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+==========
+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).
+
+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.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.