Full Stack Python

# WSGI Servers

A Web Server Gateway Interface (WSGI) server implements the web server side of the WSGI interface for running Python web applications. The WSGI standard v1.0 is specified in PEP 0333. As of September 2010, WSGI v1.0 is superseded by PEP 3333, which defines the v1.0.1 WSGI standard.

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:

    # 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;
        }
      }
    }

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 Underwear web server templates on GitHub.

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

- what WSGI stands for (Web Server Gateway Inteface)
- that a WSGI container is a separate running process that runs on a different port than your web server
- 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

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.

## WSGI's Purpose

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

- 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 PEP 3333:

  > 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.

- 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.

## WSGI Resources

PEP 0333 WSGI v1.0 and PEP 3333 WSGI v1.0.1 specifications.

Green Unicorn, mod_wsgi, uWSGI, and gevent are common WSGI server implementations.

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

Complete single server Django stack tutorial is thorough and informative for non-paas hosting choices.

This detailed post entitled The Beautiful Simplicity of an nginx and uWSGI Deployments is great reading for understanding Nginx and uWSGI configurations.

Next read the web frameworks section.