With the tree-based strategy, the parser keeps the data to itself until the very end, when it presents a complete model of the document to your program. Instead of a pipeline, it's like a camera that takes a picture and transmits the replica to you. The model is usually in a much more convenient state than raw XML. For example, nested elements may be represented in native Perl structures like lists or hashes, as we saw in an earlier example. Even more useful are trees of blessed objects with methods that help navigate the structure from one place to another. The whole point to this strategy is that your program can pull out any data it needs, in any order.

Why would you prefer one over the other? Each has strong and weak points. Event streams are fast and often have a much slimmer memory footprint, but at the expense of greater code complexity and impermanent data. Tree building, on the other hand, lets the data stick around for as long as you need it, and your code is usually simple because you don't need special tricks to do things like backwards searching. However, trees wither when it comes to economical use of processor time and memory.

All of this is relative, of course. Small documents don't cause much hardship to a typical computer, especially since CPU cycles and megabytes are getting cheaper every day. Maybe the convenience of a persistent data structure will outweigh any drawbacks. On the other hand, when working with Godzilla-sized documents like books, or huge numbers of documents all at once, you'll definitely notice the crunch. Then the agility of event stream processors will start to look better. It's impossible to give you any hard-and-fast rules, so we'll leave the decision up to you.

An interesting thing to note about the stream-based and tree-based strategies is that one is the basis for the other. That's right, an event stream drives the process of building a tree data structure. Thus, most low-level parsers are event streams because you can always write a tree building layer on top. This is how XML::Parser and most other parsers work.

In a related, more recent, and very cool development, XML event streams can also turn any kind of document into some form of XML by writing stream-based parsers that generate XML events from whatever data structures lurk in that document type.