Real-Time Python Web: Gevent and Socket.io

Real-Time Web: Gevent and Socket.io Rick Copeland @rick446 [email_address]

Getting started with Gevent
AJAX, push, WebSockets, wha?
ZeroMQ for fun and multiprocessing
Putting it all together

A (very) Brief Survey of Python Asynchronous Programming
AsynCore
In stdlib, used for stdlib SMTP server
Nobody cares about it anymore 
Twisted
Large community, vast amounts of code
Callbacks hurt my brain
Stackless
Cool, cooperative multithreading
Needs a custom Python
Event-based green threads
Like stackless, but in regular Python
Know when you yield

Let’s Go Green: Async that Doesn’t Hurt Your Brain
Greenlets: Cooperative, lightweight threads
Very forgiving – mutexes rarely needed
Use it for IO!

Gevent: Greenlets
Spawn helpers
spawn(my_python_function, *args, **kwargs)
Also spawn_later(), spawn_link(), etc.
Greenlet class
Like threads but cooperative
Useful properties: .get(), .join(), .kill(), .link()
Timeouts
Timeout(seconds, exception).start()
Pools: for limiting concurrency, use Pool.spawn

Gevent: Communication
Event
set()
clear()
wait()
Queue
Modeled after Queue.Queue
.get()
.put()
__iter__()
PriorityQueue, LifoQueue, JoinableQueue

Gevent: Networking
“ Green” versions of sockets, select(), ssl, and dns
Quick and dirty:
import gevent.monkey gevent .monkey.patch_all()

Gevent: Servers
Simple callback interface
Creates one greenlet per connection (but remember, that’s OK!)
def handle(socket, address): print 'new connection!’ server = StreamServer( ( '127.0.0.1', 1234), handle) # creates a new server server .start() # start accepting new connections

Gevent: WSGI
gevent.wsgi
Fast (~4k requests/s)
No streaming, pipelining, or ssl 
gevent.pywsgi:
Full featured
Slower (“only” 3k requests/s)
from gevent import pywsgi def hello_world(env, start_response): start_response( '200 OK', [('Content-Type', 'text/html')]) yield '<b>Hello world</b>’ server = pywsgi.WSGIServer( ( '0.0.0.0', 8080), hello_world) server .serve_forever()

What is the real-time web?
No page refreshes
Server push
Examples: chat, realtime analytics, …
Implementation
Flash (eww…)
Polling (2x eww…)
Long polling (wow – that’s clever :-/ )
HTML5 WebSockets ( Super-easy! Awesome! Security vulnerabilities! Immature spec! )

SocketIO to the Rescue “ Socket.IO aims to make realtime apps possible in every browser and mobile device, blurring the differences between the different transport mechanisms.”

Socket.io Example <script src="/socket.io/socket.io.js" ></script> <script> var socket = io.connect( 'http://localhost'); socket.on('news', function (data) { console.log(data); socket.emit( 'my other event’, { my : 'data' }); }); </script>

gevent_socketio def hello_world(environ, start_response): if not environ[ 'PATH_INFO'] .startswith( '/socket.io'): return serve_file(environ, start_response) socketio = environ[ 'socketio'] while True: socketio .send( 'Hello, world') gevent .sleep( 2)

ZeroMQ Overview
C library with Python bindings
ZMQ “sockets” are message based, delivery is via a dedicated communication thread
ZMQ transports (tcp, inproc, unix, multicast)
ZMQ socket types
REQ/RES
PUSH/PULL
PUB/SUB
…

pyzmq and gevent_zmq
pyzmq works great for threading
gevent_zmq is necessary for gevent (single-threaded)
Be careful when forking or otherwise using multiprocessing!
Gevent has a global “hub” of greenlets
ZeroMQ has a global thread & “context” for communication
Best to wait till done forking before initializing ZeroMQ

ZeroMQ: bind/connect and pub/sub
from gevent_zeromq import zmq
context = zmq.Context()
sock_queue = context.socket(zmq.PUB)
sock_queue.bind( 'inproc://chat')
zmq_sock = context.socket(zmq.SUB) zmq_sock.setsockopt(zmq.SUBSCRIBE, "") zmq_sock .connect( 'inproc://chat')

WebChat: Design Incoming Greenlet ZMQ send Outgoing Greenlet Socket.io ZMQ recv JSON Messages JSON Messages Socket.io

WebChat: HTML <h1> Socket.io Chatterbox </h1> <div id="status" style="border:1px solid black;" > Disconnected </div> <form> <input id="input" style="width: 35em;" > </form> <div id="data" style="border:1px solid black;" > </div> <script src="/js/jquery.min.js" ></script> <script src="/js/socket.io.js" ></script> <script src="/js/test.js" ></script>

WebChat: Javascript Setup ( function () { // Create and connect socket var socket = new io.Socket( 'localhost'); socket.connect(); // Socket status var $status = $( '#status'); socket.on('connect', function () { $status.html( '<b>Connected: ' + socket.transport.type + '</b>'); }); socket.on('error', function () { $status.html( '<b>Error</b>'); }); socket.on('disconnect', function () { $status.html( '<b>Closed</b>'); });

WebChat: Javascript Communication // Send data to the server var $form = $( 'form'); var $input = $( '#input'); $form.bind('submit', function () { socket.send($input.val()); $input.val( ''); return false ; }); // Get data back from the server var $data = $( '#data'); socket.on('message', function (msg) { msg = $.parseJSON(msg) ; var u = msg.u || 'SYSTEM’; $data.prepend($( '<em>' + u + '</em>: ' + msg.m + '<br/>')); }); })();

WebChat: Server def chat(environ, start_response): if not environ[ 'PATH_INFO'] .startswith( '/socket.io): return serve_file(environ, start_response) socketio = environ[ 'socketio'] #... handle auth ... zmq_sock = context.socket(zmq.SUB) zmq_sock.setsockopt(zmq.SUBSCRIBE, "") zmq_sock .connect( 'inproc://chat') greenlets = [ gevent.spawn(incoming, uname, socketio), gevent.spawn(outgoing, zmq_sock, socketio) ] gevent .joinall(greenlets)

WebChat: Greenlets def incoming(uname, socketio): while True: for part in socketio .recv(): sock_queue.send(json.dumps( dict( u =uname, m=part))) def outgoing(zmq_sock, socketio): while True: socketio.send(zmq_sock.recv())

Get the Code! Socket.io http://socket.io MIT License Chatterbox http://sf.net/u/rick446/pygotham Apache License ZeroMQ http://www.zeromq.org LGPL License Gevent http://gevent.org MIT License

Rick Copeland @rick446 [email_address]

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Real-Time Python Web: Gevent and Socket.io

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