3.3. Stream-Based Versus Tree-Based Processing

Remember the Perl mantra, "There's more than one way to do it"? It is also true when working with XML. Depending on how you want to work and what kind of resources you have, many options are available. One developer may prefer a low-maintenance parsing job and is prepared to be loose and sloppy with memory to get it. Another will need to squeeze out faster and leaner performance at the expense of more complex code. XML processing tasks vary widely, so you should be free to choose the shortest path to a solution.

There are a lot of different XML processing strategies. Most fall into two categories: stream-based and tree-based. With the stream-based strategy, the parser continuously alerts a program to patterns in the XML. The parser functions like a pipeline, taking XML markup on one end and pumping out processed nuggets of data to your program. We call this pipeline an event stream because each chunk of data sent to the program signals something new and interesting in the XML stream. For example, the beginning of a new element is a significant event. So is the discovery of a processing instruction in the markup. With each update, your program does something new—perhaps translating the data and sending it to another place, testing it for some specific content, or sticking it onto a growing heap of data.