Re: comparing XML and RDF data models

Hi Tim,

I'm not sure that this is where the differences lie.

In my view the key point is that with RDF we have unique identifiers
for concepts--whether that is the things we're talking about, or the
vocabulary we're using to talk about them.

So continuing your example, we could say that something has a
foaf:name, and in RDF we really know what we mean:

  _:a foaf:name "John" .

XML doesn't have an equivalent, and even if you bend things to say
that it has, by saying 'here is a universal element that we all agree
means something specific', like this:

  <person>
    <foaf:name>John</foaf:name>
  </person>

there is actually no easy way to share that 'well-known element'
amongst other XML users. Just try it with XML Schema, for example!
It's a nightmare.

Since we know what foaf:name means, wherever it occurs in RDF, we can
have a flexible document structure and still make sense of it. XML
also has a flexible document structure as people have pointed out, but
the problem is that you have no idea what anything means if it moves
out of its precisely defined position.

(Another way to look at it is that with RDF meaning is defined for a
concept based on its unique identifier, whilst in XML, meaning is
defined based on syntactic position, which makes its flakey.)

My introductory tutorial at SemTech on RDF and RDFS spent quite a bit
of time on why XML is not enough when dealing with knowledge and
semantics, even though many people think it is. The slides are on
Slideshare:

  <http://www.slideshare.net/mark.birbeck/a-first-course-in-rdf-and-rdfs-resource-description-framework-and-resource-description-framework-schema/>

(If you want the speaker notes you'll have to download the
presentation, since Slideshare doesn't show them in the online/web
view.)

Regards,

Mark


On Wed, Jul 2, 2008 at 11:25 AM,  <tim.glover@bt.com> wrote:
>
>
> I think there is a valid observation to be made about XML and RDF or
> relational representations.
>
> Take the simplest atom of information, "There is a Person named John"
>
> In RDF, there is only one representation;
>
> <Person name John>
>
> In RDBMS there is only one representation
>
> Person
> |------|
> | name |
> |======|
> | John |
> |------|
>
> In XML there are several possible representations, eg
>
> <Person name="John"/>
>
> <Person>
>  <name>John</name>
> </Person>
>
> <Person>
>  <name value="John">
> </Person>
>
>
> With more complicated data, the possible XML representations vary in
> different ways, and increase exponentially w.r.t. the number of atoms of
> information.  To extract the data from the XML we have to know the
> detailed representation chosen. Saying we can UNION different queries
> misses the point - we still have to write 3 queries. Saying we can use
> transformations misses the point - we still have to write
> transformations.
>
> The issue here is that XML fails to abstract the data from the
> representation as effectively as RDF and RDBMS. In this sense, RDF and
> RDBMS are better data representations than XML.
>
> Tim.
>
>
>
>
> -----Original Message-----
> From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org]
> On Behalf Of Bijan Parsia
> Sent: 02 July 2008 10:29
> To: Olivier Rossel
> Cc: semantic-web@w3.org
> Subject: Re: comparing XML and RDF data models
>
>
> On 2 Jul 2008, at 08:43, Olivier Rossel wrote:
>
>>> Any XML instance can be considered a compact, early-bound
>>> serialization of an infoset RDF graph.
>>
>> +1.
>> XML is very powerful when it comes to presenting data (because it
>> details how data imbricate with each other). But XML is very unnatural
>
> No, please no. Don't make such claims without backup. What's
> unnatural for you may be very natural to other people. And
> naturalness doesn't matter if *effectiveness* is at issue.
>
>> when it comes to crawling the data in an unexpected and ever-changing
>> manner (because XML tree structure is chosen once for all,
> [snip]
>
> And this is just false. Google for "open content model". Look at XML
> Schema's "lax" and "skip" validation modes. Consider transformations.
> (I.e., many XML people are perfectly comfortable treating the "input
> tree" as just one step, not a fixed one)
>
> RDF structure is similarly fixed in advanced (by and large).
>
> This kind of talk, aside from being wrong, helps marginalize the
> semantic web and related technologies.
>
> In general, if you are inclined to make a general "betterness" claim
> based on some abstract feature, don't. If you are going to anyway, make
> sure you have every detail nailed with concrete, preferably real
> examples ready to hand. Even then, one is better off just presenting the
> goodness without contrast. If it's good enough, people will come.
>
> Cheers,
> Bijan.
>
>
>
> -----Original Message-----
> From: semantic-web-request@w3.org [mailto:semantic-web-request@w3.org]
> On Behalf Of Bijan Parsia
> Sent: 02 July 2008 10:29
> To: Olivier Rossel
> Cc: semantic-web@w3.org
> Subject: Re: comparing XML and RDF data models
>
>
> On 2 Jul 2008, at 08:43, Olivier Rossel wrote:
>
>>> Any XML instance can be considered a compact, early-bound
>>> serialization of an infoset RDF graph.
>>
>> +1.
>> XML is very powerful when it comes to presenting data (because it
>> details how data imbricate with each other). But XML is very unnatural
>
> No, please no. Don't make such claims without backup. What's
> unnatural for you may be very natural to other people. And
> naturalness doesn't matter if *effectiveness* is at issue.
>
>> when it comes to crawling the data in an unexpected and ever-changing
>> manner (because XML tree structure is chosen once for all,
> [snip]
>
> And this is just false. Google for "open content model". Look at XML
> Schema's "lax" and "skip" validation modes. Consider transformations.
> (I.e., many XML people are perfectly comfortable treating the "input
> tree" as just one step, not a fixed one)
>
> RDF structure is similarly fixed in advanced (by and large).
>
> This kind of talk, aside from being wrong, helps marginalize the
> semantic web and related technologies.
>
> In general, if you are inclined to make a general "betterness" claim
> based on some abstract feature, don't. If you are going to anyway, make
> sure you have every detail nailed with concrete, preferably real
> examples ready to hand. Even then, one is better off just presenting the
> goodness without contrast. If it's good enough, people will come.
>
> Cheers,
> Bijan.
>
>
>
>



-- 
Mark Birbeck, webBackplane

mark.birbeck@webBackplane.com

http://webBackplane.com/mark-birbeck

webBackplane is a trading name of Backplane Ltd. (company number
05972288, registered office: 2nd Floor, 69/85 Tabernacle Street,
London, EC2A 4RR)

Received on Wednesday, 2 July 2008 11:20:15 UTC