django/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.middleware.doc.XViewMiddleware",
    )

The default admin site has the following ``MIDDLEWARE_CLASSES`` set::

    MIDDLEWARE_CLASSES = (
        "django.middleware.sessions.SessionMiddleware",
        "django.middleware.admin.AdminUserRequired",
        "django.middleware.common.CommonMiddleware",
    )

Django applies middleware in the order it's defined in ``MIDDLEWARE_CLASSES``.

For a regular (i.e., non-admin) Django installation, no middleware is required,
but it's strongly suggested that you use ``CommonMiddleware``. For a Django
admin site, ``SessionMiddleware`` and ``AdminUserRequired`` (in that order) are
required.

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

``django.middleware.admin.AdminUserRequired``
    Limits site access to valid users with the ``is_staff`` flag set. This is
    required by Django's admin, and this middleware requires ``SessionMiddleware``.

``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. 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.

    * Handles flat pages. Every time Django encounters a 404 -- either within
      a view or as a result of no URLconfs matching -- it will check the
      database of flat pages based on the current URL.

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

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

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, param_dict)``

``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.) ``param_dict`` is a dictionary of keyword arguments
that will be passed to the view -- NOT including the first 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``.

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
      default Django middleware classes are in ``django/middleware/`` in the
      Django distribution.

    * 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.