Cart
|
[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: RDF == the "binary relational model"? was Re: M
 Michael Champion wrote: > > > I just wonder whether RDF advocates are aware of these other > triples-like approaches, what they think they've learned from them, or > whether they think the analogy is misleading? > > I can't speak for all RDF "advocates", but some us that have been working with RDF are aware of the various binary-relational models (I'm basically a database guy, and these models have a pretty substantial database literature). NB: Strictly speaking, we're talking about binary-relational models with surrogates as keys/identifiers (URIs in the case of RDF). I think Chris Date's response in the reference you cite describes the gist of the situation, although his reponse restricts itself to various alternative *relational* approaches. Somewhat generalized, his points might be described as: 1. You need to have data structures that are convenient to manipulate (for the manipulation tasks you are doing) and present to users. These need not be restricted to n-ary relations; they also include XML documents that reflect the real structures of data that people are used to dealing with, programming language objects, and so on. 2. You want to be able to boil those structures down into highly normalized "information atoms", e.g., to eliminate redundancies of various kinds, or to try to capture the essential information that may be held in common by two data structures that differ only in the fact that they've chosen different "higher level" structuring techniques for the purposes of #1 (e.g., two different n-ary relational schemas, or an n-ary relational schema vs. a highly nested XML structure). In Date's version of point #1, he notes the need for n-ary relations to represent the results of join operations. This can be generalized as the need to support aggregating data from the various structures you start off with. In the case of RDF, this means that you have to be prepared to deal not just with triples as individuals, but with whole graphs (e.g., to deal with all the information you have about some person or other entity). The "irreducible" relations Chris mentions in his point #2 would, in RDF (and in the earlier binary-relational models), need to be broken down into binaries by introducing additional URIs (or blank nodes) to serve as identiifers for the additional entities you'd need to talk about, and by introducing additional predicates (relation names). Some of these can seem pretty artificial from an intuitive point of view when you look at the data, but no data model seems totally "natural" in every application. And this, I think, is a key point: what is the application of the data model? Date, for example, notes the distinction between the use of n-ary relations in general, and the desirability of more highly-normalized relations (often binary, but at least irreducable) as base relations (together, of course, with the ability to go between one form and the other). Similar considerations, it seems to me, apply to RDF. In cases, say, where a number of apps need to communicate, and they "understand" a mutually agreed-on XML vocabulary and collection of XML structures, there's no obvious need for RDF or anything like it. But what if we need to aggregate those communications with those of several other communities with their own separate and distinct XML vocabularies and structures? In that case, it seems to me I'd like to be able to boil down those communications into things like how many entities are being referred to, what is being said about each one, and so on. I think RDF helps with that as a common way of modeling (thinking about) those kinds of "information atoms". This doesn't require that the original data be in RDF, but does require that it be convertable to an RDF-like form (making sure that conversion can be done may require that some additional information be provided in the original XML about which things identify entities, and which things are properties; this has been touched on already in earlier discussion of "striping"). The usefulness of being able to boil data from heterogeneous data structures / schemas down into a more normalized binary-like form in order to do aggregation has been explored fairly well in the literature on heterogeneous database integration (using "heterogeneous" in two senses: integrating data from different schemas in the *same* data model, and integrating data from different schemas in *different* data models, such as relational and hierarchical databases). There's also currently a lot of research going on in the database community under the name "model management" to this end. All of the model management work I'm aware of uses some form of binary (graph) model. --Frank 
|
PURCHASE STYLUS STUDIO ONLINE TODAY!Purchasing Stylus Studio from our online shop is Easy, Secure and Value Priced! 
Download The World's Best XML IDE!Accelerate XML development with our award-winning XML IDE - Download a free trial today! Subscribe in XML format
|
|||||||||
