XML Programming Tools (Perl and XML)

We'll look more closely at one of these tools, as its subject matter has some interesting implications involving recent developments, before we move on to this chapter's final section.

9.3.1. XML::Generator::DBI

XML::Generator::DBI is a fine example of a glue module, a simple piece of software whose only job is to take two existing (but not entirely unrelated) pieces of software and let them talk to one another. In this case, when you construct an object of this class, you hand it your additional objects: a DBI-flavored database handle and a SAX-speaking handler object.

XML::Generator::DBI does not know or care how or where the objects came from, but only trusts that they respond to the standard method calls of their respective families (either DBI, SAX, or SAX2). Then you can call an execute method on the XML::Generator::DBI object with an ordinary SQL statement, much as you would with a DBI-created database handle.

The following example shows this module in action. The SAX handler in question is an instance of Michael Koehne's XML::Handler::YAWriter module, a pleasantly configurable module that turns SAX events into textual output. Using this program, we can turn, say, a SQL table of CDs into well-formed XML and then have it printed to standard output:

#!/usr/bin/perl

use warnings;
use strict;

use XML::Generator::DBI;
use XML::Handler::YAWriter;

use DBI;

my $ya = XML::Handler::YAWriter->new(AsFile => "-");
my $dbh = DBI->connect("dbi:mysql:dbname=test", "jmac", "");
my $generator = XML::Generator::DBI->new(
                               Handler => $ya,
                               dbh => $dbh
                               );
my $sql = "select * from cds";

$generator->execute($sql);

The result is this:

<?xml version="1.0" encoding="UTF-8"?><database>
 <select query="select * from cds">
  <row>
   <id>1</id>
   <artist>Donald and the Knuths</artist>
   <title>Greatest Hits Vol. 3.14159</title>
   <genre>Rock</genre>
  </row>
  <row>
   <id>2</id>
   <artist>The Hypnocrats</artist>
   <title>Cybernetic Grandmother Attack</title>
   <genre>Electronic</genre>
  </row>
  <row>
   <id>3</id>
   <artist>The Sam Handwich Quartet</artist>
   <title>Handwich a la Yogurt</title>
   <genre>Jazz</genre>
  </row>
 </select>
</database>

This example isn't very interesting, but it looks good in print. The point is that we didn't have to use YAWriter. We could have used any SAX handler Perl package on our system, including ones we wrote ourselves, and tossed them into the mix when baking a new XML::Generator::DBI object. Given the same database table as the example above used, when the $genenerator object's execute method is called, it would act as if it had just parsed the previous XML document (modulo the whitespace that YAWriter inserted to make things more human-readable). It would act this way even though the actual source isn't an XML document at all, but a database table.

9.3. XML Programming Tools

9.3.1. XML::Generator::DBI

9.3.2. Further Ruminations on DBI and SAX