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The event-driven model of SAX is useful for XML parsing, but it does have certain drawbacks.SAX provides a mechanism for reading data from an XML document that is an alternative to that provided by the Document Object Model (DOM).Where the DOM operates on the document as a whole, SAX parsers operate on each piece of the XML document sequentially.Other tasks, such as sorting, rearranging sections, getting from a link to its target, looking up information on one element to help process a later one, and the like, require accessing the document structure in complex orders and will be much faster with DOM than with multiple SAX passes.Some implementations do not neatly fit either category: a DOM approach can keep its persistent data on disk, cleverly organized for speed (editors such as Soft Quad Author/Editor and large-document browser/indexers such as Dyna Text do this); while a SAX approach can cleverly cache information for later use (any validating SAX parser keeps more information than described above).This takes considerable time and space for large documents (memory allocation and data-structure construction take time).
Because of the event-driven nature of SAX, processing documents is generally far faster than DOM-style parsers, so long as the processing can be done in a start-to-end pass.
Many tasks, such as indexing, conversion to other formats, very simple formatting, and the like, can be done that way.
A SAX parser only needs to report each parsing event as it happens, and normally discards almost all of that information once reported (it does, however, keep some things, for example a list of all elements that have not been closed yet, in order to catch later errors such as end-tags in the wrong order).
Thus, the minimum memory required for a SAX parser is proportional to the maximum depth of the XML file (i.e., of the XML tree) and the maximum data involved in a single XML event (such as the name and attributes of a single start-tag, or the content of a processing instruction, etc.). A DOM parser, in contrast, has to build a tree representation of the entire document in memory to begin with, thus using memory that increases with the entire document length.
Such implementations blur the DOM/SAX tradeoffs, but are often very effective in practice.
Due to the nature of DOM, streamed reading from disk requires techniques such as lazy evaluation, caches, virtual memory, persistent data structures, or other techniques (one such technique is disclosed in ).