How Python web frameworks, WSGI and CGI fit together

I think Florian's answer answers the part of your question about "what is WSGI", especially if you read the PEP.

As for the questions you pose towards the end:

WSGI, CGI, FastCGI etc. are all protocols for a web server to run code, and deliver the dynamic content that is produced. Compare this to static web serving, where a plain HTML file is basically delivered as is to the client.

CGI, FastCGI and SCGI are language agnostic. You can write CGI scripts in Perl, Python, C, bash, whatever. CGI defines which executable will be called, based on the URL, and how it will be called: the arguments and environment. It also defines how the return value should be passed back to the web server once your executable is finished. The variations are basically optimisations to be able to handle more requests, reduce latency and so on; the basic concept is the same.

WSGI is Python only. Rather than a language agnostic protocol, a standard function signature is defined:

def simple_app(environ, start_response):
    """Simplest possible application object"""
    status = '200 OK'
    response_headers = [('Content-type','text/plain')]
    start_response(status, response_headers)
    return ['Hello world!\n']

That is a complete (if limited) WSGI application. A web server with WSGI support (such as Apache with mod_wsgi) can invoke this function whenever a request arrives.

The reason this is so great is that we can avoid the messy step of converting from a HTTP GET/POST to CGI to Python, and back again on the way out. It's a much more direct, clean and efficient linkage.

It also makes it much easier to have long-running frameworks running behind web servers, if all that needs to be done for a request is a function call. With plain CGI, you'd have to start your whole framework up for each individual request.

To have WSGI support, you'll need to have installed a WSGI module (like mod_wsgi), or use a web server with WSGI baked in (like CherryPy). If neither of those are possible, you could use the CGI-WSGI bridge given in the PEP.

How WSGI, CGI, and the frameworks are all connected?

Apache listens on port 80. It gets an HTTP request. It parses the request to find a way to respond. Apache has a LOT of choices for responding. One way to respond is to use CGI to run a script. Another way to respond is to simply serve a file.

In the case of CGI, Apache prepares an environment and invokes the script through the CGI protocol. This is a standard Unix Fork/Exec situation -- the CGI subprocess inherits an OS environment including the socket and stdout. The CGI subprocess writes a response, which goes back to Apache; Apache sends this response to the browser.

CGI is primitive and annoying. Mostly because it forks a subprocess for every request, and subprocess must exit or close stdout and stderr to signify end of response.

WSGI is an interface that is based on the CGI design pattern. It is not necessarily CGI -- it does not have to fork a subprocess for each request. It can be CGI, but it doesn't have to be.

WSGI adds to the CGI design pattern in several important ways. It parses the HTTP Request Headers for you and adds these to the environment. It supplies any POST-oriented input as a file-like object in the environment. It also provides you a function that will formulate the response, saving you from a lot of formatting details.

What do I need to know / install / do if I want to run a web framework (say web.py or cherrypy) on my basic CGI configuration?

Recall that forking a subprocess is expensive. There are two ways to work around this.

1. Embedded mod_wsgi or mod_python embeds Python inside Apache; no process is forked. Apache runs the Django application directly.

2. Daemon mod_wsgi or mod_fastcgi allows Apache to interact with a separate daemon (or "long-running process"), using the WSGI protocol. You start your long-running Django process, then you configure Apache's mod_fastcgi to communicate with this process.

Note that mod_wsgi can work in either mode: embedded or daemon.

When you read up on mod_fastcgi, you'll see that Django uses flup to create a WSGI-compatible interface from the information provided by mod_fastcgi. The pipeline works like this.

Apache -> mod_fastcgi -> FLUP (via FastCGI protocol) -> Django (via WSGI protocol)

Django has several "django.core.handlers" for the various interfaces.

For mod_fastcgi, Django provides a manage.py runfcgi that integrates FLUP and the handler.

For mod_wsgi, there's a core handler for this.

How to install WSGI support?

Follow these instructions.

https://code.google.com/archive/p/modwsgi/wikis/IntegrationWithDjango.wiki

For background see this

http://docs.djangoproject.com/en/dev/howto/deployment/#howto-deployment-index

You can run WSGI over CGI as Pep333 demonstrates as an example. However every time there is a request a new Python interpreter is started and the whole context (database connections, etc.) needs to be build which all take time.

The best if you want to run WSGI would be if your host would install mod_wsgi and made an appropriate configuration to defer control to an application of yours.

Flup is another way to run with WSGI for any webserver that can speak FCGI, SCGI or AJP. From my experience only FCGI really works, and it can be used in Apache either via mod_fastcgi or if you can run a separate Python daemon with mod_proxy_fcgi.

WSGI is a protocol much like CGI, which defines a set of rules how webserver and Python code can interact, it is defined as Pep333. It makes it possible that many different webservers can use many different frameworks and applications using the same application protocol. This is very beneficial and makes it so useful.