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Re: [Question] How to do incremental parsing?

On 04/07/2001 01:27:28 "Xu, Mousheng  (SEA)" wrote:

>A problem of all the current XML parsers is that they at least read the
>whole XML document into the input stream, which can consume a lot of memory
>when the XML is big (e.g. 1 GB).

You will generally be told "use SAX not DOM" for large files/streams.  That's OK if your application can deal with the data in your XML in a localised fashion.  And, it has to be said, designing your XML formats to work within the constraints of SAX can be a good exercise in avoiding structures that require backtracking through the document when they are processed.

Still, it often is necessary to backtrack, or make connections between parts of a document that may be widely separated in the file/stream.  In this case, you want to be able to use something more like DOM, because the SAX alternative here would require you to build a store of the information that has been parsed, and that means (a) writing more code than you might like to, and (b) possibly storing as much information as a DOM tree would anyway.  What does seem to be a useful way forward for these kinds of problems are persistent DOMs built into databases, such as have been appearing recently.  The DOM tree is then paged into memory as required.  Of course, this is slower than holding the whole DOM tree in memory, but the fact is that databases are fast enough to do real stuff with, and if that is true for relational tables, it should hold true for persistent DOMs.

So, "use SAX or a persistent DOM" for large XML files/streams is what I would suggest.

Anthony B. Coates
Leader of XML Architecture & Design
Chief Technology Office
Reuters Plc, London.

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