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Historical - Re: Proposed IETF/W3C task force: "Resource meaning" Review of new HTTPbis text for 303 See Other

From: Tim Berners-Lee <timbl@w3.org>
Date: Sat, 1 Aug 2009 22:14:14 -0400
Cc: "Roy T. Fielding" <fielding@gbiv.com>, Larry Masinter <masinter@adobe.com>, Julian Reschke <julian.reschke@gmx.de>, Mark Nottingham <mnot@mnot.net>, W3C TAG <www-tag@w3.org>
Message-Id: <8618F212-C191-4CCC-9F27-6BF7829622FE@w3.org>
To: Pat Hayes <phayes@ihmc.us>

On 2009-07 -20, at 16:27, Pat Hayes wrote:
> [...]

> . But this thread started because HTTPbis explicitly disagrees with  
> RFC 3986 on what a resource is. Surely these various documents  
> should at least agree on their uses of the basic technical  
> terminology.

I agree.

Historically, URIs were used to point to thinks like web pages and  
files and movies, on the web, useful documents, or "online resources"  
in the sense of useful things out there. FTP. Gopher and HTTP sites  
served up various types of online resources.  People got used to http://example.com/ 
  being a web page and http://example.com/#contact being an anchor  
within it.

The Online Information community, into whose domain the web stuff was  
put for standardization at the IETF, referred to these things like web  
pages as resources, and changed the original "D" for "Document"  in  
"UDI" to "R".
Some felt that resource was more appropriate term, maybe because  
"document" wasn't wide enough to include things like movies.

Now the URI spec actually allowed URIs for completely different  
things, such as telephone end points, and wisely the URI spec does not  
make any arbitrary constraint on what a resource should be, especially  
a resource denoted by a URI in a new scheme to be invented.

Meanwhile, the HTTP spec was polished and elaborated basically as a  
document delivery system, plus other methods for updating documents,  
plus POST.  (POST started historically as a way of introducing a new  
web page  y posting it to a list, just as in NNTP.  It then almost  
immediately got used as a catch-all extension method. I will ignore it  
in this overview).

There was no real definition of what a resource or document was --  
maybe because it seemed obvious. The HTTP spec did not even specify  
whether the URI denoted a person or a document about them, it just  
explained that the thing returned representation of the resource.

Roy's REST work then came along to formalize HTTP as REST and declared  
that a resource was a time-varying mapping between URI and  
representation. That was good enough for HTTP. It didn't have enough  
for the AWWW, when it came along, to be able to describe how the web  
worked.

In fact, the AWWW document, to explain how to use the web properly,  
had to add in a bunch of stuff about the social expectations -- things  
like, yes, the mapping from URI to representation is a function of  
time, but not just any old one -- a random function is not typically  
very useful. There are expectations about it can change with time.   
Persistence, consistency, with various common patterns which allow the  
web to be a useful medium.  The AWWW decided to use the term  
"Information Resource" for a thing like a web page which contains  
information, and "Resource" for any old thing at all.

So HTTP and the REST work of was done very much in this space of  
document delivery, editing and update.  There was no philosophical  
need to talk about what he URI denoted (the person, the web page about  
the person) until RDF came along, when there was an immediate need.

When RDF was first developed, it was motivated by the need for data  
about resources very much in the online information sense: data about  
documents, or 'metadata'.  In fact it was designed to be able to  
describe anything, but many early users of RDF referred to it as  
metadata technology.  RDF used the word "resource" rather awkwardly in  
fact as it turned out.  In the beginning, many of the things being  
described were documents, and so the online information meaning of  
resource made sense. But in fact in RDF the resource was allowed to be  
anything at all. A class, rdf:Resource even used the term as the  
universal class of all things.  A little later, the Web Ontology  
Language decided to use Thing for that.

RDF came along in what I think was a neat way.  It used completely  
existing web protocol extension devices to introduce a new system  
which was fundamentally different from the old HTTP+HTML one.  The  
HTML web was a hypertext model, which pages and anchors. The RDF model  
was a knowledge representation one of arbitrary things.  It did this  
by using the fact that a new language can define whatever it likes as  
what a local identifier denotes.  A graphic language might use local  
identifier to denote lines and points. HTML used local identifiers to  
identify hypertext anchors.  RDF used them to identify arbitrary  
concepts, people, whatever.

The web architecture gave all these languages a common way of building  
a global identifier for the thing denoted by a local identifier in a  
given document.   The semantics of the hash sign are defined web-wide  
to mean that "a#b" can be used to denote whatever is denoted by "b" in  
the document denoted by "a".

Worked a treat.  At the beginning of the century, people played around  
and gave all kinds of things URIs like "http://example.com/ 
foo.rdf#color".  Some of us did lots of work and made all kinds of  
systems which exchanged and integrated data in this way.

Two snags occurred, as the years passed.  One was that a bunch of RDF  
users got the fact that it was good to use HTTP URIs, but didn't get  
the fact that you should put the foo.rdf online so that people can  
look up what #color means in it.  And as they didn't do that, they  
didn't actually bother with the "#" at all.  The second fly in the  
ointment was that some people wanting to use RDF for large systems  
found that they didn't want to use the "#". This was sometimes because  
the number of things defined in the same file was too low (like 1) or  
too large (like a million) and it was difficult to divide up the  
information into middle-sized chunks. Or they just didn't like the "#"  
because it looks weird. But for one reason or another people demanded  
the right to be able to use http://example.net/people/Pat to denote  
Pat rather than a web page about Pat.

This potentially led to huge failures in the whole RDF world, with  
systems already built which just used   "http://example.net/people/ 
Pat" to identify the document whether you like it or not.
I among others pushed back against using non-hash URIs for arbitrary  
things his but eventually gave in.

So in response to this, the HTTP protocol was, in fact, changed.

The spec wasn't changed.  The spec editors were not brought on board  
to the new model.  The spec was interpreted.  The TAG negotiated in a  
way a truce between the existing HTTP spec, RDF systems, and people  
who wanted to use HTTP URIs without "#" to identify people.  That  
truce was HTTPRange-14, which said that yoiu don't a priory know that  
a hashless HTTP URI denoted a document, but if the server responded  
with a 200 then you did, and you had a representation of the  
document.   If you did a get on one of these new URIs which identified  
things were not documents (people, RDF properties, classes, etc) them  
the server must not return 200, it can return 303 pointing to a  
document which explains more.

So the HTTP protocol was, effectively,  changed.  The HTTP protocol as  
extended now allows HTTP to be used not only for Documents but for  
arbitrary Things.  It extends the set of things which you can ask a  
web server about from documents to anything.  It isn't a very bad  
design, nor very beautiful.  Other designs would have worked, but that  
one was the only one which didn't have major problems for some  
community.  It could be extended, but basically it works. It would be  
very expensive to reverse it in terms of systems which have been  
deployed.

It is also very expensive to go on debating it as though it is an open  
issue. It is reasonable to try to make the documents more consistent.

Anyway, that is a simplified version of the history of all this as I  
saw it.

I would like to see what the documents all look like if edited to use  
the words Document and Thing, and eliminate Resource. That's my best  
bet as to two english words which mean as close as we can get to what  
we want. Note however that the web is a new system, a design in which  
new concepts are created, so we can't expect english words to exist to  
capture exactly the concepts. So we take those nearby and abuse them  
as little as we can as far as we can tell at the time, and then write  
them in initial caps to recognize that that is what we have done.

Tim
Received on Sunday, 2 August 2009 02:14:56 GMT

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