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Java, RDF, XML ?

From: Sandro Hawke <sandro@w3.org>
Date: Thu, 07 Jul 2005 11:46:20 -0400
To: "Waleed Abdulla" <Waleed_Abdulla@xrules.org>
Cc: "'Anthony Finkelstein'" <anthony@systemwire.com>, public-rule-workshop-discuss@w3.org
Message-Id: <20050707154623.2A6284F015@homer.w3.org>

Waleed Abdulla writes:
> Another dimension to divide rules by is the type of data they work with. I
> see three here:
>     * Rules that work with Java or C# objects (the kind common in some 
>       of the popular rules engines). These can be inference, validation,
>       or execution rules.
>     * Ruels that work with RDF, such as N3 in CWM. These are typically
>       inference rules, but they can also be used for validation (think
>       of validation as inferring weather a set of data is valid by 
>       some criteria). I haven't seen examples of RDF rules used for 
>       execution, but I can't think of a reason why they can't be used
>       to trigger events that an application intercepts to execute tasks.
>     * Rules that work with XML documents. Again, these can be used to 
>       validate XML documents, inference values of nodes from values of
>       other nodes, or trigger execution when certain conditions apply.

This is an interesting issue, which I didn't notice being discussed at
the workshop.  I suspect a lot of people were thinking either "of
course it'll be in [Java/RDF/XML]" (for whichever form they are used
to), or like me they thought "let's not open this can of worms yet!".
But I agree it needs to be addressed.  (I'd probably add SQL data to
the above list, but I'll leave it out for now, for simplicity.)

As with the three types of functionality in the previous mail [1], I
think these data forms mostly map to each other; they present very
different views to the world, but their basic expressive power is the
same, or nearly the same.  That said, Java expressions, SPARQL, and
XPath feel about as different as three languages possibly could!

Let me work through the mapping a bit here, and see if our options
become more clear.

== 1.  Use Programming-Language Objects as Fundamental Data
  It's easy enough to see how to access RDF or XML data through Java
  expressions: look at the relevent APIs.  

  The difficulties here are: 

   (1) Do we use Java, C#, Python, C++, ECMA Script, ... or what?
       Picking one doesn't seem feasible, and supporting more than one
       means we don't have much of a standard. 

   (2) Even if we could pick a programming language, we'd also have to
       pick an API, right?   I don't know the relevant markets well
       enough to know if there are defacto standards or not.

  I guess the strawman here would be Java + Jena + DOM.  That doesn't
  strike me as a suitable basis for a standard, but we can talk about it
  more if anyone really likes it.

== 2.  Use RDF Triples as Fundamental Data

  Data held in programming-language objects can be presented as RDF
  triples.  The mapping is usually quite natural -- a data member of
  an instance is pretty much the same as a value for a property of a
  resource.  Side-effect-free member functions can appear as
  predicates (properties) as well, although some approach to doing
  n-ary predicates in RDF is required if the function takes any
  parameters.  Functions/methods with side-effects need to be handled
  as actions, which I think are a different topic.

  XML documents can be presented in RDF fairly naturally too.  An XML
  element is a resource with some properties (XML attributes) and some
  content -- an RDF list of more XML element (as above, recursively)
  and text elements (literals in RDF).

  The downsides here are:

    (1) N-ary predicates and lists need syntactic sugar, at least, to
        be practical in RDF.   N3's [] and () might do the trick.

    (2) This is a fairly novel approach. 

== 3.  Use XML Documents as Fundamental Data

  Java, etc, can already serialize their data as XML, and of course
  RDF has an XML serialization.  So one could use XPath to access Java
  or RDF data via an XML infoset.


    (1) Getting at RDF triples via XPath from the RDF/XML syntax is
        extremely difficult.   This could be addressed with either an
        XPath extension (eg "treehugger") or a new XML syntax for RDF
        (eg "TriX").   Or maybe one could extend XPath with rules in
        the rule language, so one could write an RDF/XML parser in the
        rule language and then use it.  (Rereading this, that sounds
        kind of scary, but it's also kind of exciting.)

    (2) Relationship to XSLT: how is XSLT not the rule language people
        are looking for?   How would this one be different?


So that's my quick analysis to get us started.  What issues have I

> > Each area has its own typical language: "if condition then condition",
> > "if [not] condition then error", and "if condition then action", but
> > they obviously have a lot in common, too.
> > 
> > Do you see a big problem with establishing a standard language of "if
> > condition then condition/error/action" and defining conformant
> > implementations in terms of the above functions?  
> I think the real challenge is more about data and expression language.
> Basically, "what data do our rules work with" and "what language the rules
> are expressed in". As you suggested, all rules can be expressed as 
> "If condition then condition/error/action", but are we going to write the
> condition in Java, XML, XPath, RDF, or something else? If we just define a
> skeleton for If .. Then and leave the language of expression
> un-standardized, then we haven't really done much.
>     Obviously, the answer depends on the type of data we're targeting. If
> our data is Java objects, then XPath is probably not a good choice. If our
> data is RDF, then some sort of RDF extension might be the right choice.
> Weather one language can be written to express conditions and actions for
> all of our Java objects, RDF, and XML is still not clear to me. Any ideas
> about how this language might look like would be a great contribution (and
> I'm not talking about a rules language, just an expression language to
> express the conditions and actions). If we get that, we're half way done.

It's not clear to me whether this question needs to be settled
pre-chartering, or if it can be handled by the Working Group.  It does
seem like it would be helpful to give it some discussion now, though.

      -- sandro

[1] http://lists.w3.org/Archives/Public/public-rule-workshop-discuss/2005Jul/0007
Received on Thursday, 7 July 2005 15:46:27 UTC

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