XSL Transformation in Raise Fault

Nope.

In Apigee Edge, XSL and RaiseFault are two distinct policies.

If you want to Raise a fault, then ... (surprise!) use the RaiseFault policy.

If you want to transform some XML, then use the XSL policy.

In the case that you want to Raise a Fault and transmit some XML there, and that XML needs to be the result of a transform of some OTHER XML. . . . . then what I would do is use the same condition to wrap the XSL, and then the Raise Fault.

Let's look at an example. Suppose your design calls for raising a fault explicitly, if the value of a specific query parameter does not match a specific regular expression pattern. Suppose the pattern is AMG-XXXXX , where the XXXXX is replaced with decimal digits. Not everyone speaks Regular Expressions, so I'll just say: the regex for that pattern is

^AMG-[0-9]{5}$

A condition checking for a match of that is:

request.queryparam.id ~~ "^AMG-[0-9]{5}$"

But we want to raise a fault when that match is NOT found. So we need to negate that condition. Therefore the steps and conditions might look like this:

  <Step>
    <Condition>NOT (request.queryparam.id ~~ "^AMG-[0-9]{5}$")</Condition>
    <Name>XSL-TransformMyXml</Name> 
  </Step>
  <Step> 
    <Condition>NOT (request.queryparam.id ~~ "^AMG-[0-9]{5}$")</Condition>
    <Name>RaiseFault-InvalidValue</Name> 
  </Step>

And presumably the RaiseFault-InvalidValue will reference the output of the XSL step.

Maybe like this:

<RaiseFault name='RaiseFault-InvalidValue'>
  <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
  <FaultResponse>
    <Set>
      <Payload contentType='application/xml'>{xml_document}</Payload>
      <StatusCode>400</StatusCode>
      <ReasonPhrase>Bad Request</ReasonPhrase>
    </Set>
  </FaultResponse>
</RaiseFault>

Does this make sense?