[Python-Dev] PEP 333: Python Web Server Gateway Interface v1.0

[Phillip J. Eby]

(Separately posted to python-list to avoid cross-traffic.)

PEP: 333
Title: Python Web Server Gateway Interface v1.0
Version: $Revision: 1.1 $
Last-Modified: $Date: 2004/08/27 17:30:09 $
Author: Phillip J. Eby <pje at telecommunity.com>
Discussions-To: Python Web-SIG <web-sig at python.org>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 07-Dec-2003
Post-History: 07-Dec-2003, 08-Aug-2004, 20-Aug-2004, 27-Aug-2004


Abstract
========

This document specifies a proposed standard interface between web
servers and Python web applications or frameworks, to promote web
application portability across a variety of web servers.


Rationale and Goals
===================

Python currently boasts a wide variety of web application frameworks,
such as Zope, Quixote, Webware, SkunkWeb, PSO, and Twisted Web -- to
name just a few [1]_.  This wide variety of choices can be a problem
for new Python users, because generally speaking, their choice of web
framework will limit their choice of usable web servers, and vice
versa.

By contrast, although Java has just as many web application frameworks
available, Java's "servlet" API makes it possible for applications
written with any Java web application framework to run in any web
server that supports the servlet API.

The availability and widespread use of such an API in web servers for
Python -- whether those servers are written in Python (e.g. Medusa),
embed Python (e.g. mod_python), or invoke Python via a gateway
protocol (e.g. CGI, FastCGI, etc.) -- 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.

This PEP, therefore, proposes a simple and universal interface between
web servers and web applications or frameworks: the Python Web Server
Gateway Interface (WSGI).

But the mere existence of a WSGI spec does nothing to address the
existing state of servers and frameworks for Python web applications.
Server and framework authors and maintainers must actually implement
WSGI for there to be any effect.

However, since no existing servers or frameworks support WSGI, there
is little immediate reward for an author who implements WSGI support.
Thus, WSGI *must* be easy to implement, so that an author's initial
investment in the interface can be reasonably low.

Thus, simplicity of implementation on *both* the server and framework
sides of the interface is absolutely critical to the utility of the
WSGI interface, and is therefore the principal criterion for any
design decisions.

Note, however, that simplicity of implementation for a framework
author is not the same thing as ease of use for a web application
author.  WSGI presents an absolutely "no frills" interface to the
framework author, because bells and whistles like response objects and
cookie handling would just get in the way of existing frameworks'
handling of these issues.  Again, the goal of WSGI is to facilitate
easy interconnection of existing servers and applications or
frameworks, not to create a new web framework.

Note also that this goal precludes WSGI from requiring anything that
is not already available in deployed versions of Python.  Therefore,
new standard library modules are not proposed or required by this
specification, and nothing in WSGI requires a Python version greater
than 2.2.2.  (It would be a good idea, however, for future versions
of Python to include support for this interface in web servers
provided by the standard library.)

In addition to ease of implementation for existing and future
frameworks and servers, it should also be easy to create request
preprocessors, response postprocessors, and other WSGI-based
"middleware" components that look like an application to their
containing server, while acting as a server for their contained
applications.

If middleware can be both simple and robust, and WSGI is widely
available in servers and frameworks, it allows for the possibility
of an entirely new kind of Python web application framework: one
consisting of loosely-coupled WSGI middleware components.  Indeed,
existing framework authors may even choose to refactor their
frameworks' existing services to be provided in this way, becoming
more like libraries used with WSGI, and less like monolithic
frameworks.  This would then allow application developers to choose
"best-of-breed" components for specific functionality, rather than
having to commit to all the pros and cons of a single framework.

Of course, as of this writing, that day is doubtless quite far off.
In the meantime, it is a sufficient short-term goal for WSGI to
enable the use of any framework with any server.

Finally, it should be mentioned that the current version of WSGI
does not prescribe any particular mechanism for "deploying" an
application for use with a web server or server gateway.  At the
present time, this is necessarily implementation-defined by the
server or gateway.  After a sufficient number of servers and
frameworks have implemented WSGI to provide field experience with
varying deployment requirements, it may make sense to create
another PEP, describing a deployment standard for WSGI servers and
application frameworks.


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