The Pull Model with XSLT

In a pull model stylesheet, the XML source document acts purely as a data source and its structure is largely irrelevant. The stylesheet itself provides the structure of the output document.

Let's look at a simple example based on the full source of the play, Hamlet.xml, which is provided in the code download for the book. Here is our stylesheet, count.xsl:

This is the result of applying the stylesheet to the play (for example, by using XT to apply the stylesheet to Hamlet.xml and produce the HTML result):

This bears little resemblance to anything in the play. Instead, the stylesheet has provided the structure of the output, pulling in data as it is needed. You will also notice the use of the built-in count() function in this example. We will meet this later when we look at the elements and functions in detail.

The pull model is characterized by a few large templates and use of the <xsl:value-of> element so that the stylesheet controls the order of items in the output. Compare this to the push model, where we had more and smaller templates with the output largely following the structure of the XML source document.

I mentioned earlier that XSLT is often thought of as a declarative language. However, it also contains the flow control and looping instructions typical of a procedural language. Typically, a push model stylesheet emphasizes the declarative aspects of the language, while the pull model emphasizes the procedural aspects.

Of course, these definitions are not absolute. Most stylesheets will contain elements of the push and elements of the pull models. However, it is useful to keep the two models in mind as it can make your stylesheet development simpler.

We have now looked at how XSLT stylesheets are created and seen examples of both push and pull models. In doing this, we have come across several of the XSLT elements and their attributes, and the built-in count() function. We will look at these and other elements in more detail, shortly.