berners-Lee</span>
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</a>
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</span>
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<br />
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Date: <span property="dc:date">2007-10-23</span>,
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last change: $Date: 2010/06/15 13:54:51 $<br />
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Status: personal view only. Editing status: draft.
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Written in response to another round of circular discussions
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of web architecture.
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</address>
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<p>
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<a href="./">Up to Design Issues</a>
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</p>
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<hr />
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<h1>
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Levels of Abstraction: Net, Web, Graph
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</h1>
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<p style="text-align: center">
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<img width="30%" align="left" src="diagrams/layers/link0.png"
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alt="link level" />
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</p>
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<p>
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Progress in communications technology has ben characterizsed
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by a movement from lower to higher levels of abstraction.
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</p>
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<p>
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When, first, computers were connected by telephone wires,
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then you would have to run a special program to make one
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connect to another. Then you could make the second connect to
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a third, but you had to know how to use the second one. Mail
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and news would be passed around by computters calling each
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other late at night. Email addresses for a while contained a
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list of computers to pass the message through
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(timbl@mcvax!cernvax!cernvms)
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</p>
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<h3>
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It's not the wires -- it's the computers
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</h3>
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<p style="text-align: center">
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<img width="30%" align="right" src="diagrams/layers/net.png"
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alt="net level" />
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</p>
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<p>
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The ability to use this communication power between computers
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wasn't powerfully useful until the Internet. The internet
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allowed one to forget about the individual connections. It
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was thought of as the "Internet Cloud". Messages went in and
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appeared ad another computer, without (when things worked)
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one having to worry about how they were broken into packets,
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and the packets routed from computer to computer. The Domain
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Name System gives computers names, and the TCP and IP
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protocols allows a program on one computer to talk to a
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program on another computer.
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</p>
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<p>
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This made life very much easier. It takes the wires out of
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the picture, and allows the programs to talk as though the
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computers were directly connected.
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</p>
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<p>
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Of course, when things went wrong, one did have to do run
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diagnostic programs to find out whether the connection has
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broken between one's own computer and the WiFi base unit, or
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between that and the router, the cable modem, or somewhere in
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the middle of the Internet, or at the other end. But that was
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the exception.
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</p>
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<h3>
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It's not the computers -- it's the documents
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</h3>
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<p style="text-align: center">
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<img width="30%" align="left" src="diagrams/layers/web.png"
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alt="web level" />
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</p>
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<p>
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This power of communication between computers wasn't really
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easily usable by normal people until the Web came along. The
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realization of the web is: "It's not the computers which are
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interesting, it is the documents!" The WWW protocols (URI,
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HTTP, HTML) defined how documents could be sent between Web
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servers and Web browsers.
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</p>
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<p style="text-align: center">
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<img width="30%" align="right" src="diagrams/layers/web2.png"
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alt="web level" />
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</p>
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<p>
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Now the user is apparently in a web of interconnected
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documents. She does not have to worry about how the protocols
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work underneath, with two exceptions.
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</p>
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<p>
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When things go wrong, she has to be able to figure out
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whether it as a problem with her connection to the internet,
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with the URI in the link she was following, or an error on
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the server end. This involves looking under the hood. But
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that is the exception.
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</p>
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<p>
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There is another reason to be aware of what is happening. The
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information you are browsing comes from a particular server,
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whose name has registered against a particular person or
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organization. The trust you put in that information is
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related to who that organization is. It is the serving
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organization which is responsible for keeping the URIs you
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bookmark today alive tomorrow -- and some are better at it
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than others. Phishing attacks succeed when people are fooled
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into thinking it is an organiztion you trust when it isn't.
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</p>
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<p>
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So the web is just a web of documents, except one one has to
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lift the hood for debugging or questions of trust.
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</p>
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<p>
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Note that the connection between the net of computers and the
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web of documents is clear in the URI:
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</p>
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<pre>
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http://acme.excample.com/products/machin/truc
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_________________
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</pre>
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<p>
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The computer owner name (acme.example.com) is part of the
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name of the document. The Acme Example company is responsible
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for supporting the document on the web.
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</p>
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<h3>
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It's not the documents -- It's the things
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</h3>
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<p style="text-align: center">
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<img width="30%" align="left" src="diagrams/layers/sweb2.png"
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alt="graph level" />
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</p>
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<p>
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The power of the web was still not totally used to its full
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potential until the semantic web came along. The Semantic
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Web's realization is: <em>It is isn't the documents which are
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actually interesting, it is the things they are about!</em>
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</p>
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<p>
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A person who is interested in a web page on something is
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usually primarily interested in the thing rather than the
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document. There are exceptions, of course -- documents are
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certainly interesting in their own right. However, when it
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comes to the business and science, the customers, the
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products, or the proteins and the genes, are the things of
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interest. A good Semantic Web browser, then, shows a user
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information about the thing, which may have been merged from
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many sources. Primarily, the user is aware of the abstract
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web of connections between the things -- this person is a
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customer who made this order which includes this item which
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is manufactured by this facility ... and so on.
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</p>
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<p style="text-align: center">
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<img width="30%" align="right" src=
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"diagrams/layers/graph.png" alt="graph level" />
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</p>
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<p>
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There are again the same two exceptions. When things go
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wrong, the user must be able to look under the hood to see
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whether the document was fetched OK but had missing data, or
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the document was not fetched OK, in which case what was the
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underlying web problem.
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</p>
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<p>
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And again, when the user is looking at a bit of data in the
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data view, perhaps a point on a map or a cell in a table,
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then she must be able to see easily which document that
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information came from.
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</p>
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<p>
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Note that the connection between the web of documents and the
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web of things is clear in the URI:
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</p>
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<pre>
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http://acme.excample.com/products/machin/truc#part3
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_____________________________________________
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</pre>The name of the document is part of the name of the thing. A
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given thing may have many URIs of course. But when URIS have this
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form, it is clear that we are talking about a thing as described by
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a given document. This is a gene as defined in the Gene Ontology.
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This is a protein as defined in this taxonomy. A citizen as defined
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by the Immigration and Naturalization Service glossary. And so on.
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<p>
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(There are (since 2005) URIS for things which are not
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explicitly bound to a document. These require the server to
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respond with the name of a suitable document at runtime. This
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is more complicated)
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</p>
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<h2>
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Conclusion
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</h2>
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<p>
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The web of things is built on the web of documents, which is
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built on the web of computers controlled by Domain Name
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owners, which itself is build on a set of interconnected
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cables. This is an architecture which provides a social
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backing to the names for things. It allows people to find out
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the social aspects of the things they are dealing with, such
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as provenance, trust, persistence, licensing and appropriate
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use as well as the raw data. It allows people to figure out
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what has gone wrong when things don't work, by making the
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responsibility clear.
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</p>
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<p>
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The value of this architecture is that each layer leverages
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the social components of the lower layer's architecture.
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</p>
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<hr />
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<p>
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<a href="Overview.html">Up to Design Issues</a>
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</p>
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<p>
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<a href="../People/Berners-Lee/">Tim BL</a>
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</p>
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</body>
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</html>
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