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Re: URI Fragments

From: Richard Cyganiak <richard@cyganiak.de>
Date: Fri, 12 Mar 2010 19:54:03 +0000
Cc: Linked Data community <public-lod@w3.org>
Message-Id: <9E3891B9-2954-4477-8088-9FAD4B2F6F2E@cyganiak.de>
To: nathan@webr3.org
Hi Nathan,

On 12 Mar 2010, at 16:46, Nathan wrote:
>>> Then if I delete a Primary resource, the secondary resources must  
>>> also
>>> be deleted, true / false (?).
>>
>> The web is about representations of information resources. If you add
>> RDF to the picture, then it's also about descriptions of arbitrary
>> entities.
>>
>> On the web, you can create and delete representations. You can create
>> and delete descriptions. But you cannot create or delete resources.
>
> I'd argue that a resource is anything that can be named (or assigned a
> URI), regardless of whether it has a representation or not. Even  
> without
> a representation a resource could still be reserved (which allows
> references to be made to a concept before any realization of that
> concept exists - although I've yet to confirm if 204 could be used for
> this..);

I would agree with everything above. But I'd say that from the web  
POV, you can't do anything useful with a resource that is reserved but  
doesn't have a representation or provides some other useful response  
(such as 303) when resolved.

> in another use-case though a resource like /news/latest may be
> nothing more than a conceptual map to another resource (served via a  
> 3xx
> code) -

Side note: I interpret 301, 302 and 307 as “try over there to get a  
representation of this resource”. So if you get a representation from  
the target, then I'd consider that a representation of the original  
resource. This interpretation is not backed by any spec or other  
authoritative document, but for me it makes the picture cleaner. 303  
of course is explicit about that the representation is of a  
*different* resource.

> this is a resource with no representation, which can be both
> created and deleted surely?

Well it can be created and deleted, but the web (by which I mean, HTTP  
and URIs) provides no standard way of creating or deleting resources  
that have no representation. You have to use some nonstandard  
mechanism of your own invention for this (which of course can be built  
on standard operations, e.g. POST). Hence my view that the creation or  
deletion of the resource is a side effect of something that you invoke  
that's “outside” of the web.

> In another case; let's say planned to lease a /London_Office  
> (resource)
> which I then described with a representation and 303'd to; then I
> decided not to lease the /London_Office so deleted the representation
> /and/ the resource because /London_Office isn't something that can be
> named because it no longer exists, was never realized, and moreover I
> want it removed because it was a painful loss.
>
> Thus, can you delete resources? or another way, can you delete a
> conceptual map?

I suppose you can delete them, but there is no operation for doing so  
in HTTP. Hence you'll have to devise your own mechanism for this. And  
wether deleting <foo> also deletes <foo#bar> is something that depends  
on how you model your domain concepts as resources, representations  
and descriptions. In general, the HTTP and URI specs don't constrain  
this.

>> So, you can't really “delete” those primary and secondary  
>> resources. But
>> if you delete all the representations of a primary resource, then  
>> this
>> will delete the authoritative descriptions of the secondary  
>> resources,
>> because those live inside the representations.
>
> if I remove the section and the reference test.html#whatever from
> test.html; have I not deleted that secondary resource? it can't be  
> named
> any more, or referenced, or.. and so on

Good point, in this case you are right. For HTML documents,  
<foo.html#whatever>, if defined at all, will be a named element within  
the HTML document (see RFC 2854). It's pretty clear what it means to  
delete a named element in an HTML document (there are even DOM  
operations for doing so), it's pretty safe to say that you are  
deleting the secondary resource.

This is because RFC 2854 constrains the semantics of fragment IDs on  
HTML documents in such a way that it becomes clear what creating and  
deleting them means. The same would apply to any other media type  
where fragments identify parts of the document.

This is not the case for RDF media types, where the semantics of  
fragment IDs is essentially: “They identify whatever the full hash URI  
identifies according to the RDF graph in the document” (too lazy to  
dig out the reference -- the media type registration RFC for  
application/rdf+xml, which points to some section of some RDF spec  
that according to my understanding can be summarised as above).

So perhaps I should say that *in general* you cannot really delete  
resources, unless some spec (especially media type specs) defines the  
semantics of the resources in such a way that you can delete them.

Feel free to challenge some more ;-)

All the best,
Richard


>
>> Best,
>> Richard
>>
>
> Thanks again,
>
> Nathan
>
>>> Here are some examples, which may seem like over kill but some are
>>> interesting and generally I *feel* rules like this should be either
>>> always true, or always false, never varying.
>>>
>>> examples:
>>> if I remove a database table, then all it's rows also no longer  
>>> exist.
>>> if I remove London then the Tower of London also no longer exists.
>>> if somebody removes me, then my arms also no longer exist.
>>> if I remove test.html then test.html#whatever no longer exists.
>>> if I remove test.rdf then test.rdf#this no longer exists
>>> if I remove http://www.w3.org/People/Berners-Lee/card then
>>> http://www.w3.org/People/Berners-Lee/card#i no longer exists.
>>>
>>> conversely:
>>> if I remove a row, the table still exists
>>> if I remove the Tower of London, London still exists
>>> if you remove my arms, I still exists and I'll find another way to  
>>> type.
>>> if I remove test.html#whatever test.html still exists
>>> if I remove test.rdf#this, test.rdf still exists
>>> if I remove http://www.w3.org/People/Berners-Lee/card#i then
>>> http://www.w3.org/People/Berners-Lee/card still exists.
>>>
>>> If the above is true (secondary resource must also be deleted on  
>>> removal
>>> of primary resource), then I should never use a fragment  
>>> Identifier to
>>> refer to a non-virtual object (i.e. "me" a Person) - because I  
>>> can't be
>>> deleted by simply removing a resource. (?)
>>>
>>> Regards!
>>>
>>> Nathan
>>>
>>
>>
>>
>
Received on Friday, 12 March 2010 19:54:38 UTC

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