I was a little distracted today with the proceedings of the DAWG.
Over the last few days Jeff Pollock has expressed frustration that the DAWG has not made it a formal requirement to commit to a language based on XQuery. Personally, I've been a little surprised at this, given that the group seems quite happy to consider such a proposal, but wants to investigate the ramifications first. The relevant section of the minutes says:
We discussed Proposed XQuery requirement and/or objective without reaching critical mass around any particular wording. ACTION SimonR: write a document discussing tradeoffs with adapting XQuery as an RDF query language for discussion thru the September meeting in Bristol.
The issue seems to be that Jeff insists that a commitment to XQuery as a "requirement or objective" must be made now. Of course, Jeff has his own agenda on this, as does everyone else. That is the point of a committee after all. However, I haven't found his arguments to be persuasive.
Jeff made 4 points for using XQuery as the basis of the query language to be proposed by the DAWG. The first 3 of these are quite valid, but miss the point. In two of them he points out that XQuery is modular, general purpose, and well structured. In the other he points out that XQuery is a W3C specification, and describes the importance of supporting such standards. In other words, he describes many of the strengths of XQuery as a language, with no reference to its applicability to RDF.
However, it was the fourth point that frustrated me. Excerpting from his message:
the output of RDF and OWL (and most likely SWRL) specifications was solidly grounded upon XML inside the SemWeb layer cake
RDF can certainly be represented as XML, but to claim that the RDF specification is "solidly grounded" on XML is incorrect. XML describes a tree structure, while RDF describes a directed graph. Many RDF documents have a simple tree structure and are easily represented in XML, but when an RDF document contains loops it cannot be directly represented in XML. One method to overcome this is to label a branch of the XML with an ID attribute, and to refer to that ID from elsewhere in the document. While this works, it circumvents standard XML structure.
For XQuery to deal with RDF XML is certainly possible, though not as trivial as one might expect from an XML-based structure. To claim that XQuery is appropriate for RDF and OWL because they are solidly grounded on XML is incorrect.
That said, I have no personal objection to the use of XQuery. I'm looking forward to reading Simon's document on the benefits and problems associated with using it. In the meantime, Jeff should consider his arguments more carefully in future, as his current ones don't carry any weight.
My initial idea to work around the problem of duplicated variables described yesterday was to replace the rule objects which use them with a different object type that did the workaround. Once I got to implementing the new class, I realised that I really wanted to make all the rule objects appear the same in the .drl file, so the same tests could be called on them. That meant that both classes should implement a common interface. After considering the operations classes needed by these classes, I finally opted for an abstract class instead.
The classes are nearly done, but I'm still working on the new insertion code for the workaround class. I also realised that rule XI which needs namespace string matching, can also be done with an extension of the abstract class. Up until now I'd been avoiding rule XI because we didn't have iTQL that could be used for it, but now we need to provide workarounds for other missing iTQL functionality it makes sense to implement this rule as well.
I also spent a little time documenting after-the-fact requirements for the paging pre-fetching.
I had an extended lunch while I went into UQ again. I've been told by both Bob and the nice lady in ITEE postgraduate administration (Kathy) that my proposal will be accepted by the university, but that they are notoriously slow at getting through these things. Kathy explained that she spends a significant portion of every week trying to get a response out of the main university administration on student applications, so I may be waiting a while to get back an official response. In the meantime, the application form from the university says:
I understand that I will be enrolled as a student on the commencement date I specify above, and that I have agreed to start my research project on this date even if I have not received written advice from the University about my admission.
This is a little annoying, as there are a few texts in the Physical Sciences library that I'd like to borrow, but I can't until the application is processed. Having the application accepted would give me a little piece of mind as well. Kathy promised to help. At least I'm more fortunate that overseas applicants, whose visas typically require an acceptance.
I spent most of my time in a discussion with Bob about what I should be doing to start with. At this point he just seems to be happy to help me find my feet while I work out the specific direction I should be going in. He also lent me a few PhD and Masters theses to provide a rough guide of the sorts of things expected eventually.
Probably the most useful remark Bob made was not to let myself get caught up with coding. It usually has little bearing on the thesis, and a student can fool themselves into thinking that they are getting something useful done, when they're really standing still. I still have some ideas which need me to write code, but I'll be careful to keep this warning in mind.
We also spent a little time discussing Bob's current research. He is working with a group out of DSTC along with people from IBM and Sandpiper Software. They are building a translation mapping from one ontology description framework to another. These include UML, OWL, E-R diagrams, and others. It certainly touches on what I'm interested in, although it is in a slightly different direction, particularly given that my emphasis is squarely on OWL. Still, I'm interested in learning a little more, so I'll read up on it in the coming weeks.
Tuesday, July 20, 2004