# Interrupted Responses

The previous example had a Resource that 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. This example shows how to be notified that the connection has been lost.

This example will build upon the asynchronous responses example which simply (if not very realistically) generated its response after a fixed delay. We will expand that resource so that as soon as the client connection is lost, the delayed event is cancelled 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:

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    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, we're 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`.

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    ...
        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.

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    ...
        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, we 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:

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    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()

        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!

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Version: 13.0.0