Python Webservice, part 1 : a Twisted solution

As mentioned previously I’ve been spending some time recently working on a SOAP server mock. For this I’ve used Python Twisted and ZSI. Since the interaction between the 2 seems rather poorly documented (even Google doesn’t know much about it), I thought I’d share what I’ve investigated and done.

Disclaimer: you should be familiar with SOAP and Python Twisted. I’ll not cover anything in these areas.

For the sake of simplicity, I’ll take a classical minimal server: an Echo service.

Let’s start from the definition of such a service, as a WSDL. This one was randomly picked from there. Anyway the implementation details are not so important here.

<?xml version="1.0" encoding="UTF-8"?>
    targetNamespace="urn:ZSI" >
    <xsd:schema elementFormDefault="qualified" targetNamespace="urn:ZSI">
      <xsd:element name="Echo">
            <xsd:element name="value" type="xsd:anyType"/>

  <message name="EchoRequest">
    <part name="parameters" element="tns:Echo" />

  <message name="EchoResponse">
    <part name="parameters" element="tns:Echo"/>
  <portType name="EchoServer">
    <operation name="Echo">
      <input message="tns:EchoRequest"/>
      <output message="tns:EchoResponse"/>

  <binding name="EchoServer" type="tns:EchoServer">
    <soap:binding style="document"
    <operation name="Echo">
      <soap:operation soapAction="Echo"/>
        <soap:body use="literal"/>
        <soap:body use="literal"/>

  <service name="EchoServer">
    <port name="EchoServer" binding="tns:EchoServer">
      <soap:address location="http://localhost:8080/echo"/>


So really nothing fancy here. Just the definitions and shape of the messages/operations (only trivial ones).

ZSI offers the wsdl2py tool, that can process such WSDL files, and produce client bindings and server stub. In this article, the server part is what we want. (note the -w flag will trigger Twisted code generation)

$ wsdl2py -w echo.wsdl now contains the code for the service stub. Not much in there, the main part being the following method:

def soap_Echo(self, ps, **kw):
    request = ps.Parse(EchoRequest.typecode)
    return request,EchoResponse()

This is the one we should implement to make the service concrete. Here is a first attempt:

class EchoService(EchoServer):

    def __init__(self):

    def soap_Echo(self, ps, **kw):
        request, _ = EchoServer.soap_Echo(self, ps, **kw)
        return request, request

That will work. If you register this Service as a Twisted Resource, it will provide a functional SOAP Echo service, on which you can execute the following.

$ POST http://localhost:8080/echo
Please enter content (application/x-www-form-urlencoded) to be POSTed:
  <soapenv:Body xmlns:ns1="urn:ZSI">
      <ns1:value xsi:type="xsd:string">plop</ns1:value>
<soapenv:Envelope xmlns:ZSI="" xmlns:soapenc="" xmlns:soapenv="" xmlns:xsd="" xmlns:xsi="">
<soapenv:Header></soapenv:Header><soapenv:Body xmlns:ns1="urn:ZSI"><ns1:Echo><ns1:value xsi:type="xsd:string">plop</ns1:value></ns1:Echo></soapenv:Body></soapenv:Envelope>

Still, this is not very Twisted I mean we don’t take advantage of the asynchronous framework here, as the only method we wrote is definitely synchronous and we treat each call after one another. Would the soap_Echo() take time for whatever reason, that would introduce unacceptable delays for the user. Of course, there is no magic in the Deferred objects, which means than simply returning a Deferred in soap_Echo() will absolutely not work.

Digging a bit more in the ZSI Twisted glue, one can find a promising piece of code, appropriately named DeferHandlerChain. This is an alternative to the DefaultHandlerChain that’s used (by default) by WSresource objects (hence EchoServer ones). The nice property of this handler chain is to put in place a callback chain from processRequest() to processResponse(), actually supporting Deferred objects returned by soap methods. In a word, what we need

The code can become something like:

class EchoDeferHandlerChainFactory:
    protocol = DeferHandlerChain

    def newInstance(cls):
        return cls.protocol(DefaultCallbackHandler, DataHandler)

class EchoService(EchoServer):

    def __init__(self):
        EchoServer.factory = EchoDeferHandlerChainFactory

    def soap_Echo(self, ps, **kw):
        request, _ = EchoServer.soap_Echo(self, ps, **kw)

        def _answer():
            return request

        # yeah, we have a very busy echo service... 5 seconds delay !
        d = deferLater(reactor, 5, _answer)
        return request, d

You can now try triggering several parallel Echo calls, and see that they will not be treated in sequence !