.. _ref-middleware:

=============================
Built-in middleware reference
=============================

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

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

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

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

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

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

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

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

    * 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/``.

      **New in Django development version:** The behavior of
      :setting:`APPEND_SLASH` has changed slightly in the development version.
      It didn't used to check whether the pattern was matched in the URLconf.

      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.

View metadata middleware
------------------------

.. module:: django.middleware.doc
   :synopsis: Middleware to help your app self-document.

.. class:: django.middleware.doc.XViewMiddleware

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

GZIP middleware
---------------

.. module:: django.middleware.gzip
   :synopsis: Middleware to serve gziped content for performance.
   
.. class:: 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 compatibility), or
responses that have the ``Content-Encoding`` header already specified.

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

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

.. class:: 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
:class:`~django.http.HttpNotModified`.

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

Reverse proxy middleware
------------------------

.. class:: 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``.

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

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

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

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

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

.. class:: django.contrib.sessions.middleware.SessionMiddleware

Enables session support. See the :ref:`session documentation
<topics-http-sessions>`.

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

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

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

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

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

.. class:: django.contrib.csrf.middleware.CsrfMiddleware

**New in Django development version**

Adds protection against Cross Site Request Forgeries by adding hidden form
fields to POST forms and checking requests for the correct value. See the
:ref:`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:: 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 :ref:`transaction management documentation <topics-db-transactions>`.

