root/django/branches/schema-evolution-ng/docs/middleware.txt

==========
Middleware
==========

Middleware is a framework of hooks into Django's request/response processing.
It's a light, low-level "plugin" system for globally altering Django's input
and/or output.

Each middleware component is responsible for doing some specific function. For
example, Django includes a middleware component, ``XViewMiddleware``, that adds
an ``"X-View"`` HTTP header to every response to a ``HEAD`` request.

This document explains all middleware components that come with Django, how to
use them, and how to write your own middleware.

Activating middleware
=====================

To activate a middleware component, add it to the ``MIDDLEWARE_CLASSES`` list
in your Django settings. In ``MIDDLEWARE_CLASSES``, each middleware component
is represented by a string: the full Python path to the middleware's class
name. For example, here's the default ``MIDDLEWARE_CLASSES`` created by
``django-admin.py startproject``::

    MIDDLEWARE_CLASSES = (
        'django.middleware.common.CommonMiddleware',
        'django.contrib.sessions.middleware.SessionMiddleware',
        'django.contrib.auth.middleware.AuthenticationMiddleware',
        'django.middleware.doc.XViewMiddleware',
    )

Django applies middleware in the order it's defined in ``MIDDLEWARE_CLASSES``,
except in the case of response and exception middleware, which is applied in
reverse order.

A Django installation doesn't require any middleware -- e.g.,
``MIDDLEWARE_CLASSES`` can be empty, if you'd like -- but it's strongly
suggested that you use ``CommonMiddleware``.

Available middleware
====================

django.middleware.cache.CacheMiddleware
---------------------------------------

Enables site-wide cache. If this is enabled, each Django-powered page will be
cached for as long as the ``CACHE_MIDDLEWARE_SECONDS`` setting defines. See
the `cache documentation`_.

.. _`cache documentation`: http://www.djangoproject.com/documentation/cache/#the-per-site-cache

django.middleware.common.CommonMiddleware
-----------------------------------------

Adds a few conveniences for perfectionists:

* Forbids access to user agents in the ``DISALLOWED_USER_AGENTS`` setting,
  which should be a list of strings.

* Performs URL rewriting based on the ``APPEND_SLASH`` and ``PREPEND_WWW``
  settings. If ``APPEND_SLASH`` is ``True``, URLs that lack a trailing
  slash will be redirected to the same URL with a trailing slash, unless the
  last component in the path contains a period. So ``foo.com/bar`` is
  redirected to ``foo.com/bar/``, but ``foo.com/bar/file.txt`` is passed
  through unchanged.

  If ``PREPEND_WWW`` is ``True``, URLs that lack a leading "www." will be
  redirected to the same URL with a leading "www."

  Both of these options are meant to normalize URLs. The philosophy is that
  each URL should exist in one, and only one, place. Technically a URL
  ``foo.com/bar`` is distinct from ``foo.com/bar/`` -- a search-engine
  indexer would treat them as separate URLs -- so it's best practice to
  normalize URLs.

* Handles ETags based on the ``USE_ETAGS`` setting. If ``USE_ETAGS`` is set
  to ``True``, Django will calculate an ETag for each request by
  MD5-hashing the page content, and it'll take care of sending
  ``Not Modified`` responses, if appropriate.

django.middleware.doc.XViewMiddleware
-------------------------------------

Sends custom ``X-View`` HTTP headers to HEAD requests that come from IP
addresses defined in the ``INTERNAL_IPS`` setting. This is used by Django's
automatic documentation system.

django.middleware.gzip.GZipMiddleware
-------------------------------------

Compresses content for browsers that understand gzip compression (all modern
browsers).

django.middleware.http.ConditionalGetMiddleware
-----------------------------------------------

Handles conditional GET operations. If the response has a ``ETag`` or
``Last-Modified`` header, and the request has ``If-None-Match`` or
``If-Modified-Since``, the response is replaced by an HttpNotModified.

Also removes the content from any response to a HEAD request and sets the
``Date`` and ``Content-Length`` response-headers.

django.middleware.http.SetRemoteAddrFromForwardedFor
----------------------------------------------------

**New in Django development version**

Sets ``request.META['REMOTE_ADDR']`` based on
``request.META['HTTP_X_FORWARDED_FOR']``, if the latter is set. This is useful
if you're sitting behind a reverse proxy that causes each request's
``REMOTE_ADDR`` to be set to ``127.0.0.1``.

**Important note:** This does NOT validate ``HTTP_X_FORWARDED_FOR``. If you're
not behind a reverse proxy that sets ``HTTP_X_FORWARDED_FOR`` automatically, do
not use this middleware. Anybody can spoof the value of
``HTTP_X_FORWARDED_FOR``, and because this sets ``REMOTE_ADDR`` based on
``HTTP_X_FORWARDED_FOR``, that means anybody can "fake" their IP address. Only
use this when you can absolutely trust the value of ``HTTP_X_FORWARDED_FOR``.

django.contrib.sessions.middleware.SessionMiddleware
----------------------------------------------------

Enables session support. See the `session documentation`_.

.. _`session documentation`: http://www.djangoproject.com/documentation/sessions/

django.contrib.auth.middleware.AuthenticationMiddleware
-------------------------------------------------------

Adds the ``user`` attribute, representing the currently-logged-in user, to
every incoming ``HttpRequest`` object. See `Authentication in Web requests`_.

.. _Authentication in Web requests: http://www.djangoproject.com/documentation/authentication/#authentication-in-web-requests

django.middleware.transaction.TransactionMiddleware
---------------------------------------------------

Binds commit and rollback to the request/response phase. If a view function runs
successfully, a commit is done. If it fails with an exception, a rollback is
done.

The order of this middleware in the stack is important: middleware modules
running outside of it run with commit-on-save - the default Django behavior.
Middleware modules running inside it (coming later in the stack) will be under
the same transaction control as the view functions.

See the `transaction management documentation`_.

.. _`transaction management documentation`: http://www.djangoproject.com/documentation/transactions/

Writing your own middleware
===========================

Writing your own middleware is easy. Each middleware component is a single
Python class that defines one or more of the following methods:

process_request
---------------

Interface: ``process_request(self, request)``

``request`` is an ``HttpRequest`` object. This method is called on each
request, before Django decides which view to execute.

``process_request()`` should return either ``None`` or an ``HttpResponse``
object. If it returns ``None``, Django will continue processing this request,
executing any other middleware and, then, the appropriate view. If it returns
an ``HttpResponse`` object, Django won't bother calling ANY other middleware or
the appropriate view; it'll return that ``HttpResponse``.

process_view
------------

Interface: ``process_view(self, request, view_func, view_args, view_kwargs)``

``request`` is an ``HttpRequest`` object. ``view_func`` is the Python function
that Django is about to use. (It's the actual function object, not the name of
the function as a string.) ``view_args`` is a list of positional arguments that
will be passed to the view, and ``view_kwargs`` is a dictionary of keyword
arguments that will be passed to the view. Neither ``view_args`` nor
``view_kwargs`` include the first view argument (``request``).

``process_view()`` is called just before Django calls the view. It should
return either ``None`` or an ``HttpResponse`` object. If it returns ``None``,
Django will continue processing this request, executing any other
``process_view()`` middleware and, then, the appropriate view. If it returns an
``HttpResponse`` object, Django won't bother calling ANY other middleware or
the appropriate view; it'll return that ``HttpResponse``.

process_response
----------------

Interface: ``process_response(self, request, response)``

``request`` is an ``HttpRequest`` object. ``response`` is the ``HttpResponse``
object returned by a Django view.

``process_response()`` should return an ``HttpResponse`` object. It could alter
the given ``response``, or it could create and return a brand-new
``HttpResponse``.

process_exception
-----------------

Interface: ``process_exception(self, request, exception)``

``request`` is an ``HttpRequest`` object. ``exception`` is an ``Exception``
object raised by the view function.

Django calls ``process_exception()`` when a view raises an exception.
``process_exception()`` should return either ``None`` or an ``HttpResponse``
object. If it returns an ``HttpResponse`` object, the response will be returned
to the browser. Otherwise, default exception handling kicks in.

Guidelines
----------

    * Middleware classes don't have to subclass anything.

    * The middleware class can live anywhere on your Python path. All Django
      cares about is that the ``MIDDLEWARE_CLASSES`` setting includes the path
      to it.

    * Feel free to look at Django's available middleware for examples. The
      core Django middleware classes are in ``django/middleware/`` in the
      Django distribution. The session middleware is in ``django/contrib/sessions``.

    * If you write a middleware component that you think would be useful to
      other people, contribute to the community! Let us know, and we'll
      consider adding it to Django.