You want to work with a large XML file, but you can''t read it into memory to form a DOM or other kind of tree because it''s too big.
Use SAX (as described in Recipe 22.3) to process events instead of building a tree.
Alternatively, use XML::Twig to build trees only for the parts of the document you want to work with (as specified by XPath expressions):
use XML::Twig;
my $twig = XML::Twig->new( twig_handlers => {
$XPATH_EXPRESSION => \&HANDLER,
# ...
});
$twig->parsefile($FILENAME);
$twig->flush( );You can call a lot of DOM-like functions from within a handler, but only the elements identified by the XPath expression (and whatever those elements enclose) go into a tree.
DOM modules turn the entire document into a tree, regardless of whether you use all of it. With SAX modules, there are no trees built—if your task depends on document structure, you must keep track of that structure yourself. A happy middle ground is XML::Twig, which creates DOM trees only for the bits of the file that you''re interested in. Because you work with files a piece at a time, you can cope with very large files by processing pieces that fit in memory.
For example, to print the titles of books in books.xml (Example 22-1), you could write:
use XML::Twig;
my $twig = XML::Twig->new( twig_roots => { ''/books/book'' => \&do_book });
$twig->parsefile("books.xml");
$twig->purge( );
sub do_book {
my($title) = $_->find_nodes("title");
print $title->text, "\n";
}For each book element, XML::Twig calls do_book on its contents. That subroutine finds the title node and prints its text. Rather than having the entire file parsed into a DOM structure, we keep only one book element at a time.
Consult the XML::Twig manpages for details on how much DOM and XPath the module supports—it''s not complete, but it''s growing all the time. XML::Twig uses XML::Parser for its XML parsing, and as a result the functions available on nodes are slightly different from those provided by XML::LibXSLT''s DOM parsing.
Recipe 22.6; the documentation for the module XML::Twig