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Twisted Web in 60 seconds: interrupted responses
jcalderone

Welcome to the eleventh installment of "Twisted Web in 60 seconds". Previously I gave an example of a Resource which generates its response asynchronously rather than immediately upon the call to its render method. When generating responses asynchronously, the possibility is introduced that the connection to the client may be lost before the response is generated. In such a case, it is often desirable to abandon the response generation entirely, since there is nothing to do with the data once it is produced. In this installment, I'll show you how to be notified that the connection has been lost.

This example will build upon the example from installment nine which simply (if not very realistically) generated its response after a fixed delay. I will expand that resource so that as soon as the client connection is lost, the delayed event is canceled and the response is never generated.

The feature this example relies on is provided by another Request method: notifyFinish. This method returns a new Deferred which will fire with None if the request is successfully responded to or with an error otherwise - for example if the connection is lost before the response is sent.

The example starts in a familiar way, with the requisite Twisted imports and a resource class with the same _delayedRender used previously:

  from twisted.web.resource import Resource
 from twisted.web.server import NOT_DONE_YET
 from twisted.internet import reactor

 class DelayedResource(Resource):
     def _delayedRender(self, request):
         request.write("<html><body>Sorry to keep you waiting.</body></html>")
         request.finish()

Before defining the render method, I'm going to define an errback (an errback being a callback that gets called when there's an error), though. This will be the errback attached to the Deferred returned by Request.notifyFinish. It will cancel the delayed call to _delayedRender.

      def _responseFailed(self, err, call):
         call.cancel()

Finally, the render method will set up the delayed call just as it did before, and return NOT_DONE_YET likewise. However, it will also use Request.notifyFinish to make sure _responseFailed is called if appropriate.

      def render_GET(self, request):
         call = reactor.callLater(5, self._delayedRender, request)
         request.notifyFinish().addErrback(self._responseFailed, call)
         return NOT_DONE_YET

Notice that since _responseFailed needs a reference to the delayed call object in order to cancel it, I passed that object to addErrback. Any additional arguments passed to addErrback (or addCallback) will be passed along to the errback after the Failure instance which is always passed as the first argument. Passing call here means it will be passed to _responseFailed, where it is expected and required.

That covers almost all the code for this example. Here's the entire example without interruptions, as an rpy script:

from twisted.web.resource import Resource
from twisted.web.server import NOT_DONE_YET
from twisted.internet import reactor

class DelayedResource(Resource):
   def _delayedRender(self, request):
       request.write("Sorry to keep you waiting.")
       request.finish()

   def _responseFailed(self, err, call):
       call.cancel()

   def render_GET(self, request):
       call = reactor.callLater(5, self._delayedRender, request)
       request.notifyFinish().addErrback(self._responseFailed, call)
       return NOT_DONE_YET

resource = DelayedResource()

Toss this into example.rpy, fire it up with twistd -n web --path ., and hit http://localhost:8080/example.rpy. If you wait five seconds, you'll get the page content. If you interrupt the request before then, say by hitting escape (in Firefox, at least), then you'll see perhaps the most boring demonstration ever - no page content, and nothing in the server logs. Success!

Next time I'll digress slightly to cover the basics of Twisted logging and expand this example to use it to show when clients fail to receive the response they requested.


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