# Asynchronous Responses

In all of the previous examples, the resource examples presented generated responses immediately. One of the features of prime interest of Twisted Web, though, is the ability to generate a response over a longer period of time while leaving the server free to respond to other requests. In other words, asynchronously. In this installment, we’ll write a resource like this.

A resource that generates a response asynchronously looks like one that generates a response synchronously in many ways. The same base class, `Resource` , is used either way; the same render methods are used. There are three basic differences, though.

First, instead of returning the string which will be used as the body of the response, the resource uses `Request.write` . This method can be called repeatedly. Each call appends another string to the response body. Second, when the entire response body has been passed to `Request.write` , the application must call `Request.finish` . As you might expect from the name, this ends the response. Finally, in order to make Twisted Web not end the response as soon as the render method returns, the render method must return `NOT_DONE_YET` . Consider this example:

    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(b"<html><body>Sorry to keep you waiting.</body></html>")
            request.finish()

        def render_GET(self, request):
            reactor.callLater(5, self._delayedRender, request)
            return NOT_DONE_YET

If you’re not familiar with the reactor `callLater` method, all you really need to know about it to understand this example is that the above usage of it arranges to have `self._delayedRender(request)` run about 5 seconds after `callLater` is invoked from this render method and that it returns immediately.

All three of the elements mentioned earlier can be seen in this example. The resource uses `Request.write` to set the response body. It uses `Request.finish` after the entire body has been specified (all with just one call to write in this case). Lastly, it returns `NOT_DONE_YET` from its render method. So there you have it, asynchronous rendering with Twisted Web.

Here’s a complete rpy script based on this resource class (see the previous example if you need a reminder about rpy scripts):

    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(b"<html><body>Sorry to keep you waiting.</body></html>")
            request.finish()

        def render_GET(self, request):
            reactor.callLater(5, self._delayedRender, request)
            return NOT_DONE_YET

    resource = DelayedResource()

Drop this source into a `.rpy` file and fire up a server using `twistd -n web --path /directory/containing/script/.` You’ll see that loading the page takes 5 seconds. If you try to load a second before the first completes, it will also take 5 seconds from the time you request it (but it won’t be delayed by any other outstanding requests).

Something else to consider when generating responses asynchronously is that the client may not wait around to get the response to its request. A subsequent example demonstrates how to detect that the client has abandoned the request and that the server shouldn’t bother to finish generating its response.