xml-dev - RE: Why XML Over the Relational Model?

["Paul Butkiewicz" <arabbit@earthlink.net>]

>The simplest case is when your data store is keeping a bunch of documents
>for you, be they memos, lab notebooks, or documentation.  While you can
>store documents in RDBMS systems, it... well, it kind of sucks, even on a
>good day.  If you happen to be lucky enough to be dealing with the portion
>of data that all fits neatly into relational structures, without needed to
>build messy scaffolding around it, XML probably isn't that exciting.  For
>those of us who would like to perform complex operations on documents and
>other information that doesn't fit relational systems neatly, XML is
>revelation.

>...

But using XML for persistence in this case presents its own perils.
Consider the case of modifying the database which is now implemented as an
XML document.  If I add an attribute, a node, or make some text longer, I
may very well need to re-write the *entire* file to accommodate the changes
I've just made and make them persistent.  Of course, I could pad various
parts of the document with spaces to get around this, but given that XML has
no notion of field lengths this certainly isn't a sure-fire fix --- I'm
still going to need to rewrite the whole document now and then, especially
if I'm adding nodes ad hoc.  And in addition to the overhead of actually
writing out the whole document (which *must* be done as part of the
transaction), when writing that document I would need to lock the *entire*
document.  This wouldn't be acceptable at all in a situation where many
concurrent writes are going to data in the same document, even if on
different nodes.

And then there's the obvious problem that if I want to go directly to
Article 14, Section 3, Subsection 41, Chapter 27, Paragraph 6, and I don't
have that part of the document cached in memory, I may have to iterate
through the entire document get that item.

Maybe this part of the discussion should be XML vs RDBMS vs ODBMS.  :)

Paul


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