WebSockets in Python
Since the dawn of AJAX, web developers have longed for persistent server-side connections. For a while Comet was hailed as the bastion of “server push”, but deep down we knew it was just a hack. Now finally, years later, we have an API and a protocol being standardized for socket connections between the browser and the server – aptly named, WebSockets.
WebSockets are bi-directional communication channels that run on single TCP sockets allowing communication between the client and the server. Since they behave like regular INET sockets, we should be able to easily implement them with existing tools. However, when I was looking for example implementations in Python, I didn’t find anything that quite satisfied me.
Python sockets module
import socket
websocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
websocket.bind(("localhost", 9999))
websocket.listen(5)That’s all you need to get the WebSocket up and running. Granted, it’s not very useful since you can’t connect to it (no handshake), but it’s a WebSocket nonetheless. When a client connects to the socket, it initiates the handshake with the following
GET / HTTP/1.1
Upgrade: WebSocket
Connection: Upgrade
Host: localhost:9999
Origin: file://
Sec-WebSocket-Key1: d3L703 2 {63 k L1( 90
Sec-WebSocket-Key2: 14 +40Z7R<12om I8 0[
??????????????And expects a response in a similar form:
HTTP/1.1 101 Web Socket Protocol Handshake
Upgrade: WebSocket
Connection: Upgrade
WebSocket-Origin: file://
WebSocket-Location: ws://localhost:9999/
Sec-Websocket-Origin: file://
Sec-Websocket-Location: ws://localhost:9999/
??????????????The “?” are random bits used in the challenge/response part of the handshake. Interesting note: In addition to failing to do the Challenge/Response, Chrome looks for the “Websocket-X” headers, while Safari (correctly) looks for the “Sec-Websocket-X” headers.
Example of WebSocket server: http://gist.github.com/512987
import struct
import socket
import hashlib
import sys
from select import select
import re
import logging
from threading import Thread
import signal
class WebSocket(object):
handshake = (
"HTTP/1.1 101 Web Socket Protocol Handshake\r\n"
"Upgrade: WebSocket\r\n"
"Connection: Upgrade\r\n"
"WebSocket-Origin: %(origin)s\r\n"
"WebSocket-Location: ws://%(bind)s:%(port)s/\r\n"
"Sec-Websocket-Origin: %(origin)s\r\n"
"Sec-Websocket-Location: ws://%(bind)s:%(port)s/\r\n"
"\r\n"
)
def __init__(self, client, server):
self.client = client
self.server = server
self.handshaken = False
self.header = ""
self.data = ""
def feed(self, data):
if not self.handshaken:
self.header += data
if self.header.find('\r\n\r\n') != -1:
parts = self.header.split('\r\n\r\n', 1)
self.header = parts[0]
if self.dohandshake(self.header, parts[1]):
logging.info("Handshake successful")
self.handshaken = True
else:
self.data += data
validated = []
msgs = self.data.split('\xff')
self.data = msgs.pop()
for msg in msgs:
if msg[0] == '\x00':
self.onmessage(msg[1:])
def dohandshake(self, header, key=None):
logging.debug("Begin handshake: %s" % header)
digitRe = re.compile(r'[^0-9]')
spacesRe = re.compile(r'\s')
part_1 = part_2 = origin = None
for line in header.split('\r\n')[1:]:
name, value = line.split(': ', 1)
if name.lower() == "sec-websocket-key1":
key_number_1 = int(digitRe.sub('', value))
spaces_1 = len(spacesRe.findall(value))
if spaces_1 == 0:
return False
if key_number_1 % spaces_1 != 0:
return False
part_1 = key_number_1 / spaces_1
elif name.lower() == "sec-websocket-key2":
key_number_2 = int(digitRe.sub('', value))
spaces_2 = len(spacesRe.findall(value))
if spaces_2 == 0:
return False
if key_number_2 % spaces_2 != 0:
return False
part_2 = key_number_2 / spaces_2
elif name.lower() == "origin":
origin = value
if part_1 and part_2:
logging.debug("Using challenge + response")
challenge = struct.pack('!I', part_1) + struct.pack('!I', part_2) + key
response = hashlib.md5(challenge).digest()
handshake = WebSocket.handshake + response
else:
logging.warning("Not using challenge + response")
handshake = WebSocket.handshake
handshake = handshake % {'origin': origin, 'port': self.server.port,
'bind': self.server.bind}
logging.debug("Sending handshake %s" % handshake)
self.client.send(handshake)
return True
def onmessage(self, data):
logging.info("Got message: %s" % data)
def send(self, data):
logging.info("Sent message: %s" % data)
self.client.send("\x00%s\xff" % data)
def close(self):
self.client.close()
class WebSocketServer(object):
def __init__(self, bind, port, cls):
self.socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.socket.bind((bind, port))
self.bind = bind
self.port = port
self.cls = cls
self.connections = {}
self.listeners = [self.socket]
def listen(self, backlog=5):
self.socket.listen(backlog)
logging.info("Listening on %s" % self.port)
self.running = True
while self.running:
rList, wList, xList = select(self.listeners, [], self.listeners, 1)
for ready in rList:
if ready == self.socket:
logging.debug("New client connection")
client, address = self.socket.accept()
fileno = client.fileno()
self.listeners.append(fileno)
self.connections[fileno] = self.cls(client, self)
else:
logging.debug("Client ready for reading %s" % ready)
client = self.connections[ready].client
data = client.recv(1024)
fileno = client.fileno()
if data:
self.connections[fileno].feed(data)
else:
logging.debug("Closing client %s" % ready)
self.connections[fileno].close()
del self.connections[fileno]
self.listeners.remove(ready)
for failed in xList:
if failed == self.socket:
logging.error("Socket broke")
for fileno, conn in self.connections:
conn.close()
self.running = False
if __name__ == "__main__":
logging.basicConfig(level=logging.DEBUG, format="%(asctime)s - %(levelname)s - %(message)s")
server = WebSocketServer("localhost", 9999, WebSocket)
server_thread = Thread(target=server.listen, args=[5])
server_thread.start()
# Add SIGINT handler for killing the threads
def signal_handler(signal, frame):
logging.info("Caught Ctrl+C, shutting down...")
server.running = False
sys.exit()
signal.signal(signal.SIGINT, signal_handler)
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