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<!DOCTYPE html>
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<h1>WebSockets</h1>
<p>A WebSocket is a <a href="http://tools.ietf.org/html/rfc6455">standard protocol</a> for
two-way data transfer between a client and server. The WebSockets protocol
does not run over HTTP, instead it is a separate implementation on top of
<a href="http://en.wikipedia.org/wiki/Transmission_Control_Protocol">TCP</a>.</p>
<h2>Why use WebSockets?</h2>
<p>A WebSocket connection allows full-duplex communication between a client
and server so that either side can push data to the other through an
established connection. The reason why WebSockets, along with the related
technologies of
<a href="http://en.wikipedia.org/wiki/Server-sent_events">Server-sent Events</a> (SSE)
and
<a href="https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-12">WebRTC data channels</a>,
are important is that HTTP is not meant for keeping open a connection for
the server to frequently push data to a web browser. Previously, most web
applications would implement long polling via frequent
Asynchronous JavaScript and XML (AJAX) requests as shown in the below diagram. </p>
<p><img src="theme/img/ajax-long-polling.png" width="100%" alt="Long polling via AJAX is incredibly inefficient for some applications." class="technical-diagram" /></p>
<p>Server push is more efficient and scalable than long polling because the
web browser does not have to constantly ask for updates through a stream
of AJAX requests.</p>
<p><img src="theme/img/websockets-flow.png" width="100%" alt="WebSockets are more efficient than long polling for server sent updates." class="technical-diagram" /></p>
<p>While the above diagram shows a server pushing data to the client, WebSockets
is a full-duplex connection so the client can also push data to the server
as shown in the diagram below.</p>
<p><img src="theme/img/websockets-flow-with-client-push.png" width="100%" alt="WebSockets also allow client push in addition to server pushed updates." class="technical-diagram" /></p>
<p>The WebSockets approach for server- and client-pushed updates works well for
certain categories of web applications such as chat room, which is why that's
often an example application for a WebSocket library.</p>
<h2>Implementing WebSockets</h2>
<p>Both the web browser and the server must implement the WebSockets protocol
to establish and maintain the connection. There are important implications for
servers since WebSockets connections are long lived, unlike typical HTTP
connections. </p>
<p>A multi-threaded or multi-process based server cannot scale appropriately for
WebSockets because it is designed to open a connection, handle a request as
quickly as possible and then close the connection. An asynchronous server such
as <a href="http://www.tornadoweb.org/en/stable/">Tornado</a> or
<a href="http://gunicorn.org/">Green Unicorn</a> monkey patched with
<a href="http://www.gevent.org/">gevent</a> is necessary for any practical WebSockets
server-side implementation.</p>
<p>On the client side, it is not necessary to use a JavaScript library for
WebSockets. Web browsers that implement WebSockets will expose all necessary
client-side functionality through the
<a href="http://www.w3.org/TR/2011/WD-websockets-20110419/">WebSockets object</a>. </p>
<p>However, a JavaScript wrapper library can make a developer's life easier by
implementing graceful degradation (often falling back to long-polling when
WebSockets are not supported) and by providing a wrapper around
browser-specific WebSocket quirks. Examples of JavaScript client libraries
and Python implementations are found below.</p>
<h2>JavaScript client libraries</h2>
<ul>
<li>
<p><a href="http://socket.io/">Socket.io</a>'s client side JavaScript library can be
used to connect to a server side WebSockets implementation.</p>
</li>
<li>
<p><a href="https://github.com/gimite/web-socket-js">web-socket-js</a> is a Flash-based
client-side WebSockets implementation.</p>
</li>
</ul>
<h2>Python implementations</h2>
<ul>
<li>
<p><a href="http://autobahn.ws/python/">Autobahn</a> uses Twisted or asyncio to implement
the WebSockets protocol.</p>
</li>
<li>
<p><a href="http://crossbar.io/">Crossbar.io</a> builds upon Autobahn and includes a
separate server for handling the WebSockets connections if desired by
the web app developer.</p>
</li>
</ul>
<h2>Nginx WebSocket proxying</h2>
<p>Nginx officially supports WebSocket proxying as of
<a href="http://nginx.com/blog/websocket-nginx/">version 1.3</a>. However, you have
to configure the Upgrade and Connection headers to ensure requests are
passed through Nginx to your WSGI server. It can be tricky to set this up
the first time. </p>
<p>Here are the configuration settings I use in my Nginx file as part of my
WebSockets proxy.</p>
<div class="highlight"><pre><span class="err">#</span> <span class="nt">this</span> <span class="nt">is</span> <span class="nt">where</span> <span class="nt">my</span> <span class="nt">WSGI</span> <span class="nt">server</span> <span class="nt">sits</span> <span class="nt">answering</span> <span class="nt">only</span> <span class="nt">on</span> <span class="nt">localhost</span>
<span class="err">#</span> <span class="nt">usually</span> <span class="nt">this</span> <span class="nt">is</span> <span class="nt">Gunicorn</span> <span class="nt">monkey</span> <span class="nt">patched</span> <span class="nt">with</span> <span class="nt">gevent</span>
<span class="nt">upstream</span> <span class="nt">app_server_wsgiapp</span> <span class="p">{</span>
<span class="n">server</span> <span class="n">localhost</span><span class="o">:</span><span class="m">5000</span> <span class="n">fail_timeout</span><span class="o">=</span><span class="m">0</span><span class="p">;</span>
<span class="p">}</span>
<span class="nt">server</span> <span class="p">{</span>
<span class="err">#</span> <span class="n">typical</span> <span class="n">web</span> <span class="n">server</span> <span class="n">configuration</span> <span class="n">goes</span> <span class="n">here</span>
<span class="err">#</span> <span class="n">this</span> <span class="n">section</span> <span class="n">is</span> <span class="n">specific</span> <span class="n">to</span> <span class="n">the</span> <span class="n">WebSockets</span> <span class="n">proxying</span>
<span class="n">location</span> <span class="o">/</span><span class="n">socket</span><span class="o">.</span><span class="n">io</span> <span class="err">{</span>
<span class="n">proxy_pass</span> <span class="n">http</span><span class="o">://</span><span class="n">app_server_wsgiapp</span><span class="o">/</span><span class="n">socket</span><span class="o">.</span><span class="n">io</span><span class="p">;</span>
<span class="n">proxy_redirect</span> <span class="n">off</span><span class="p">;</span>
<span class="n">proxy_set_header</span> <span class="n">Host</span> <span class="err">$</span><span class="n">host</span><span class="p">;</span>
<span class="n">proxy_set_header</span> <span class="n">X</span><span class="o">-</span><span class="n">Real</span><span class="o">-</span><span class="n">IP</span> <span class="err">$</span><span class="n">remote_addr</span><span class="p">;</span>
<span class="n">proxy_set_header</span> <span class="n">X</span><span class="o">-</span><span class="n">Forwarded</span><span class="o">-</span><span class="n">For</span> <span class="err">$</span><span class="n">proxy_add_x_forwarded_for</span><span class="p">;</span>
<span class="n">proxy_http_version</span> <span class="m">1</span><span class="o">.</span><span class="m">1</span><span class="p">;</span>
<span class="n">proxy_set_header</span> <span class="n">Upgrade</span> <span class="err">$</span><span class="n">http_upgrade</span><span class="p">;</span>
<span class="n">proxy_set_header</span> <span class="n">Connection</span> <span class="s2">"upgrade"</span><span class="p">;</span>
<span class="n">proxy_read_timeout</span> <span class="m">600</span><span class="p">;</span>
<span class="p">}</span>
<span class="err">}</span>
</pre></div>
<p>The following resources are also helpful for setting up the configuration
properly.</p>
<ul>
<li>
<p>Nginx has <a href="http://nginx.org/en/docs/http/websocket.html">an official page for WebSocket proxying</a>.</p>
</li>
<li>
<p><a href="http://blog.martinfjordvald.com/2013/02/websockets-in-nginx/">WebSockets in Nginx</a>
walks through the Nginx WebSockets configuration directives.</p>
</li>
<li>
<p><a href="https://chrislea.com/2013/02/23/proxying-websockets-with-nginx/">Proxying WebSockets with Nginx</a>
shows how to proxy with Socket.io.</p>
</li>
</ul>
<h2>Open source Python examples with WebSockets</h2>
<ul>
<li>
<p>The
<a href="https://github.com/makaimc/python-websockets-example">python-websockets-example</a>
contains code to create a simple web application that provides WebSockets
using Flask, Flask-SocketIO and gevent.</p>
</li>
<li>
<p>The Flask-SocketIO project has a
<a href="https://github.com/miguelgrinberg/Flask-SocketIO/tree/master/example">chat web application</a>
that demos sending server generated events as well as input from users
via a text box input on a form.</p>
</li>
</ul>
<h2>General WebSockets resources</h2>
<ul>
<li>
<p>The official W3C
<a href="http://www.w3.org/TR/websockets/">candidate draft for WebSockets API</a>
and the
<a href="http://dev.w3.org/html5/websockets/">working draft for WebSockets</a> are
good reference material but can be tough for those new to the WebSockets
concepts. I recommend reading the working draft after looking through some
of the more beginner-friendly resources list below.</p>
</li>
<li>
<p><a href="http://lucumr.pocoo.org/2012/9/24/websockets-101/">WebSockets 101</a> by
Armin Ronacher provides a detailed assessment of the subpar state of HTTP
proxying in regards to WebSockets. He also discusses the complexities of
the WebSockets protocol including the packet implementation.</p>
</li>
<li>
<p>The "Can I Use?" website has a
<a href="http://caniuse.com/#feat=websockets">handy WebSockets reference chart</a>
for which web browsers and specific versions support WebSockets.</p>
</li>
<li>
<p>Mozilla's
<a href="https://developer.mozilla.org/en-US/docs/WebSockets">Developer Resources for WebSockets</a>
is a good place to find documentation and tools for developing with
WebSockets.</p>
</li>
<li>
<p><a href="http://websocketd.com/">websocketd</a> is a WebSockets server aiming to be
the "CGI of WebSockets". Worth a look.</p>
</li>
</ul>
<h2>Python-specific WebSockets resources</h2>
<ul>
<li>
<p><a href="http://mrjoes.github.io/2013/06/21/python-realtime.html">Real-time in Python</a>
provides Python-specific context for how the server push updates were
implemented in the past and how Python's tools have evolved to perform
server side updates.</p>
</li>
<li>
<p><a href="https://github.com/aaugustin/websockets">websockets</a> is a WebSockets
implementation for Python 3.3+ written with the
<a href="https://docs.python.org/3.4/library/asyncio.html">asyncio</a> module (or with
<a href="https://code.google.com/p/tulip/">Tulip</a> if you're working with
Python 3.3).</p>
</li>
<li>
<p>The <a href="https://www.twilio.com/blog/2014/11/choose-your-own-adventure-presentations-with-reveal-js-python-and-websockets.html">Choose Your Own Adventure Presentations</a>
tutorial uses WebSockets via gevent on the server and socketio.js for
pushing vote count updates from the server to the client. </p>
</li>
<li>
<p><a href="http://crossbar.io/docs/Adding-Real-Time-to-Django-Applications/">Adding Real Time to Django Applications</a>
shows how to use Django and Crossbar.io to implement a publish/subscribe
feature in the application.</p>
</li>
<li>
<p><a href="http://bottlepy.org/docs/dev/async.html">Async with Bottle</a> shows how to
use greenlets to support WebSockets with the Bottle web framework.</p>
</li>
<li>
<p>If you're deploying to Heroku, there is a
<a href="https://devcenter.heroku.com/articles/python-websockets">specific WebSockets guide</a>
for getting your Python application up and running.</p>
</li>
<li>
<p>The
<a href="http://www.reddit.com/r/Python/comments/2ujqd7/an_overview_of_using_websockets_in_python/">Reddit thread for this page</a>
has some interesting comments on what's missing from the above content that
I'm working to address.</p>
</li>
</ul>
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