The XSLT Debugging Problem for Logic Apps Developers

#xsl #logicapps #vscode #azure

Azure Logic Apps uses XSLT for XML transformations. While the Azure Data Mapper allows basic testing, it lacks true debugging capabilities — no message capture, no variable inspection, no execution tracing.

XSLT's Debug Tool: `xsl:message`

XSLT provides xsl:message for diagnostic logging:

<xsl:message>Processing started</xsl:message>
<xsl:message select="'Found ' || count(//book) || ' books'" />

Messages output to a separate stream without affecting transformation results. However, different processors handle it differently:

.NET (XslCompiledTransform)

transform.XsltMessageEncountered += (sender, e) => {
    Console.WriteLine($"[XSLT] {e.Message}");
};

XSLT 1.0 only
Event-based capture
Silent if no handler registered

📘 Microsoft Docs

Saxon (XSLT 2.0/3.0)

transformer.setMessageListener((content, terminate, location) -> {
    System.out.println("[XSLT] " + content);
});

XSLT 2.0/3.0 support
Listener interface
Includes location info

📘 Saxonica Docs

Feature	.NET	Saxon
XSLT Version	1.0	2.0 / 3.0
Location Info	❌	✅
`select` Attribute	❌	✅

Existing Tools: Azure Data Mapper

Azure provides the Data Mapper for testing XSLT transformations:

📘 Logic Apps Data Mapper - Compiling and Testing

What it does:

Run transformations locally
View output results

What it doesn't do:

No xsl:message capture — Can't view debug logs
No variable inspection — Can't see intermediate values
No execution tracing — Can't track template calls or logic flow
Limited error context — Basic error messages without detailed diagnostics

The Debugging Gap

What's still missing for effective XSLT debugging:

Message capture — Can't view xsl:message output
Variable inspection — Can't see values during execution
Execution tracing — Can't understand transformation flow
Rich error diagnostics — Need detailed context and line numbers

What's Needed

A proper XSLT debugger for Logic Apps should:

Run transformations locally in VS Code
Capture and display xsl:message output
Support both .NET and Saxon processors
Provide fast edit-test cycles
Show clear errors with line numbers

This is what the XSLT Debugger extension delivers — built on .NET with open-source Saxon to provide debugging capabilities for Logic Apps developers without additional licensing costs.