title: Twisted deferred vs blocking in web services
body: <p>I'm struggling to produce the same behavior in web service code that uses Deferred objects as in code that does not.  My objective is to write a decorator that will delegate processing of any method (which is decoupled from Twisted) to the Twisted thread pool, so that the reactor is not blocked, without changing any of that method's semantics.</p>
  
  <p>When an instance of class echo below is exposed as a web service, this code:</p>
  
  <pre><code>from twisted.web import server, resource
  from twisted.internet import defer, threads
  from cgi import escape
  from itertools import count
  
  class echo(resource.Resource):
    isLeaf = True
    def errback(self, failure): return failure
    def callback1(self, request, value):
      #raise ValueError  # E1
      lines = ['&lt;html&gt;&lt;body&gt;\n',
               '&lt;p&gt;Page view #%s in this session&lt;/p&gt;\n' % (value,),
               '&lt;/body&gt;&lt;/html&gt;\n']
      return ''.join(lines)
    def callback2(self, request, encoding):
      def execute(message):
        #raise ValueError  # E2
        request.write(message.encode(encoding))
        #raise ValueError  # E3
        request.finish()
        #raise ValueError  # E4
        return server.NOT_DONE_YET
      return execute
    def render_GET(self, request):
      content_type, encoding = 'text/html', 'UTF-8'
      request.setHeader('Content-Type', '%s; charset=%s' %
          tuple(map(str, (content_type, encoding))))
      s = request.getSession()
      if not hasattr(s, 'counter'):
        s.counter = count(1)
      d = threads.deferToThread(self.callback1, request, s.counter.next())
      d.addCallback(self.callback2(request, encoding))
      d.addErrback(self.errback)
      #raise ValueError  # E5
      return server.NOT_DONE_YET
  </code></pre>
  
  <p>will display an HTML document to the browser when all the raise statements are commented out, and display a nicely formatted stack trace (which Twisted does for me) when the raise statement labelled "E5" is included.  That is what I want.  Likewise, if I do not use Deferred objects at all and place all the behavior from callback1 and callback2 within render_GET(), an exception raised anywhere within render_GET will produce the desired stack trace.</p>
  
  <p>I am trying to write code that will respond to the browser immediately, not cause resource leaks within Twisted, and cause the browser stack trace to also be displayed in the cases where any of the raise statements "E1" to "E3" is included in the deferred code--though of course I understand that the stack traces themselves will be different.  (The "E4" case I don't care about as much.)  After reading the Twisted documentation and other questions on this site I am unsure how to achieve this.  I would have thought that adding an errback should facilitate this, but evidently not.  There must be something about Deferred objects and the twisted.web stack that I'm not understanding.</p>
  
  <p>The effects on logging I document here may be affected by my use of the PythonLoggingObserver to bridge Twisted logging to the standard logging module.</p>
  
  <p>When "E1" is included, the browser waits until the reactor is shut down, at which point the ValueError exception with stack trace is logged and the browser receives an empty document.</p>
  
  <p>When "E2" is included, the ValueError exception with stack trace is logged immediately, but the browser waits until the reactor shuts down at which point it receives an empty document.</p>
  
  <p>When "E3" is included, the ValueError exception with stack trace is logged immediately, the browser waits until the reactor shuts down, and at that point receives the intended document.</p>
  
  <p>When raise statement "E4" is included, the intended document is returned to the browser immediately, and the ValueError exception with stack trace is logged immediately.  (Is there any possibility of a resource leak in this case?)</p>