==========
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`: ../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).

It is suggested to place this first in the middleware list, so that the
compression of the response content is the last thing that happens. Will not
compress content bodies less than 200 bytes long, when the response code is
something other than 200, Javascript files (for IE compatibitility), or
responses that have the ``Content-Encoding`` header already specified.

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
----------------------------------------------------

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.middleware.locale.LocaleMiddleware
-----------------------------------------

Enables language selection based on data from the request. It customizes content
for each user. See the `internationalization documentation`_.

.. _`internationalization documentation`: ../i18n/

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

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

.. _`session 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: ../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`: ../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.
