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<!DOCTYPE html>

<title>Full Stack Python: WSGI Servers</title>

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<a href="/">Full Stack Python</a>

</div>

<h1>WSGI Servers</h1>

A <a href="http://wsgi.readthedocs.org/en/latest/">Web Server Gateway Interface</a>

(WSGI) server implements the web server side of the WSGI interface for

running Python web applications.

<h2>Why are WSGI servers necessary?</h2>

A traditional web server does not understand or have any way to run Python

applications. In the late 1990s, a developer named Grisha Trubetskoy

<a href="http://grisha.org/blog/2013/10/25/mod-python-the-long-story/">came up with an Apache module called mod_python</a>

to execute arbitrary Python code. For several years in the late 1990s

and early 2000s, Apache configured with mod_python ran most Python web

applications.

However, mod_python wasn't a actual standard. It was just an implementation

that allowed Python code to run on a server. As mod_python's

development stalled and security vulnerabilities were discovered there

was recognition by the community that a consistent way to execute Python

code for web applications was needed.

Therefore the Python community came up with WSGI as a standard interface that

modules and containers could implement. WSGI is now the accepted approach

for running Python web applications.

<h2>WSGI's Purpose</h2>

Why use WSGI and not just point a web server directly at an application?

<ul>

<li>

WSGI gives you flexibility. Application developers can swap out

web stack components for others. For example, a developer can switch from

Green Unicorn to uWSGI without modifying the application or framework

that implements WSGI.

From <a href="http://www.python.org/dev/peps/pep-3333/">PEP 3333</a>:

The availability and widespread use of such an API in web servers for

Python [...] would separate choice of framework from choice of web

server, freeing users to choose a pairing that suits them, while

freeing framework and server developers to focus on their preferred

area of specialization.

</li>

<li>

WSGI servers promote scaling. Serving thousands of requests for dynamic

content at once is the domain of WSGI servers, not frameworks.

WSGI servers handle processing requests from the web server and deciding

how to communicate those requests to an application framework's process.

The segregation of responsibilities is important for efficiently scaling

web traffic.

</li>

</ul>

<img src="theme/img/web-browser-server-wsgi.png" alt="WSGI Server <-> Web server <-> Browser" width="100%" class="technical-diagram" />

WSGI is by design a simple standard interface for running Python code. As

a web developer you won't need to know much more than

<ul>

<li>

what WSGI stands for (Web Server Gateway Inteface)

</li>

<li>

that a WSGI container is a separate running process that runs on a

different port than your web server

</li>

<li>

your web server is configured to pass requests to the WSGI container which

runs your web application, then pass the response (in the form of HTML)

back to the requester

</li>

</ul>

If you're using a standard web framework such as Django, Flask, or

Bottle, or almost any other current Python framework, you don't need to worry

about how frameworks implement the application side of the WSGI standard.

Likewise, if you're using a standard WSGI container such as Green Unicorn,

uWSGI, mod_wsgi, or gevent, you can get them running without worrying about

how they implement the WSGI standard.

However, knowing the WSGI standard and how these frameworks and containers

implement WSGI should be on your learning checklist though as you become

a more experienced Python web developer.

<h2>Official WSGI specifications</h2>

The WSGI standard v1.0 is specified in

<a href="http://www.python.org/dev/peps/pep-0333/">PEP 0333</a>. As of September 2010,

WSGI v1.0 is superseded by

<a href="http://www.python.org/dev/peps/pep-3333/">PEP 3333</a>, which defines the

v1.0.1 WSGI standard.

<h2>Example configuration</h2>

A web server's configuration specifies what requests should be passed to

the WSGI server to process. Once a request is processed and generated by the

WSGI server, the response is passed back through the web server and onto

the browser.

For example, this Nginx web server's configuration specifics

Nginx should handle static assets (such as images, JavaScript, and CSS

files) under the /static directory and pass all other requests to the WSGI

server running on port 8000:

<div class="codehilite"><pre># this specifies that there is a WSGI server running on port 8000

upstream app_server_djangoapp {

server localhost:8000 fail_timeout=0;

}

# Nginx is set up to run on the standard HTTP port and listen for requests

server {

listen 80;

# nginx should serve up static files and never send to the WSGI server

location /static {

autoindex on;

alias /srv/www/assets;

}

# requests that do not fall under /static are passed on to the WSGI

# server that was specified above running on port 8000

location / {

proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

proxy_set_header Host $http_host;

proxy_redirect off;

if (!-f $request_filename) {

proxy_pass http://app_server_djangoapp;

break;

}

</pre></div>

Note that the above code is a simplified version of a production-ready Nginx

configuration. For real SSL and non-SSL templates, take a look at the

<a href="https://github.com/makaimc/underwear/tree/master/underwear/roles/web/templates">Underwear web server templates</a> on GitHub.

<h2>WSGI Resources</h2>

<ul>

<li>

and

specifications.

</li>

<li>

<a href="http://gunicorn.org/">Green Unicorn</a>,

<a href="http://code.google.com/p/modwsgi/">mod_wsgi</a>,

<a href="https://github.com/unbit/uwsgi-docs">uWSGI</a> and

<a href="http://www.gevent.org/">gevent</a> are common WSGI server implementations.

</li>

<li>

This <a href="http://agiliq.com/blog/2013/07/basics-wsgi/">basics of WSGI</a> post

contains a simple example of how a WSGI-compatible application works.

</li>

<li>

A thorough and informative post for LAMP-stack hosting chocies is

presented in the

"<a href="http://www.apreche.net/complete-single-server-django-stack-tutorial/>">complete single server Django stack tutorial</a>."

</li>

<li>

This detailed post entitled

<a href="http://bartek.im/blog/2012/07/08/simplicity-nginx-uwsgi-deployment.html">The Beautiful Simplicity of an nginx and uWSGI Deployments</a>

is great reading for understanding Nginx and uWSGI configurations.

</li>

<li>

The Python community made a long effort to

<a href="http://blog.dscpl.com.au/2010/05/modpython-project-soon-to-be-officially.html">transition from mod_python</a>

to the WSGI standard. That transition period is now complete and an

implementation of WSGI should always be used instead mod_python.

</li>

<li>

Nicholas Piël wrote an interesting benchmark blog post of

<a href="http://nichol.as/benchmark-of-python-web-servers">Python WSGI servers</a>.

Note that the post is a few years old. Benchmarks should be considered

for their specific tested scenarios and not quickly extrapolated as general

"this server is faster than this other server" results.

</li>

</ul>

Next read the

<a href="/web-frameworks.html">web frameworks</a> section.

</div>

<a href="/introduction.html" class="list-group-item ">Introduction</a>

<a href="/servers.html" class="list-group-item ">Servers</a>

<a href="/operating-systems.html" class="list-group-item ">Operating Systems</a>

<a href="/web-servers.html" class="list-group-item ">Web Servers</a>

<a href="/platform-as-a-service.html" class="list-group-item ">Platform-as-a-service</a>

<a href="/databases.html" class="list-group-item ">Databases</a>

<a href="/wsgi-servers.html" class="list-group-item active">WSGI Servers</a>

<a href="/web-frameworks.html" class="list-group-item ">Web Frameworks</a>

<a href="/application-dependencies.html" class="list-group-item ">Application Dependencies</a>

<a href="/cascading-style-sheets.html" class="list-group-item ">Cascading Style Sheets</a>

<a href="/static-content.html" class="list-group-item ">Static Content</a>

<a href="/source-control.html" class="list-group-item ">Source Control</a>

<a href="/caching.html" class="list-group-item ">Caching</a>

<a href="/task-queues.html" class="list-group-item ">Task Queues</a>

<a href="/api-integration.html" class="list-group-item ">API Integration</a>

<a href="/no-sql-datastore.html" class="list-group-item ">NoSQL Data Stores</a>

<a href="/web-application-security.html" class="list-group-item ">Web Security</a>

<a href="/logging.html" class="list-group-item ">Logging</a>

<a href="/monitoring.html" class="list-group-item ">Monitoring</a>

<a href="/configuration-management.html" class="list-group-item ">Configuration Management</a>

<a href="/web-analytics.html" class="list-group-item ">Web Analytics</a>

<a href="/best-python-resources.html" class="list-group-item ">Best Python Resources</a>

<a href="/about-author.html" class="list-group-item ">About the Author</a>

<a href="/change-log.html" class="list-group-item ">Change Log</a>

</div>

</div></div>

<hr/>

<a href="http://www.mattmakai.com/" class="underline">Matt Makai</a> 2014

</div>

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