Many real-world transformations do not run on a single XML document. You often merge a primary payload with reference data, catalog lookups, or environment configuration. Done well, this results in a clean, predictable transform. Done poorly, it becomes a maze of document() calls and hidden dependencies. The difference is in how you model inputs and parameters from the start. As an integration engineer, I treat input selection and parameter design as first-class API design for the stylesheet.
Start by naming every input. Instead of embedding document('config.xml') in multiple templates, load each external document once near the top of the stylesheet and bind it to a global variable. This makes dependencies explicit and keeps the rest of the code focused on mapping. It also helps with testing, because you can override the URI with a parameter. A clean pattern is to define xsl:param values for input URIs and then bind them to xsl:variable values that hold the parsed documents.
The same clarity applies to parameters. Keep parameters primitive and predictable, and avoid passing in node sets unless you truly need them. A parameter should be an external knob: region, language, a feature flag, or an output format. If you have a complex decision tree, consider using a lookup XML or JSON input and then query it inside the stylesheet. This approach keeps the invocation interface stable while still letting you evolve business rules.
