[oslc-core] OSLC Compact representation, titles with markup
Arthur Ryman
ryman at ca.ibm.com
Thu Aug 25 16:32:02 EDT 2011
Dave,
1. XML Namespaces.
RDF/XML is well-formed XML so it must support namespaces correctly. For
triples whose datatype is XML Literal, the value of this literal is a
well-formed XML fragment, and therefore the namespaces should be present
in the content. If there is an enclosing <span> element, then the
namespace should be there. Otherwise, each element in the content should
have the namespace.
The spec doesn't say "for XHTML, you need to insert an xmlns attribute for
http://www.w3.org/1999/xhtml" because that is part of the XHTML standard,
i.e. it's not XHTML unless the elements are in the XHTML namespace.
2. Jena
I loaded the sample RDF/XML into Fuseki which uses Jena and it produced
the correct result. I assume the Jena API lets you get an XML DOM from the
literal value.
The input contained: <dcterms:title rdf:parseType="Literal" xmlns="
http://www.w3.org/1999/xhtml"> 12345: <s>Null pointer exception during
startup</s> </dcterms:title>
The output value is: " 12345: <s xmlns="http://www.w3.org/1999/xhtml
">Null pointer exception during startup</s> "^^<
http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral>
3. XHTML versus HTML
The primary reason is that RDF supports XHTML via the XMLLiteral datatype.
There is no parsing support for HTML built into RDF.
Another strong reason is that the syntax of HTML is very irregular and
hard to parse correctly - that is one of the reasons XML was invented.
This is very important from a security viewpoint. To guard against script
injection attacks, you really should parse the input and remove any
<script> elements or Javascript attributes. Doing that correctly for HTML
requires a full HTML parser. On the other hand, the XHTML is given to you
as a DOM which you can easily traverse or process using XSLT or XPATH.
4. Datatypes
The specs do specify the datatypes for some properties. Look at the
Value-Type column of the tables, e.g. [1]. You need to include the
datatype explicitly for ints, dates, XML. etc. You specify that using
rdf:datatype in RDF/XML, or using ^^ in Turtle.
I don't know what the state of adoption is. We really should get some test
suites written for the specs.
5. Inventing new Datatypes
The RDF spec defines the XSD datatypes and the XMLLiteral datatype. RDF
parsers know how to parse those. If someone introduces a new datatype URI,
it could break parsers since they won't know how to parse the contents.
There is no standard way to define new datatypes.
Try it with the RDF Validation service [2]
[1] http://open-services.net/bin/view/Main/OSLCCoreSpecAppendixA
[2] http://www.w3.org/RDF/Validator/
Regards,
___________________________________________________________________________
Arthur Ryman
DE, PPM & Reporting Chief Architect
IBM Software, Rational
Toronto Lab | +1-905-413-3077 (office) | +1-416-939-5063 (mobile)
From:
Dave Steinberg/Toronto/IBM at IBMCA
To:
oslc-core at open-services.net
Date:
08/24/2011 03:05 PM
Subject:
Re: [oslc-core] OSLC Compact representation, titles with markup
Sent by:
oslc-core-bounces at open-services.net
Hi Arthur,
Thanks for the response. Apologies for being slow in replying; I've been
out sick for the last day and a half.
I agree that putting the XML namespace on the enclosing element would be a
convenience, but only if tools supported that. As far as I could find,
Jena provides no fine-grained access to namespace declarations (i.e. other
than at the model level), so I believe that one couldn't use it to produce
or consume the fragment that you suggested. Moreover, the other RDF
representations offer no such convenience, even in theory.
So, it seems to me that the suggestion to use a namespace was actually a
pretty significant one, and not one that's reflected in the specs, since
you'd always need an enclosing element for your XML content.
Thanks for the suggestion of using Tidy to convert from HTML to XHTML.
That was very helpful for me. But I must admit, I'm still left wondering
what makes XHTML superior to HTML for interchanging formatted text,
especially in light of the compact representation example and my own
experiences, where the opposite seems to be true.
One last thing that I'll emphasize is that I mentioned a lack of guidance
in the OSLC specs specifically about plain vs. typed literals. It seems so
odd to me that plain literals seem to be favoured everywhere, except when
in comes to using XMLLiteral with rdf:parseType="literal", but none of
this is acknowledged or explained anywhere. It looks like using a typed
literal in this one case is accepted merely as a requirement to benefit
from the prettier RDF/XML syntax for XML content. However, I view things
completely in the opposite light. To me, typed literals are a powerful
benefit of RDF. You can use a typed literal to decide how to handle a
literal value, without looking at the value itself, but that advantage is
lost without a sufficiently specific type. Thus, I don't understand how
defining and using a new RDF datatype to identify something as widely
recognized and understood as HTML would impair interoperability. I think
it would do the opposite.
Cheers,
Dave
--
Dave Steinberg
IBM Rational Software
davidms at ca.ibm.com
Arthur Ryman---08/23/2011 10:09:55 AM---Dave, Thx for the comments.
From:
Arthur Ryman/Toronto/IBM
To:
Dave Steinberg/Toronto/IBM at IBMCA
Cc:
oslc-core at open-services.net, oslc-core-bounces at open-services.net
Date:
08/23/2011 10:09 AM
Subject:
Re: [oslc-core] OSLC Compact representation, titles with markup
Dave,
Thx for the comments.
I agree that the guidance on using XMLLiteral is not very clear in the
spec. There was a lot of discussion about this at the time the spec was
under development, but not much of that discussion survived the editorial
process. The only place I see it is in the appendix on standard properties
- dcterms:title and dcterms:description. [1]
The guidance was that dcterms:title should be valid XHTML <span> content
and dcterms:description valid XHTML <div> content. This means that the RDF
datatype should be XMLLiteral and that appropriate namespaces should be
used for XHTML content.
Putting the XHTML namespace on the enclosing element is a convenience. The
parser should propagate that to the content, i.,e. when you look at the
triples, the XML literal node should have the inherited namespace.
If you wanted the namespace directly in the content then you could enclose
the content in a <div> or <span> and put the namespace there.
Using XHTML is the best way to achieve interchange of formatted text.
There are converter from HTML to XHTML, e.g. Tidy. However, in the case of
preview, why would conversion be needed? Shouldn't we be defining content
that is XHTML?
In another use case, people wanted to use native Wiki text as the content.
However, that would cause a big interop problem since there are many Wiki
syntaxes. All of these are convertible to XHTML since that is what the
Wikis do to display the formatted result. In another use case, people
wanted to include Rich Text.
The general theme is that developers want to use whatever native format
their tool supports, e,g, HTML, wiki text, and Rich Text, since it avoids
conversions. However, this would couple the resource to the tool. OSLC is
trying to achieve interoperability among heterogeneous tools. Therefore a
common rich text format is needed.
The alternative of defining new RDF datatypes for HTML, wiki text, RTF
etc. would mean that OSLC resources would not be understood by other
applications. In general, the creation of new RDF datatypes is discouraged
since it impairs interoperability.
[1]
http://open-services.net/bin/view/Main/OSLCCoreSpecAppendixA?sortcol=table;up=#Dublin_Core_Properties
Regards,
___________________________________________________________________________
Arthur Ryman
DE, PPM Chief Architect
IBM Software, Rational
Toronto Lab | +1-905-413-3077
Twitter | Facebook | YouTube
Dave Steinberg---08/23/2011 12:06:32 AM---Hi all, I've been following this
thread with interest, as it touches on some of the
From:
Dave Steinberg/Toronto/IBM at IBMCA
To:
oslc-core at open-services.net
Date:
08/23/2011 12:06 AM
Subject:
Re: [oslc-core] OSLC Compact representation, titles with markup
Sent by:
oslc-core-bounces at open-services.net
Hi all,
I've been following this thread with interest, as it touches on some of
the more general confusion/discomfort I've been developing over the past
several weeks or months about the use of XMLLiteral with
rdf:parseType="Literal" for HTML content.
Adam's comments below are particularly interesting. In general, it's not
clear to me who benefits from the use of the unescaped literal
representation, or in what scenario. And that approach, then, requires the
use of the XMLLiteral type, which I also wonder about (as I'll explain
further). If there is some benefit that I don't know about, perhaps it
derails this whole line of thought. But if there isn't, could this be a
case of the concrete representation tail wagging the abstract syntax dog?
One thing that always struck me as odd was that rdf:parseType="Literal"
examples were the only ones I could find anywhere in OSLC that use typed
literals (the XMLLiteral type is implicit with this special RDF/XML
syntax). Moreover, I couldn't find any guidance in the specs about the use
of plain vs. typed literals at all. From the perspective of a client,
anyway, it would seem a very nice thing if a particular provider would use
a typed literal to tell you that a title, for example, should be treated
as a simple string or as HTML content. And that's the very thing that
typed literals do. It could be that the presence of an XMLLiteral type is
supposed to signal the use of XHTML content, and the absence of any type
is supposed to signal plain text. But I couldn't find that spelled out
anywhere -- if it is, perhaps it's hard to find, or perhaps I just did a
poor job of looking -- and I'd argue it would be better to include types
in both cases. [1]
It's this line of thinking that leads me to question the use of XMLLiteral
in the first place. I saw in some old discussions that the intention in
OSLC was not for XMLLiteral to imply XHTML necessarily. Using it for other
XML languages was considered and endorsed, in principle. But where does
that leave XHTML? With a type that doesn't really say what it is or what
you can do with it. We have specs that communicate the XHMTL intent in
words, but we also have a mechanism built into RDF that could communicate
this, and we're not really using it fully. Thus, I think it would be
preferable to define and use a type that specifically represents HTML. And
note, I suggest HTML, not XHTML, since using any type other than
XMLLiteral eliminates the "benefit" of the special rdf:parseType="Literal"
syntax. And without that, I don't see a particular benefit in the stricter
XHTML syntax.
One other possibility that I've considered, which Arthur suggested
previously, is using a namespace to identify that the XML is XHTML, in
particular, instead of doing it directly in the literal type. And I
believe that, strictly, the XHTML namespace is required for the elements
to be valid XHTML. But I found no hint of this in the spec or any
examples, and certainly RTC doesn't do this (I haven't checked other
providers). Moreover, I believe it's also a worse approach, since there's
no guarantee that your RDF runtime of choice will give you access to
namespaces declared on the property element (I don't believe Jena does),
and detecting a namespace inside the element content would require
actually parsing the value as XML. If all you want to do is pass markup
along for display in a browser, it would be unfortunate to have to
actually parse the content to determine that it's XHTML.
And this is where I close the loop on my thinking, by coming back to how a
consumer might actually want to make use of HTML content. Even outside of
the compact rendering scenario, ultimately it's probably going to get
displayed by a browser, whether as part of a larger Web page or in a
browser-backed widget in a rich client. And for that, HTML is probably
just as good as, if not better than, XHTML. Rather than worrying about
whether the content is well-formed XML, it's probably sufficient to just
give it to the browser and see what it can do with it. I would assert that
"something a browser can render" has been the working definition of HTML
for a good number of years now, while XHTML has largely faded in
importance.
Going the other way, the appeal of HTML really shows. If a provider
natively deals with HTML (without concern for XML well-formedness), it
would be attractive to not have to convert that into XHTML to expose it
via OSLC. Likewise, a consumer may use a rich text control that yields
HTML. Generalized parsing of HTML for conversion to XHTML is non-trivial,
and it seems unfortunate to impose that conversion task onto everyone,
just so that we can use rdf:parseType="Literal" in RDF/XML and avoid
applying normal XML encoding to markup content (of course, some encoding
will likely be required for other RDF syntaxes anyway).
So, those are my thoughts on this (admittedly enlarged) topic. Even if
they all do make perfect sense (and I'm not necessarily claiming they do),
I realize we may be well past the point of being able to act on them.
Still, I thought I'd put them out there and see what others make of them.
Cheers,
Dave
[1] In fact, I think that the consistent use of typed literals in general
would be beneficial. You could even imagine exploiting them as a
compatibility measure, if it was decided that the type of a property
needed to change. This is a related, but separate, topic, which I'd be
thrilled to discuss further, but I don't want to open too many cans of
worms at once.
[2] Or, perhaps, a less kind way of putting that is that the XHTML
namespace is required for the elements to
--
Dave Steinberg
IBM Rational Software
davidms at ca.ibm.com
Adam Archer---08/22/2011 06:20:05 PM---The big concern to me is not the
ability to process the RDF/XML with XPath, it's the ability to do
From:
Adam Archer/Toronto/IBM at IBMCA
To:
Arthur Ryman/Toronto/IBM at IBMCA
Cc:
"oslc-core at open-services.net" <oslc-core at open-services.net>, Randy Hudson
<hudsonr at us.ibm.com>, oslc-core-bounces at open-services.net
Date:
08/22/2011 06:20 PM
Subject:
Re: [oslc-core] OSLC Compact representation, titles with markup
Sent by:
oslc-core-bounces at open-services.net
The big concern to me is not the ability to process the RDF/XML with
XPath, it's the ability to do so in a browser environment. Currently all
implementations of all rich hovers in all Jazz based products encode any
html tags in their dcterms:title attributes (and doubly encode special
characters). For the consumer on the browser side, this means simply
taking the content of the attribute, decoding it (which browsers are very
good at) and slapping the result into the dom (which browsers are also
very good at).
The alternative would be a total consumability nightmare from the point of
view of a browser (which is the most important consumer of this entire
spec). If the tags are actually child nodes in the xml representation, it
means we will have child elements in the resulting document that we get
back from the xml http request which means we will have to traverse a dom
tree and recreate a structure which could easily be represented as an
escaped string, like everyone is doing today.
I realize that implementation is not supposed to lead the spec, but I
don't even think that would be the case here. The oslc compact spec grew
organically out of the old jazz compact rendering spec which can be found
here:
https://jazz.net/wiki/bin/view/Sandbox/CompactRenderingV1P1
If we look at the semantic description of the dc:title and jp:abbreviation
it states explicitly that the content MUST be escaped:
> The HTML markup MUST be escaped; for example, "<b>" as "<b>".
This decision was made consciously for very well defined technical reasons
(discussed above) in the original spec. If that decision was reversed in
the creation of the OSLC compact spec then I believe that to have been a
huge mistake and would like to see the spec fixed rather than for all
providers to have to change how their compact documents are served and all
consumers to have to go to the trouble of walking the dom to determine
what the provider is actually trying to show.
Adam Archer
Jazz Developer
IBM Toronto Lab
From: Arthur Ryman/Toronto/IBM
To: Samuel Padgett <spadgett at us.ibm.com>
Cc: Adam Archer/Toronto/IBM at IBMCA, Randy Hudson <hudsonr at us.ibm.com>,
"oslc-core at open-services.net" <oslc-core at open-services.net>,
oslc-core-bounces at open-services.net
Date: 08/22/2011 04:40 PM
Subject: Re: [oslc-core] OSLC Compact representation, titles with markup
Sam,
You wrote:
It's very difficult to parse the former using XPath. For instance, the
expression "/oslc:Compact/dcterms:title" takes out the "<s>" and "</s>"
I don't think problems using XPath are a valid reason to encode markup
since RDF/XML itselt is very difficult to process using XPath. At one
point we tried to define an OSLC-variant of RDF/XML that looked like
"normal" XML. However, we abandonned that and now require support for
generic RDF/XML.
The are many equivalent ways to represent a given set of triples in
RDF/XML. It would therefore be very problematic to use XPath, XSLT, or
XQuery to process RDF/XML. The safe way to process RDF/XML is to use an
RDF toolkit like Jena.
Regards,
___________________________________________________________________________
Arthur Ryman
DE, PPM Chief Architect
IBM Software, Rational
Toronto Lab | +1-905-413-3077
Twitter | Facebook | YouTube
From:
Samuel Padgett <spadgett at us.ibm.com>
To:
"oslc-core at open-services.net" <oslc-core at open-services.net>
Cc:
Adam Archer/Toronto/IBM at IBMCA, Randy Hudson <hudsonr at us.ibm.com>
Date:
08/07/2011 01:01 PM
Subject:
[oslc-core] OSLC Compact representation, titles with markup
Sent by:
oslc-core-bounces at open-services.net
I believe the spec is a bit confusing when it comes to titles with markup
for UI Preview.
The Compact representation has a dcterms:title property. It's defined as
an
XML Literal that can contain XHTML markup [1]. My understanding of XML
Literals as discussed in the RDF Primer [2] means a title with markup
would
look like this,
<dcterms:title>12345: <s>Null pointer exception during
startup</s></dcterms:title>
The example [3] of this resource has a title like this, however,
<dcterms:title> 12345: <s>Null pointer exception during
startup</s> </dcterms:title>
The example doesn't seem to fit with the description.
It's very difficult to parse the former using XPath. For instance, the
expression "/oslc:Compact/dcterms:title" takes out the "<s>" and "</s>"
Most implementations I'm aware also follow the example where markup is
encoded. It means special characters need to be "double encoded." For
instance, "12345: Values > 1000 incorrectly calculated" would be,
<dcterms:title>12345: Values > 1000 incorrectly
calculated</dcterms:title>
I think we should add more clarity to the spec here, as getting this wrong
can open up consumers to cross-site scripting attacks. I'd also suggest we
say that providers MUST NOT use any markup with a <script> tag and
consumer
MUST NOT display any markup with a <script> tag to guard against this
problem.
Best Regards,
Sam
[1]
http://open-services.net/bin/view/Main/OslcCoreUiPreview?sortcol=table;up=#Representation_Compact
[2] http://www.w3.org/TR/rdf-syntax/#xmlliterals
[3]
http://open-services.net/bin/view/Main/OslcCoreUiPreview?sortcol=table;up=#XML_Representation_Format
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