[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Caught napping!
Subrahmanyam Allamaraju wrote: > When I look at relational databases, I know what to look for in a > product - from the logical model to the query language to other > "ilities". The same is more or less the case with document management > systems. However, I've difficulty when it comes to XML databases. They > are different in terms of level of granularity of operations, query > languages, etc. Most of these products have additional semantics for > different kind of operations (queries, updates, inserts, deletes, > imports, exports, transactions etc) on data. So my question is about the > logical framework underlying all these "features". First, it's important to remember that native XML databases model XML *documents*, not the data in those documents. Thus, it's somewhat misleading to mix the topics of data modeling and native XML databases. In the XML world, data modeling is essentially done with DTDs, XML schemas, or just plain markup in schema-less documents. It's not done in the database. Second, every native XML database defines two different models for an XML document. The first is the physical model, which determines both how the document is stored and at what level it can be round-tripped. Unlike relational databases, this is often in plain view in native XML databases. The minimal physical model is elements, attributes, text, and document order; the maximal physical "model" is the stream of bytes in the XML document. The second is the logical model, which is the underpinning of the query language. This is commonly the XPath data model and will generally be the XQuery data model in the future. Note that there is a definite possibility that the query data model is richer than the physical data model. For example, if the physical model is minimal (elements, attributes, text, and document order), an XPath query on a comment won't do much good. When thinking about the theory of native XML databases and trying to contrast them to relational theory, it's a good idea to focus on this model. Third, a lot of this stuff will coalesce as XQuery gets finished and the industry matures. In the long run, I think you will still see a variety of physical models, but very few logical models, with all serious players standardizing on XQuery's model. Similarly, I predict that an ODBC-like interface (with XQuery playing the role of SQL) will be standardized within a couple of years (XML:DB with XPath is here today). As to implementation details like transaction granularity (document- v. node-level locking), you'll probably always see differences, just as there are differences in the relational world today. -- Ron
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