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This happens to be a long mail -- I think the work on data models or rather the information in a document with schema is still not very complete, it is quite an intriguing problem. XPath 1.0 is well-defined (subset), and fairly complete, but i would classify XQuery algebra as ongoing work. i consider xml schemas as a *data modeling language*, i expect this to happen -- most of the data in the world will be wrapped with an XML wrapper, along with an xml schema. XML schemas provide regular expression types (from XDuce), or more finely -- tree types and hedge types (hedge is an ordered list of trees) -- the above along with also forest types (forest is an unordered set of trees) will form a *very* complete data model, in my opinion (though I do not care about forest types actually). I am not very familiar with RDF, but in my opinion, RDF sits on top of xml schemas, and actually is for a different purpose. I think it is more to define semantics of much larger granularity than xml schemas, and semantics that are typically not defined in xml schemas -- for example, semantics like the who created a web page, or saying something like this collection of web documents form a "community" -- so now search engines can make use of such metalevel information for better results -- this is building the semantic web... We did try to use a web-centric approach for describing our services, but a web-centric approach does not scale for sensor services well -- the services present at any instant are very transitory, also what we need more is a local service discovery protocol -- similar to what Jini provides -- based on geographical proximity. We still do not use RDF though -- it definitely seems useful for us to use RDF, but Jini templates have been satisfactory so far. For data, again, we need closure under union, just for describing the data -- we have not thought about operations w.r.to the project, but again there is no way out for us than to have closure under query operations. The current state of the art, I believe, for defining query operations is -- I *strongly* believe that we can define semantics for query operations such that they will be closed under regular tree languages -- note that if not regular tree languages, i am *very* sure we cannot define meaningful operations. Now for data storage, though the data model that the user sees will be based on xml schemas, the storage will be in a relational database -- but still, though there are tools available for doing that, I am not very satisfied with the way union types expressed in an xml schema is handled -- there is a way of normalizing and writing a regular tree grammar so that "in effect" it has no union types (no | operator) -- i am very sure this is the way of mapping from xml schemas to relational schemas -- inlining etc is good, but w/o a proper treatment of union types, it is quite incomplete. Now I will have to wait for operations to be defined, good semantics defined, then try to complete the mapping from xml data model to relational model by mapping the operations also. I think that will mostly complete my thesis, unless my advisor comes up with suggestions along the way. also all along the way, I should try to convince people to first listen to what I have to say, and then try to convince them why I believe in what I am saying... hmm, wonder how long it is going to take if I have to try to do all of the above :) but it is *very* interesting. Finally to answer Len's question -- I believe xml schemas are for data modeling -- *numerous* gaps in data modeling are filled by xml schemas. I think if we say data model, then operations are an *inherent* part of it -- operations include simple retrieval operations like projection -- w3c's proposal for xml-schema will fail at the first operation itself. I believe RDF will not meet my requirements because i think we are talking of different things and definitely different granularity. <warning>speaking for himself only</warning> cheers and regards - murali. On Fri, 8 Jun 2001, Bullard, Claude L (Len) wrote: > I have not yet looked into RELAX/TREX deeply enough > to compare. Also, I don't have your deep knowledge > of tree grammars. Isn't the XQL work creating a > data model to satisfy this requirement? > > Because this also came up on the HumanMarkup list, > let me ask, do you consider xml-schemas a data modeling > language or simply a way to define a data structure? > Does that make a difference? In other words, is > it the application or the tool? Would RDF meet > your requirements? > > Len > http://www.mp3.com/LenBullard > > Ekam sat.h, Vipraah bahudhaa vadanti. > Daamyata. Datta. Dayadhvam.h > > > -----Original Message----- > From: Murali Mani [mailto:mani@C...] > > Interesting. I also tend to believe that OOP is definitely *not* something > which should be there from the beginning. And it is *impossible* to > satisfy everyone with the set of constraints you provide. > > Let me ask how important are the following properties in favor of > RELAX NG/RELAX/TREX over xml-schemas --- > > a) Query operations are a must for xml-schemas, actually for any data > model. 1-unambiguity for any set of operators other than the usual regular > expression operators (|, ,. *) have *never* been characterized. Without > this characterization, it is impossible to do type inferencing for > operations -- note that local tree grammars etc have been characterized, > but it is the 1-unambiguity that has *never* been characterized. > > b) People do data integration -- for merger of companies etc, also for one > project I work on -- a project on sensor networks, where services provided > by sensors are *highly* transitory, and unpredictable. Data integration > benefits *enormously* from closure under union -- actually otherwise, this > problem is so difficult (trying to solve a problem with no solution except > for uncharacterized special cases) that you will *never* be satisfied. >
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