Twisted.web

As in normal twisted.web server , at TCP level, we have HTTPChannel class (a child class of T.I.protocol.Protocol) and server.Site class (which is the child class of T.I.protocol.ServerFactory). Also a Resource instance needs to be passed to server.site class, so that it can serve GET request.

Whenever a data is received, DataReceived method of HTTPChannel is invoked. Now if data starts with ‘GET’, allow the HTTPChannel handle it, which will invoke the render function of the root resource provided to Site class. Render will set 101 response code and will compute the websocket response key. During handshake do not send any raw data, because if handshake is successful this will be considered as framed binary data. Even if you want to send, frame it and send.

If data doesn’t start with ‘GET’, that means we can assume it is a binary encoded message. Now this message can be decoded using Frame.py, which is a very simple data framing module following WebSocket specification. Data send to the client by server should be unmasked as per the websocket specification.

Below is code example of an echo websocket server.

import base64, hashlib
from twisted.internet import reactor
from twisted.web.server import (Site, http, resource)

class EchoResource(resource.Resource):
    isLeaf = 1

    def render(self, request):

        # Processing the Key as per RFC 6455
        key = request.getHeader('Sec-WebSocket-Key')
        h = hashlib.sha1(key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11")
        request.setHeader('Sec-WebSocket-Accept', base64.b64encode(h.digest()))

        # setting response headers
        request.setHeader('Upgrade', 'websocket')
        request.setHeader('Connection', 'Upgrade')
        request.setResponseCode(101)
        return ''


class EchoChannel(http.HTTPChannel):

    def dataReceived(self, data):

        if data.startswith('GET'):
            # This will invoke the render method of resource provided
            http.HTTPChannel.dataReceived(self, data)

        else:
            # decoding Data using Frame module wrote by Morgan Philips.
            f = frame.Frame(bytearray(data))
            received_message = f.message()
            print received_message

            # Sending back the received message.
            msg = frame.Frame.buildMessage(received_message, mask=False)
            self.transport.write(str(msg))


class EchoSite(Site):
    def buildProtocol(self, addr):
        channel = EchoChannel()
        channel.requestFactory = self.requestFactory
        channel.site = self
        return channel

site = EchoSite(EchoResource())

if __name__ == '__main__':
    reactor.listenTCP(8080, site)
    reactor.run()