Websockets tutorial/example with pywebsocket

As everyone already knows, Google Chrome now supports websockets. In essence, this allows you to keep a connection open with a webserver indefinitely (analogous to typical sockets) and send data bi-directionally. Unfortunately Chrome is the only browser currently supporting this, but I’m pretty sure this will change.

So I decided to give this a try and experiment a bit with it. This is my step by step process on getting a web page opening a websocket to a server and receiving the server’s date and time every second. It is based on an article by Joe Armstrong, though he uses Erlang for the server, while I decided to follow the easy road and use Google’s pywebsocket - an apache module (uses mod_python) that allows you to create handlers for websocket connections in a easy-to-use fashion. The code also contains a way to start a standalone server (i.e. not requiring apache) for testing purposes.

So here are the steps I took to get this working:

1. Create a web page

This is the code I borrowed from Joe, though slightly modified to fit my purposes (it requires jquery, by the way):
<html>

<head>

<script src="jquery-1.3.2.min.js"></script> <script>

$(document).ready(function(){

var ws;

if ("WebSocket" in window) { debug("Horray you have web sockets. Trying to connect..."); ws = new WebSocket("ws://localhost:9998/echo");

ws.onopen = function() { // Web Socket is connected. You can send data by send() method. debug("connected..."); ws.send("hello from the browser"); ws.send("more from browser"); };

run = function() { var val=$("#i1").val(); // read the entry $("#i1").val("");       // and clear it ws.send(val);           // tell erlang return true;            // must do this };

ws.onmessage = function (evt) { //alert(evt.data); var data = evt.data; var i = data.indexOf("!"); var tag = data.slice(0,i); var val = data.slice(i+1); $("#" + tag).html(val); };

ws.onclose = function() { debug(" socket closed"); }; } else { alert("You have no web sockets"); };

function debug(str){ $("#debug").append("<p>" +  str); };

}); </script>

</head>

<body>

<h1>Interaction experiment</h1>

<h2>Debug</h2> <div id="debug"></div>

<fieldset> <legend>Clock</legend> <div id="clock">I am a clock</div> </fieldset>

</body>

</html>

2. Download and install pywebsocket

Checkout the code with
svn checkout http://pywebsocket.googlecode.com/svn/trunk/ pywebsocket-read-only
Then do python setup.py build and sudo python setup.py install inside the src folder. This will install it into your python environment.

3. Being lazy, means we will change an example handler

The way pywebsocket works is delegating the connections to something they call handlers. In the pywebsocket-read-only/src/example folder you will find a file named echowsh.py. They have this convention where handlers are named <entrypoint>_wsh.py. This means that when you later call (from your web page) the url http://localhost:9998/echo the server will delegate the processing of that connection to that file.

I modified that file to something like this:

# Copyright 2009, Google Inc.

All rights reserved.

Redistribution and use in source and binary forms, with or without

modification, are permitted provided that the following conditions are

met:

     * Redistributions of source code must retain the above copyright

notice, this list of conditions and the following disclaimer.

     * Redistributions in binary form must reproduce the above

copyright notice, this list of conditions and the following disclaimer

in the documentation and/or other materials provided with the

distribution.

     * Neither the name of Google Inc. nor the names of its

contributors may be used to endorse or promote products derived from

this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS

"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT

LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR

A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT

OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,

SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT

LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

from mod_pywebsocket import msgutil from datetime import datetime import time

GOODBYEMESSAGE = 'Goodbye'

def websocketdoextrahandshake(request): print 'Connected.' pass  # Always accept.

def websockettransferdata(request): while True: time.sleep(1) date = datetime.now() #try: #    line = msgutil.receivemessage(request) #except Exception, e: #    print 'Foi com os porcos' #    raise e #print 'Got something: %s' % line #msgutil.sendmessage(request, line) msgutil.sendmessage(request, 'clock!%s' % date) #if line == GOODBYEMESSAGE: #    return

So basically whenever a new connection is made to this entry point, a call to websocketdoextrahandshake is made. After that, websockettransferdata is called and it’s your responsibility to create the loop that receives messages and handles the flow (as you can see in the commented lines). I don’t care about that right now, since I only want to push the date and time to the client every second.

3. Start the standalone server

Go to the pywebsocket-read-only/src/mod
pywebsocket folder and run the following command:
sudo python standalone.py -p 9998 -w ../example/
This will start the server in port 9998 and use the handlers directory specified by the -w option. That is where our echo_wsh.py lives.

4. Test it :D

So now open your browser (remember, only chrome supports websockets currently) and open the html file your created in the beginning. Voilá, server’s date and time every second in the clock div.


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