django/docs/ref/middleware.txt

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

.. module:: django.middleware
   :synopsis: Django's built-in middleware classes.

This document explains all middleware components that come with Django. For
information on how to use them and how to write your own middleware, see
the :doc:`middleware usage guide </topics/http/middleware>`.

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

Cache middleware
----------------

.. module:: django.middleware.cache
   :synopsis: Middleware for the site-wide cache.

.. class:: UpdateCacheMiddleware

.. class:: FetchFromCacheMiddleware

Enable the site-wide cache. If these are enabled, each Django-powered page will
be cached for as long as the :setting:`CACHE_MIDDLEWARE_SECONDS` setting
defines. See the :doc:`cache documentation </topics/cache>`.

"Common" middleware
-------------------

.. module:: django.middleware.common
   :synopsis: Middleware adding "common" conveniences for perfectionists.

.. class:: CommonMiddleware

Adds a few conveniences for perfectionists:

* Forbids access to user agents in the :setting:`DISALLOWED_USER_AGENTS`
  setting, which should be a list of compiled regular expression objects.

* Performs URL rewriting based on the :setting:`APPEND_SLASH` and
  :setting:`PREPEND_WWW` settings.

  If :setting:`APPEND_SLASH` is ``True`` and the initial URL doesn't end
  with a slash, and it is not found in the URLconf, then a new URL is
  formed by appending a slash at the end. If this new URL is found in the
  URLconf, then Django redirects the request to this new URL. Otherwise,
  the initial URL is processed as usual.

  For example, ``foo.com/bar`` will be redirected to ``foo.com/bar/`` if
  you don't have a valid URL pattern for ``foo.com/bar`` but *do* have a
  valid pattern for ``foo.com/bar/``.

  If :setting:`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 :setting:`USE_ETAGS` setting. If
  :setting:`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.

.. class:: BrokenLinkEmailsMiddleware

* Sends broken link notification emails to :setting:`MANAGERS` (see
  :doc:`/howto/error-reporting`).

GZip middleware
---------------

.. module:: django.middleware.gzip
   :synopsis: Middleware to serve GZipped content for performance.

.. class:: GZipMiddleware

.. warning::

    Security researchers recently revealed that when compression techniques
    (including ``GZipMiddleware``) are used on a website, the site becomes
    exposed to a number of possible attacks. These approaches can be used to
    compromise, among other things, Django's CSRF protection. Before using
    ``GZipMiddleware`` on your site, you should consider very carefully whether
    you are subject to these attacks. If you're in *any* doubt about whether
    you're affected, you should avoid using ``GZipMiddleware``. For more
    details, see the `the BREACH paper (PDF)`_ and `breachattack.com`_.

    .. _the BREACH paper (PDF): http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf
    .. _breachattack.com: http://breachattack.com

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

This middleware should be placed before any other middleware that need to
read or write the response body so that compression happens afterward.

It will NOT compress content if any of the following are true:

* The content body is less than 200 bytes long.

* The response has already set the ``Content-Encoding`` header.

* The request (the browser) hasn't sent an ``Accept-Encoding`` header
  containing ``gzip``.

* The request is from Internet Explorer and the ``Content-Type`` header
  contains ``javascript`` or starts with anything other than ``text/``.
  We do this to avoid a bug in early versions of IE that caused decompression
  not to be performed on certain content types.

You can apply GZip compression to individual views using the
:func:`~django.views.decorators.gzip.gzip_page()` decorator.

Conditional GET middleware
--------------------------

.. module:: django.middleware.http
   :synopsis: Middleware handling advanced HTTP features.

.. class:: 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
:class:`~django.http.HttpResponseNotModified`.

Also sets the ``Date`` and ``Content-Length`` response-headers.

Locale middleware
-----------------

.. module:: django.middleware.locale
   :synopsis: Middleware to enable language selection based on the request.

.. class:: LocaleMiddleware

Enables language selection based on data from the request. It customizes
content for each user. See the :doc:`internationalization documentation
</topics/i18n/translation>`.

.. attribute:: LocaleMiddleware.response_redirect_class

Defaults to :class:`~django.http.HttpResponseRedirect`. Subclass
``LocaleMiddleware`` and override the attribute to customize the redirects
issued by the middleware.

Message middleware
------------------

.. module:: django.contrib.messages.middleware
   :synopsis: Message middleware.

.. class:: MessageMiddleware

Enables cookie- and session-based message support. See the
:doc:`messages documentation </ref/contrib/messages>`.

Session middleware
------------------

.. module:: django.contrib.sessions.middleware
   :synopsis: Session middleware.

.. class:: SessionMiddleware

Enables session support. See the :doc:`session documentation
</topics/http/sessions>`.

Site middleware
---------------

.. module:: django.contrib.sites.middleware
  :synopsis: Site middleware.

.. class:: CurrentSiteMiddleware

.. versionadded:: 1.7

Adds the ``site`` attribute representing the current site to every incoming
``HttpRequest`` object. See the :ref:`sites documentation <site-middleware>`.

Authentication middleware
-------------------------

.. module:: django.contrib.auth.middleware
  :synopsis: Authentication middleware.

.. class:: AuthenticationMiddleware

Adds the ``user`` attribute, representing the currently-logged-in user, to
every incoming ``HttpRequest`` object. See :ref:`Authentication in Web requests
<auth-web-requests>`.

.. class:: RemoteUserMiddleware

Middleware for utilizing Web server provided authentication. See
:doc:`/howto/auth-remote-user` for usage details.

.. class:: SessionAuthenticationMiddleware

.. versionadded:: 1.7

Allows a user's sessions to be invalidated when their password changes. See
:ref:`session-invalidation-on-password-change` for details. This middleware must
appear after :class:`django.contrib.auth.middleware.AuthenticationMiddleware`
in :setting:`MIDDLEWARE_CLASSES`.

CSRF protection middleware
--------------------------

.. module:: django.middleware.csrf
   :synopsis: Middleware adding protection against Cross Site Request
              Forgeries.

.. class:: CsrfViewMiddleware

Adds protection against Cross Site Request Forgeries by adding hidden form
fields to POST forms and checking requests for the correct value. See the
:doc:`Cross Site Request Forgery protection documentation </ref/contrib/csrf>`.

Transaction middleware
----------------------

.. module:: django.middleware.transaction
   :synopsis: Middleware binding a database transaction to each Web request.

.. class:: TransactionMiddleware

.. versionchanged:: 1.6

    ``TransactionMiddleware`` is deprecated. The documentation of transactions
    contains :ref:`upgrade instructions <transactions-upgrading-from-1.5>`.

Binds commit and rollback of the default database 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 :doc:`transaction management documentation </topics/db/transactions>`.

X-Frame-Options middleware
--------------------------

.. module:: django.middleware.clickjacking
   :synopsis: Clickjacking protection

.. class:: XFrameOptionsMiddleware

Simple :doc:`clickjacking protection via the X-Frame-Options header </ref/clickjacking/>`.

.. _middleware-ordering:

Middleware ordering
===================

Here are some hints about the ordering of various Django middleware classes:

#. :class:`~django.middleware.cache.UpdateCacheMiddleware`

   Before those that modify the ``Vary`` header (``SessionMiddleware``,
   ``GZipMiddleware``, ``LocaleMiddleware``).

#. :class:`~django.middleware.gzip.GZipMiddleware`

   Before any middleware that may change or use the response body.

   After ``UpdateCacheMiddleware``: Modifies ``Vary`` header.

#. :class:`~django.middleware.http.ConditionalGetMiddleware`

   Before ``CommonMiddleware``: uses its ``Etag`` header when
   :setting:`USE_ETAGS` = ``True``.

#. :class:`~django.contrib.sessions.middleware.SessionMiddleware`

   After ``UpdateCacheMiddleware``: Modifies ``Vary`` header.

#. :class:`~django.middleware.locale.LocaleMiddleware`

   One of the topmost, after ``SessionMiddleware`` (uses session data) and
   ``CacheMiddleware`` (modifies ``Vary`` header).

#. :class:`~django.middleware.common.CommonMiddleware`

   Before any middleware that may change the response (it calculates ``ETags``).

   After ``GZipMiddleware`` so it won't calculate an ``ETag`` header on gzipped
   contents.

   Close to the top: it redirects when :setting:`APPEND_SLASH` or
   :setting:`PREPEND_WWW` are set to ``True``.

#. :class:`~django.middleware.csrf.CsrfViewMiddleware`

   Before any view middleware that assumes that CSRF attacks have been dealt
   with.

#. :class:`~django.contrib.auth.middleware.AuthenticationMiddleware`

   After ``SessionMiddleware``: uses session storage.

#. :class:`~django.contrib.messages.middleware.MessageMiddleware`

   After ``SessionMiddleware``: can use session-based storage.

#. :class:`~django.middleware.cache.FetchFromCacheMiddleware`

   After any middleware that modifies the ``Vary`` header: that header is used
   to pick a value for the cache hash-key.

#. :class:`~django.contrib.flatpages.middleware.FlatpageFallbackMiddleware`

   Should be near the bottom as it's a last-resort type of middleware.

#. :class:`~django.contrib.redirects.middleware.RedirectFallbackMiddleware`

   Should be near the bottom as it's a last-resort type of middleware.