XML Replay

Replaying Logged XML

When attempting to diagnose problems which occur within web service environments, it is often useful to be able to re-run the XML which triggered the problem, so as to be able to step through it in the Visual DataFlex Studio debugger. The XML passed to a web service can be logged (see XML logging for details of how this can be done) and this historic data can then be used to "replay" the offending call and see what probably happened.

Assuming you have access to the original XML (see XML logging), this can then be saved into a text file - usually with the extension ".xml". If this has been logged using the mechanism suggested in the XML logging article, it may need to be slightly doctored before being sent for replay. This is because, for obscure reasons, the message namespace gets attached to the first child element of the message XML, instead of to the parent itself, thus:

<MessageName>
  <ParameterName xmlns="http://myDomain/myURI">
    <Element1>Foo</Element1>
    <Element2>Bar</Element2>
    <Element3>Etc.</Element3>
  </ParameterName>
</MessageName>

Instead of the correct:

<MessageName xmlns="http://myDomain/myURI">
  <ParameterName>
    <Element1>Foo</Element1>
    <Element2>Bar</Element2>
    <Element3>Etc.</Element3>
  </ParameterName>
</MessageName>

Once this has been corrected (either manually or in your replay program), the XML is then ready for incorporation into a SOAP message for re-sending to you service, running under a debugger so you can set breakpoints and trace the execution through, step by step.

In order to do this however, you are going to need a client which operates at a lower level than the standard web service client class you are able to generate for your service. The following is an example of how you might do this.

  Use cXmlHttpTransfer.pkg

  Object oTfr is a cXmlHttpTransfer
  End_Object  // oTfr
  
  Procedure InvokeService
     Handle  hoXML hoXmlResponse hoEnv hoBody hoDoc hoMsg
     Boolean bOK
     String  sServer sURI

     Get Create U_cXmlDomDocument to hoDoc
     Set psDocumentName of hoDoc  to (Value(oInput(Self)))
     Get LoadXmlDocument of hoDoc to bOK

     If not bOK Begin
        Error 0 ("Could not load document" * psDocumentName(hoDoc))
        Procedure_Return
     End

     Get DocumentElement of hoDoc to hoMsg

     Get Create U_cXmlDomDocument to hoXML
     Send AddChildProcessingInstruction of hoXML "xml" 'version="1.0"'
     Get CreateDocumentElementNS of hoXML "http://schemas.xmlsoap.org/soap/envelope/" "soap:Envelope" to hoEnv
     Get AddElement of hoEnv "soap:Body" "" to hoBody
     Get AppendNode of hoBody hoMsg to hoMsg

     Send ClearHeaders of oTfr

     Set psAcceptTypes of oTfr to "text/*"
     Set psContentTypeSent of oTfr to "text/xml; charset=UTF-8"
     Get AddHeader of oTfr "SOAPAction" ('"' + Value(oOperation(Self)) + '"') to bOk

     Get Value of oServer to sServer
     Get Value of oURI    to sURI
     Set piRemotePort of oTfr to (Value(oPort(Self)))

     Get HttpPostXmlNode of oTfr sServer sURI hoXML to hoXMLResponse

     If (hoXmlResponse=0) Set Value of oReturnedXML to "NO RESPONSE"
     Else                 Set Value of oReturnedXML to (psXml(hoXmlResponse))

     Send Destroy of hoXML
     If hoXMLResponse Send Destroy of hoXMLResponse

  End_Procedure  // InvokeService

The above code obviously assumes the existence of objects (probably mostly of theclass "Form") which contain the relevant information for the operation invocation:

• oInput - the path/name of the XML file to load and send
• oOperation - the operation (function) name beng invoked
• oServer - the server or IP address the service is running on (localhost in most debugging scenarios)
• oPort - the port number to use (80 by default)
• oURI - the URI of the service within the host: e.g. "MyService/TestService.wso"
• oReturnedXML - an object of class "cTextEdit" to take and display the response XML