streaming-xml-transform.groovy

import org.mule.api.transport.OutputHandler

// store the current payload at the time the script is invoked 
// (i.e. the back-end response stream/iterator)
def itr = payload;

// return a Groovy closure / lambda
return {evt, out ->

  // define a stream XML builder object
  def xml = new groovy.xml.StreamingMarkupBuilder();

  // construct the builder function
  def xmlClosure = {
    mkp.declareNamespace(ns0: "http://mycomany.com/schema/account/v1/xsd")
    is.accountList { // XML root element
      itr.each { _acnt -> // lazily iterate over the back-end response records
        account { // XML record element
          systemId(_acnt.accountId)
          // map other fields in here as necessary
          // ...
        }
      }
    }
  }

  // write the resulting XML directly to the output stream provided by Mule as a parameter
  out << xml.bind(xmlClosure); 

} as OutputHandler // Groovy way to implement a Java interface with a single method