From the pattern: "adds outsourcing to the process of generating an
HTML page from an XML document and XSL stylesheet. With a model that transforms
XML data while keeping it completely separated from the processing instructions, External
Assistant solves the problem of how to add external computation.
Here XML data on one side and the instructions in the XSL stylesheet on the other
feed a transformation process that generates output in the form of an HTML file. In this
context, External Assistant explains how to call an external process whose results are
incorporated to the output, always keeping high modularity and clear distinction of the
responsibilities of each component."
This pattern is described in an article XML Patterns by Maria Laura Ponisio and Gustavo Rossi
This is a one of a group of the four related patterns described in the article about processing XML on a Web server.