[oslc-core] Meeting notes and using rdf:RDF as root element

Samit Mehta samit.mehta at us.ibm.com
Fri Jul 9 10:17:03 EDT 2010


Arthur wrote:
> I say we are not really making life easy for XML people because in the 
XML 
> community, people are used to getting some machine-processible 
description 
> of the format which allows them to validate the document and possibly 
even 
> generate a parser. The common description formats are XSD, DTD, Relax 
NG, 
> Schematron, etc. In our case, we provide no such description. We only 
give 
> a verbal description of how to generate "valid" representations. What 
can 
> an XML developer actually rely on in the OSLC subset? How can an XML 
> developer design XSLT or XPath so that it won't break? In the absence of 
a 
> more formal description of the OSLC subset, I think this type of 
> processing would be fragile.

I completely agree with Arthur's statement that without providing some 
industry standard way to provide "machine-processible description" of the 
XML format, we've made it really difficult for service consumers and 
providers that want to generate/parse (and validate) the XML.  Each 
provider/consumer will have to develop a set of language bindings for the 
XML.  Frankly, this was not as much of an issue with the first release of 
the OSLC CM specification, since the specification and the scenarios were 
very simple.  With the new Core Specification and OSLC CM v2, the 
scenarios can be far more complex and the amount of XML that needs to be 
generated and parsed can be significant.

However, I am not sure that the following proposed solution from Arthur 
solves this issue for those generating/parsing XML.

> I suggest that the "correct" way to serve each community is to:
> 1. not alter the meaning of application/rdf+xml, and 
> 2. allow domains to define "real" XML formats and use application/xml 
for 
> them. We could regard the OSLC RDF/XML subset as a "default" 
> application/xml representation. 

It seems that we would only be making it simpler for those 
consuming/producing RDF.

In addition to what Arthur has proposed, can we also not consider 
providing something like XSD, so consumers/producers can use industry 
standard tools to generate a parser or simply validate the XML?

____________________________________________
Samit Mehta
(512) 323-9350 - Work
mailto:samit.mehta at us.ibm.com
IBM Rational Software - Business Development


oslc-core-bounces at open-services.net wrote on 07/09/2010 08:15:46 AM:

> Jim,
> 
> I agree that we should make life easy for people who like XML and are 
> comfortable with it, just as we should make life easy for people who 
like 
> JSON and are comfortable with it. By the same token, we should make life 

> easy for people who like RDF and are comfortable with it. However, the 
> current approach fails to make life easy for XML folks, and at the same 
> time makes life hard for RDF folks. By altering the meaning of 
> application/rdf+xml, we make life hard for both the XML and RDF 
> communities. 
> 
> I say we are not really making life easy for XML people because in the 
XML 
> community, people are used to getting some machine-processible 
description 
> of the format which allows them to validate the document and possibly 
even 
> generate a parser. The common description formats are XSD, DTD, Relax 
NG, 
> Schematron, etc. In our case, we provide no such description. We only 
give 
> a verbal description of how to generate "valid" representations. What 
can 
> an XML developer actually rely on in the OSLC subset? How can an XML 
> developer design XSLT or XPath so that it won't break? In the absence of 
a 
> more formal description of the OSLC subset, I think this type of 
> processing would be fragile.
> 
> Similarly, we are not making life easy for RDF people because now they 
> have to generate RDF/XML that conforms to the OSLC subset, or else risk 
> breaking consumers. There are many equivalent ways to represent the same 

> set of triples. OSLC RDF/XML makes some of those ways invalid. Given an 
> RDF graph as input, there is no guarantee that a toolkit will generate 
an 
> RDF/XML representation of it that falls within the OSLC subset.
> 
> I suggest that the "correct" way to serve each community is to:
> 1. not alter the meaning of application/rdf+xml, and 
> 2. allow domains to define "real" XML formats and use application/xml 
for 
> them. We could regard the OSLC RDF/XML subset as a "default" 
> application/xml representation. 
> 
> We can handle this situation satisfactorily through standard HTTP 
content 
> negotiation. Let's confine the discussion to the XML-based 
> representations. Here are the principles:
> 
> 1. Use application/rdf+xml for content that conforms to the W3C RDF/XML 
> standard, without any restrictions
> 2. Use application/xml for content that is well-formed W3C XML. A 
special 
> case of this is an XML document that starts with <rdf:RDF> and conforms 
to 
> the OSLC RDF/XML subset.
> 3. If a consumer (client or service) cannot process W3C RDF/XML, then it 

> MUST NOT use application/rdf+xml in its HTTP Accept header.
> 4. If a provider (client or service) cannot generate OSLC RDF/XML, then 
it 
> MUST NOT return application/xml content.
> 
> Here are the cases:
> 1. The client can process incoming W3C RDF/XML. This is the maximally 
> interoperable case since OSLC RDF/XML is a subset of W3C RDF/XML. The 
> client sends
> 
>         Accept: application/rdf+xml
> 
> The server can return either OSLC RDF/XML or W3C RDF/XML and gives it 
> 
>         Content-type: application/rdf+xml.
> 
> 2. The client can only process incoming OSLC RDF/XML. The client sends 
> 
>         Accept: application/xml
> 
> If the server can generate OSLC RDF/XML then it returns it
> 
>         Content-type: application/xml 
> 
> Otherwise the server responds with 
> 
>         406 Not Acceptable.
> 
> --------
> 
> Instead of using application/xml as sugested above, another way to 
handle 
> this is to use the quality indicator on the Accept header. The quality 
> indicator (from 0 to 1) says how well the client can process the media 
> type. Since OSLC RDF/XML is valid W3C RDF/XML, we could assign a quality 

> level to it, e.g. 0.5.  The way to indicate that you can process only 
the 
> OSLC subset of RDF/XML would be:
> 
>         Accept: application/rdf+xml; q=0.5
> 
> The OSLC subset would then use
> 
>         Content-type: application/rdf+xml
> 
> Regards, 
> 
___________________________________________________________________________ 

> 
> Arthur Ryman, PhD, DE
> 
> 
> Chief Architect, Project and Portfolio Management
> 
> IBM Software, Rational
> 
> Markham, ON, Canada | Office: 905-413-3077, Cell: 416-939-5063
> Twitter | Facebook | YouTube
> 
> 
> 
> 
> 
> 
> 
> From:
> Jim des Rivieres/Ottawa/IBM at IBMCA
> To:
> oslc-core at open-services.net
> Date:
> 07/08/2010 05:34 PM
> Subject:
> Re: [oslc-core] Meeting notes and using rdf:RDF as root element
> Sent by:
> oslc-core-bounces at open-services.net
> 
> 
> 
> Arthur, 
> 
> > The OSLC subset and generation rules result in RDF/XML documents that 
> are a subset of all possible valid RDF/XML documents. 
> 
> > If we supported full RDF/XML I wouldn't need to spend time on the 
> syntactic details.
> 
> This is quite deliberate.  If OSLC swallows RDF whole, we end up in a 
> position where everyone consuming and providing OSLC domain specs will 
> only be able to do so using full-fledged RDF/XML parsers. No consumer 
> would ever be able to parse a resource with a regular XML parser, or use 

> simple XML tools like xpath to extract a couple of values of interest.
> 
> This is explained in 
> http://open-services.
> net/bin/view/Main/OSLCCoreSpecDRAFT#OSLC_Defined_Resource_Representa 
> 
>  Here's the relevant passage:
> 
> RDF/XML defines an extensive set of XML elements and attributes for 
> representing an RDF data model. RDF/XML provides a lot of flexibility 
and 
> if we allowed each OSLC workgroup to decide now to serialize OSLC 
> resources to and from RDF/XML, we would require each workgroup to master 

> RDF-XML, we would end-up with different serializations for each domain, 
> the XML produced would not be XML-tool friendly and in the end 
> interoperability would suffer. 
> 
> To ensure that the RDF/XML produced by OSLC services is uniform, easy to 

> understand and as simple as possible, we define a set of step-by-step 
> rules for generating the RDF/XML. We use a very limited set of RDF 
> elements and attributes, the rdf:type element and attributes rdf:about, 
> rdf:resource= and =rdf:nodeID.
> 
> I, for one, think this was the right direction for OSLC Core to go.
> 
> Regards,
> 
> Jim des Rivieres
> Rational AMC Technical Lead
> 
> ----- Forwarded by Jim des Rivieres/Ottawa/IBM on 07/08/2010 04:17 PM 
> -----
> 
> From:
> Arthur Ryman/Toronto/IBM at IBMCA
> To:
> Steve K Speicher <sspeiche at us.ibm.com>
> Cc:
> oslc-core <oslc-core at open-services.net>, 
> oslc-core-bounces at open-services.net
> Date:
> 07/08/2010 03:46 PM
> Subject:
> Re: [oslc-core] Meeting notes and using rdf:RDF as root element
> Sent by:
> oslc-core-bounces at open-services.net
> 
> 
> 
> Steve,
> 
> I am not referring to the use of <rdf:RDF> element since that is a part 
of 
> 
> 
> RDF/XML. I am referring to the exclusion of those features of RDF/XML 
that 
> 
> 
> are not part of the OSLC subset. The OSLC subset and generation rules 
> result in RDF/XML documents that are a subset of all possible valid 
> RDF/XML documents. There is no guarantee that when I serialize an RDF 
> graph using some toolkit that the result will fall within the subset 
> defined by OSLC.
> 
> For example, the document might contain multiple <rdf:Description> 
> elements for the subject nodes instead of "inlining" the triples under 
> some main subject node, or a subject node might not use the expected 
> rdf:type abbreviation if it had multiple types. There are other 
features, 
> such as rdf:parseType="Resource" and rdf:parseType="Collection" that are 

> not in the OSLC subset, but that might get generated. Those are simply 
> abbreviations that produce more compact and readable documents, but that 

> are not in the OSLC subset. A serializer could generate them.
> 
> On a related thought, consider the issue of "enforcing" conformance to 
the 
> 
> 
> OSLC subset.
> 
> Currently, the OSLC subset is described implicitly, i.e. as the result 
of 
> applying the representation rules. This means there is no programmatic 
way 
> 
> 
> to check conformance of an RDF/XML document with the OSLC rules. 
However, 
> I don't think it would be a good use of our time to create an OSLC 
> validator. We don't want to enshrine this subset since it's very likely 
to 
> 
> 
> change (and probably coincide with RDF/XML eventually).
> 
> Here's a real-world example. Today I reviewed a design for calendar 
> events, based on the RDF representation of the iCal standard. Here's a 
> sample RDF/XML representation:
> 
> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>         xmlns:jc="http://jazz.net/xmlns/prod/jazz/calendar#" xmlns="
> http://www.w3.org/2002/12/cal/icaltzd#"
>         xmlns:foaf="http://xmlns.com/foaf/0.1/">
>         <VCalendar>
>                 <jc:calendar_owner rdf:parseType="Resource">
>                         <foaf:mbox rdf:resource="mailto:user at local.net" 
/>
>                         <foaf:nick>user</foaf:nick>
>                 </jc:calendar_owner>
>                 <component>
>                         <Vevent>
>                                 <jc:ownerResource 
rdf:parseType="Resource"
> >
>                                         <foaf:nick>user</foaf:nick>
>                                 </jc:ownerResource>
>                                 <dtstart rdf:datatype="
> http://www.w3.org/2002/12/cal/icaltzd#dateTime">2010-01-01T09:00:00Z</
> dtstart>
>                                 <dtend rdf:datatype="
> http://www.w3.org/2002/12/cal/icaltzd#dateTime">2010-03-31T18:00:00Z</
> dtend>
>                                 <transp>TRANSPARENT</transp>
>                                 <rrule rdf:parseType="Resource">
>                                         <freq>WEEKLY</freq>
>                                         <byday>MO,TU,WE,TH,FR</byday>
>                                 </rrule>
>                         </Vevent>
>                 </component>
>         </VCalendar>
> </rdf:RDF>
> 
> This is valid RDF/XML. It uses standards like iCal and FOAF. However, it 

> is invalid wrt to OSLC subset. Note the use of rdf:parseType="Resource". 

> Also note the use of the iCal dateTime datatype, which is not on the 
> approved list of datatypes.  I don't think it's a good use of anyone's 
> time to try to hammer this into a shape that matches the OSCL subset. 
I'd 
> rather just focus on the data and interface. If we supported full 
RDF/XML 
> I wouldn't need to spend time on the syntactic details.
> 
> Regards, 
> 
___________________________________________________________________________ 

> 
> 
> 
> Arthur Ryman, PhD, DE
> 
> 
> Chief Architect, Project and Portfolio Management
> 
> IBM Software, Rational
> 
> Markham, ON, Canada | Office: 905-413-3077, Cell: 416-939-5063
> Twitter | Facebook | YouTube
> 
> 
> 
> 
> 
> 
> 
> From:
> Steve K Speicher <sspeiche at us.ibm.com>
> To:
> Dave <snoopdave at gmail.com>
> Cc:
> oslc-core <oslc-core at open-services.net>
> Date:
> 07/08/2010 02:39 PM
> Subject:
> Re: [oslc-core] Meeting notes and using rdf:RDF as root element
> Sent by:
> oslc-core-bounces at open-services.net
> 
> 
> 
> > > Furthermore, when I try to generate RDF using
> > > the toolkit, it will not conform to the OSLC subset so I'll have to 
> write
> > > my own serializer. We are therefore in the paradoxical situation of
> > > embracing RDF as our data model yet making life more difficult for
> > > implementers that want to use RDF toolkits.
> > 
> > This could be a real issue and probably warrants some testing with
> > Jena and other RDF serializers. Can anybody comment in this issue?
> > 
> 
> I inquired on this to a team that I know has been using RDF/XML (Jena) 
for 
> 
> 
> 
> some time, they said they didn't have this issue.  In fact, they had to 
do 
> 
> 
> 
> some unnatural acts to remove <rdf:RDF> root element, so adding that 
back 
> has made things much simpler.
> 
> - Steve
> 
> 
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