Transform Actions (XSLT)

The Transform action runs a stylesheet — most commonly XSLT — against the action's input context and writes the result to its output context. It's the classic way to reshape XML and to convert between XML and JSON.

A minimal transform

Suppose the backend wants <order> elements but the client sends a different shape. A stylesheet maps one tree to the other:

<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/request">
    <order>
      <id><xsl:value-of select="orderId"/></id>
      <customer><xsl:value-of select="customer/name"/></customer>
      <total currency="{customer/@currency}">
        <xsl:value-of select="amount"/>
      </total>
    </order>
  </xsl:template>
</xsl:stylesheet>

You upload this to local: and point a Transform action at it. On the request side, wire the action so its input is the previous action's output and its output feeds the next action (or OUTPUT).

DataPower XSLT extensions

DataPower adds extension functions and elements (in the http://www.datapower.com/extensions namespace, conventionally dp:) that make XSLT useful as a gateway language, not just a document transformer:

<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:dp="http://www.datapower.com/extensions"
    extension-element-prefixes="dp">

  <xsl:template match="/">
    <!-- Read a service variable (e.g. the request URI) -->
    <xsl:variable name="uri"
      select="dp:variable('var://service/URI')"/>

    <!-- Set the dynamic backend destination -->
    <dp:set-variable name="'var://service/routing-url'"
      value="concat('https://backend.internal', $uri)"/>

    <!-- Emit a log message -->
    <xsl:message dp:priority="info">
      Routing request for <xsl:value-of select="$uri"/>
    </xsl:message>
  </xsl:template>
</xsl:stylesheet>

Useful extensions include:

• dp:variable('name') / dp:set-variable(...) — read/write service & context variables (routing URL, headers, custom context values).
• dp:request-header(...) / dp:set-response-header(...) — manipulate HTTP headers.
• dp:url-open(...) — call out to another service mid-transform.
• dp:reject(...) — fail the transaction with a message (drives error handling).

JSON in / JSON out

Because JSON is parsed to a node tree, you can transform JSON with XSLT too — or use the dedicated JSON→XML / XML→JSON conversions and let XSLT handle the structure in between. For heavily JSON-shaped logic, prefer GatewayScript.

Common pitfalls

• Wiring: the Transform action transforms its input context. If you point it at INPUT when you meant the previous action's output, you transform the wrong thing.
• Namespaces: XML namespace mismatches are the #1 reason a template "doesn't match". Declare and use the right namespace prefixes.
• Output method: set xsl:output method (xml/text/json) to match what the next stage expects.

You'll write and wire a real stylesheet in Lab: Write an XSLT Transform.

?Knowledge Check

Which DataPower XSLT extension would you use to set the dynamic backend destination from within a stylesheet?

A template "doesn’t match" the incoming XML. What’s the most common cause on DataPower?