django1/docs/topics/http/sessions.txt

===================
How to use sessions
===================

.. module:: django.contrib.sessions
   :synopsis: Provides session management for Django projects.

Django provides full support for anonymous sessions. The session framework
lets you store and retrieve arbitrary data on a per-site-visitor basis. It
stores data on the server side and abstracts the sending and receiving of
cookies. Cookies contain a session ID -- not the data itself (unless you're
using the :ref:`cookie based backend<cookie-session-backend>`).

Enabling sessions
=================

Sessions are implemented via a piece of :doc:`middleware </ref/middleware>`.

To enable session functionality, do the following:

* Edit the :setting:`MIDDLEWARE` setting and make sure it contains
  ``'django.contrib.sessions.middleware.SessionMiddleware'``. The default
  ``settings.py`` created by ``django-admin startproject`` has
  ``SessionMiddleware`` activated.

If you don't want to use sessions, you might as well remove the
``SessionMiddleware`` line from :setting:`MIDDLEWARE` and
``'django.contrib.sessions'`` from your :setting:`INSTALLED_APPS`.
It'll save you a small bit of overhead.

.. _configuring-sessions:

Configuring the session engine
==============================

By default, Django stores sessions in your database (using the model
``django.contrib.sessions.models.Session``). Though this is convenient, in
some setups it's faster to store session data elsewhere, so Django can be
configured to store session data on your filesystem or in your cache.

Using database-backed sessions
------------------------------

If you want to use a database-backed session, you need to add
``'django.contrib.sessions'`` to your :setting:`INSTALLED_APPS` setting.

Once you have configured your installation, run ``manage.py migrate``
to install the single database table that stores session data.

.. _cached-sessions-backend:

Using cached sessions
---------------------

For better performance, you may want to use a cache-based session backend.

To store session data using Django's cache system, you'll first need to make
sure you've configured your cache; see the :doc:`cache documentation
</topics/cache>` for details.

.. warning::

    You should only use cache-based sessions if you're using the Memcached
    cache backend. The local-memory cache backend doesn't retain data long
    enough to be a good choice, and it'll be faster to use file or database
    sessions directly instead of sending everything through the file or
    database cache backends. Additionally, the local-memory cache backend is
    NOT multi-process safe, therefore probably not a good choice for production
    environments.

If you have multiple caches defined in :setting:`CACHES`, Django will use the
default cache. To use another cache, set :setting:`SESSION_CACHE_ALIAS` to the
name of that cache.

Once your cache is configured, you've got two choices for how to store data in
the cache:

* Set :setting:`SESSION_ENGINE` to
  ``"django.contrib.sessions.backends.cache"`` for a simple caching session
  store. Session data will be stored directly in your cache. However, session
  data may not be persistent: cached data can be evicted if the cache fills
  up or if the cache server is restarted.

* For persistent, cached data, set :setting:`SESSION_ENGINE` to
  ``"django.contrib.sessions.backends.cached_db"``. This uses a
  write-through cache -- every write to the cache will also be written to
  the database. Session reads only use the database if the data is not
  already in the cache.

Both session stores are quite fast, but the simple cache is faster because it
disregards persistence. In most cases, the ``cached_db`` backend will be fast
enough, but if you need that last bit of performance, and are willing to let
session data be expunged from time to time, the ``cache`` backend is for you.

If you use the ``cached_db`` session backend, you also need to follow the
configuration instructions for the `using database-backed sessions`_.

Using file-based sessions
-------------------------

To use file-based sessions, set the :setting:`SESSION_ENGINE` setting to
``"django.contrib.sessions.backends.file"``.

You might also want to set the :setting:`SESSION_FILE_PATH` setting (which
defaults to output from ``tempfile.gettempdir()``, most likely ``/tmp``) to
control where Django stores session files. Be sure to check that your Web
server has permissions to read and write to this location.

.. _cookie-session-backend:

Using cookie-based sessions
---------------------------

To use cookies-based sessions, set the :setting:`SESSION_ENGINE` setting to
``"django.contrib.sessions.backends.signed_cookies"``. The session data will be
stored using Django's tools for :doc:`cryptographic signing </topics/signing>`
and the :setting:`SECRET_KEY` setting.

.. note::

    It's recommended to leave the :setting:`SESSION_COOKIE_HTTPONLY` setting
    on ``True`` to prevent access to the stored data from JavaScript.

.. warning::

    **If the SECRET_KEY is not kept secret and you are using the**
    :class:`~django.contrib.sessions.serializers.PickleSerializer`, **this can
    lead to arbitrary remote code execution.**

    An attacker in possession of the :setting:`SECRET_KEY` can not only
    generate falsified session data, which your site will trust, but also
    remotely execute arbitrary code, as the data is serialized using pickle.

    If you use cookie-based sessions, pay extra care that your secret key is
    always kept completely secret, for any system which might be remotely
    accessible.

    **The session data is signed but not encrypted**

    When using the cookies backend the session data can be read by the client.

    A MAC (Message Authentication Code) is used to protect the data against
    changes by the client, so that the session data will be invalidated when being
    tampered with. The same invalidation happens if the client storing the
    cookie (e.g. your user's browser) can't store all of the session cookie and
    drops data. Even though Django compresses the data, it's still entirely
    possible to exceed the `common limit of 4096 bytes`_ per cookie.

    **No freshness guarantee**

    Note also that while the MAC can guarantee the authenticity of the data
    (that it was generated by your site, and not someone else), and the
    integrity of the data (that it is all there and correct), it cannot
    guarantee freshness i.e. that you are being sent back the last thing you
    sent to the client. This means that for some uses of session data, the
    cookie backend might open you up to `replay attacks`_. Unlike other session
    backends which keep a server-side record of each session and invalidate it
    when a user logs out, cookie-based sessions are not invalidated when a user
    logs out. Thus if an attacker steals a user's cookie, they can use that
    cookie to login as that user even if the user logs out. Cookies will only
    be detected as 'stale' if they are older than your
    :setting:`SESSION_COOKIE_AGE`.

    **Performance**

    Finally, the size of a cookie can have an impact on the `speed of your site`_.

.. _`common limit of 4096 bytes`: https://tools.ietf.org/html/rfc2965#section-5.3
.. _`replay attacks`: https://en.wikipedia.org/wiki/Replay_attack
.. _`speed of your site`: https://yuiblog.com/blog/2007/03/01/performance-research-part-3/

Using sessions in views
=======================

When ``SessionMiddleware`` is activated, each :class:`~django.http.HttpRequest`
object -- the first argument to any Django view function -- will have a
``session`` attribute, which is a dictionary-like object.

You can read it and write to ``request.session`` at any point in your view.
You can edit it multiple times.

.. class:: backends.base.SessionBase

    This is the base class for all session objects. It has the following
    standard dictionary methods:

    .. method:: __getitem__(key)

      Example: ``fav_color = request.session['fav_color']``

    .. method:: __setitem__(key, value)

      Example: ``request.session['fav_color'] = 'blue'``

    .. method:: __delitem__(key)

      Example: ``del request.session['fav_color']``. This raises ``KeyError``
      if the given ``key`` isn't already in the session.

    .. method:: __contains__(key)

      Example: ``'fav_color' in request.session``

    .. method:: get(key, default=None)

      Example: ``fav_color = request.session.get('fav_color', 'red')``

    .. method:: pop(key, default=__not_given)

      Example: ``fav_color = request.session.pop('fav_color', 'blue')``

    .. method:: keys()

    .. method:: items()

    .. method:: setdefault()

    .. method:: clear()

    It also has these methods:

    .. method:: flush()

      Deletes the current session data from the session and deletes the session
      cookie. This is used if you want to ensure that the previous session data
      can't be accessed again from the user's browser (for example, the
      :func:`django.contrib.auth.logout()` function calls it).

    .. method:: set_test_cookie()

      Sets a test cookie to determine whether the user's browser supports
      cookies. Due to the way cookies work, you won't be able to test this
      until the user's next page request. See `Setting test cookies`_ below for
      more information.

    .. method:: test_cookie_worked()

      Returns either ``True`` or ``False``, depending on whether the user's
      browser accepted the test cookie. Due to the way cookies work, you'll
      have to call ``set_test_cookie()`` on a previous, separate page request.
      See `Setting test cookies`_ below for more information.

    .. method:: delete_test_cookie()

      Deletes the test cookie. Use this to clean up after yourself.

    .. method:: get_session_cookie_age()

      .. versionadded:: 3.0

      Returns the age of session cookies, in seconds. Defaults to
      :setting:`SESSION_COOKIE_AGE`.

    .. method:: set_expiry(value)

      Sets the expiration time for the session. You can pass a number of
      different values:

      * If ``value`` is an integer, the session will expire after that
        many seconds of inactivity. For example, calling
        ``request.session.set_expiry(300)`` would make the session expire
        in 5 minutes.

      * If ``value`` is a ``datetime`` or ``timedelta`` object, the
        session will expire at that specific date/time. Note that ``datetime``
        and ``timedelta`` values are only serializable if you are using the
        :class:`~django.contrib.sessions.serializers.PickleSerializer`.

      * If ``value`` is ``0``, the user's session cookie will expire
        when the user's Web browser is closed.

      * If ``value`` is ``None``, the session reverts to using the global
        session expiry policy.

      Reading a session is not considered activity for expiration
      purposes. Session expiration is computed from the last time the
      session was *modified*.

    .. method:: get_expiry_age()

      Returns the number of seconds until this session expires. For sessions
      with no custom expiration (or those set to expire at browser close), this
      will equal :setting:`SESSION_COOKIE_AGE`.

      This function accepts two optional keyword arguments:

      - ``modification``: last modification of the session, as a
        :class:`~datetime.datetime` object. Defaults to the current time.
      - ``expiry``: expiry information for the session, as a
        :class:`~datetime.datetime` object, an :class:`int` (in seconds), or
        ``None``. Defaults to the value stored in the session by
        :meth:`set_expiry`, if there is one, or ``None``.

    .. method:: get_expiry_date()

      Returns the date this session will expire. For sessions with no custom
      expiration (or those set to expire at browser close), this will equal the
      date :setting:`SESSION_COOKIE_AGE` seconds from now.

      This function accepts the same keyword arguments as :meth:`get_expiry_age`.

    .. method:: get_expire_at_browser_close()

      Returns either ``True`` or ``False``, depending on whether the user's
      session cookie will expire when the user's Web browser is closed.

    .. method:: clear_expired()

      Removes expired sessions from the session store. This class method is
      called by :djadmin:`clearsessions`.

    .. method:: cycle_key()

      Creates a new session key while retaining the current session data.
      :func:`django.contrib.auth.login()` calls this method to mitigate against
      session fixation.

.. _session_serialization:

Session serialization
---------------------

By default, Django serializes session data using JSON. You can use the
:setting:`SESSION_SERIALIZER` setting to customize the session serialization
format. Even with the caveats described in :ref:`custom-serializers`, we highly
recommend sticking with JSON serialization *especially if you are using the
cookie backend*.

For example, here's an attack scenario if you use :mod:`pickle` to serialize
session data. If you're using the :ref:`signed cookie session backend
<cookie-session-backend>` and :setting:`SECRET_KEY` is known by an attacker
(there isn't an inherent vulnerability in Django that would cause it to leak),
the attacker could insert a string into their session which, when unpickled,
executes arbitrary code on the server. The technique for doing so is simple and
easily available on the internet. Although the cookie session storage signs the
cookie-stored data to prevent tampering, a :setting:`SECRET_KEY` leak
immediately escalates to a remote code execution vulnerability.

Bundled serializers
~~~~~~~~~~~~~~~~~~~

.. class:: serializers.JSONSerializer

    A wrapper around the JSON serializer from :mod:`django.core.signing`. Can
    only serialize basic data types.

    In addition, as JSON supports only string keys, note that using non-string
    keys in ``request.session`` won't work as expected::

        >>> # initial assignment
        >>> request.session[0] = 'bar'
        >>> # subsequent requests following serialization & deserialization
        >>> # of session data
        >>> request.session[0]  # KeyError
        >>> request.session['0']
        'bar'

    Similarly, data that can't be encoded in JSON, such as non-UTF8 bytes like
    ``'\xd9'`` (which raises :exc:`UnicodeDecodeError`), can't be stored.

    See the :ref:`custom-serializers` section for more details on limitations
    of JSON serialization.

.. class:: serializers.PickleSerializer

    Supports arbitrary Python objects, but, as described above, can lead to a
    remote code execution vulnerability if :setting:`SECRET_KEY` becomes known
    by an attacker.

.. _custom-serializers:

Write your own serializer
~~~~~~~~~~~~~~~~~~~~~~~~~

Note that unlike :class:`~django.contrib.sessions.serializers.PickleSerializer`,
the :class:`~django.contrib.sessions.serializers.JSONSerializer` cannot handle
arbitrary Python data types. As is often the case, there is a trade-off between
convenience and security. If you wish to store more advanced data types
including ``datetime`` and ``Decimal`` in JSON backed sessions, you will need
to write a custom serializer (or convert such values to a JSON serializable
object before storing them in ``request.session``). While serializing these
values is often straightforward
(:class:`~django.core.serializers.json.DjangoJSONEncoder` may be helpful),
writing a decoder that can reliably get back the same thing that you put in is
more fragile. For example, you run the risk of returning a ``datetime`` that
was actually a string that just happened to be in the same format chosen for
``datetime``\s).

Your serializer class must implement two methods,
``dumps(self, obj)`` and ``loads(self, data)``, to serialize and deserialize
the dictionary of session data, respectively.

Session object guidelines
-------------------------

* Use normal Python strings as dictionary keys on ``request.session``. This
  is more of a convention than a hard-and-fast rule.

* Session dictionary keys that begin with an underscore are reserved for
  internal use by Django.

* Don't override ``request.session`` with a new object, and don't access or
  set its attributes. Use it like a Python dictionary.

Examples
--------

This simplistic view sets a ``has_commented`` variable to ``True`` after a user
posts a comment. It doesn't let a user post a comment more than once::

    def post_comment(request, new_comment):
        if request.session.get('has_commented', False):
            return HttpResponse("You've already commented.")
        c = comments.Comment(comment=new_comment)
        c.save()
        request.session['has_commented'] = True
        return HttpResponse('Thanks for your comment!')

This simplistic view logs in a "member" of the site::

    def login(request):
        m = Member.objects.get(username=request.POST['username'])
        if m.password == request.POST['password']:
            request.session['member_id'] = m.id
            return HttpResponse("You're logged in.")
        else:
            return HttpResponse("Your username and password didn't match.")

...And this one logs a member out, according to ``login()`` above::

    def logout(request):
        try:
            del request.session['member_id']
        except KeyError:
            pass
        return HttpResponse("You're logged out.")

The standard :meth:`django.contrib.auth.logout` function actually does a bit
more than this to prevent inadvertent data leakage. It calls the
:meth:`~backends.base.SessionBase.flush` method of ``request.session``.
We are using this example as a demonstration of how to work with session
objects, not as a full ``logout()`` implementation.

Setting test cookies
====================

As a convenience, Django provides a way to test whether the user's browser
accepts cookies. Call the :meth:`~backends.base.SessionBase.set_test_cookie`
method of ``request.session`` in a view, and call
:meth:`~backends.base.SessionBase.test_cookie_worked` in a subsequent view --
not in the same view call.

This awkward split between ``set_test_cookie()`` and ``test_cookie_worked()``
is necessary due to the way cookies work. When you set a cookie, you can't
actually tell whether a browser accepted it until the browser's next request.

It's good practice to use
:meth:`~backends.base.SessionBase.delete_test_cookie()` to clean up after
yourself. Do this after you've verified that the test cookie worked.

Here's a typical usage example::

    from django.http import HttpResponse
    from django.shortcuts import render

    def login(request):
        if request.method == 'POST':
            if request.session.test_cookie_worked():
                request.session.delete_test_cookie()
                return HttpResponse("You're logged in.")
            else:
                return HttpResponse("Please enable cookies and try again.")
        request.session.set_test_cookie()
        return render(request, 'foo/login_form.html')

Using sessions out of views
===========================

.. note::

    The examples in this section import the ``SessionStore`` object directly
    from the ``django.contrib.sessions.backends.db`` backend. In your own code,
    you should consider importing ``SessionStore`` from the session engine
    designated by :setting:`SESSION_ENGINE`, as below:

      >>> from importlib import import_module
      >>> from django.conf import settings
      >>> SessionStore = import_module(settings.SESSION_ENGINE).SessionStore

An API is available to manipulate session data outside of a view::

    >>> from django.contrib.sessions.backends.db import SessionStore
    >>> s = SessionStore()
    >>> # stored as seconds since epoch since datetimes are not serializable in JSON.
    >>> s['last_login'] = 1376587691
    >>> s.create()
    >>> s.session_key
    '2b1189a188b44ad18c35e113ac6ceead'
    >>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')
    >>> s['last_login']
    1376587691

``SessionStore.create()`` is designed to create a new session (i.e. one not
loaded from the session store and with ``session_key=None``). ``save()`` is
designed to save an existing session (i.e. one loaded from the session store).
Calling ``save()`` on a new session may also work but has a small chance of
generating a ``session_key`` that collides with an existing one. ``create()``
calls ``save()`` and loops until an unused ``session_key`` is generated.

If you're using the ``django.contrib.sessions.backends.db`` backend, each
session is a normal Django model. The ``Session`` model is defined in
``django/contrib/sessions/models.py``. Because it's a normal model, you can
access sessions using the normal Django database API::

    >>> from django.contrib.sessions.models import Session
    >>> s = Session.objects.get(pk='2b1189a188b44ad18c35e113ac6ceead')
    >>> s.expire_date
    datetime.datetime(2005, 8, 20, 13, 35, 12)

Note that you'll need to call
:meth:`~base_session.AbstractBaseSession.get_decoded()` to get the session
dictionary. This is necessary because the dictionary is stored in an encoded
format::

    >>> s.session_data
    'KGRwMQpTJ19hdXRoX3VzZXJfaWQnCnAyCkkxCnMuMTExY2ZjODI2Yj...'
    >>> s.get_decoded()
    {'user_id': 42}

When sessions are saved
=======================

By default, Django only saves to the session database when the session has been
modified -- that is if any of its dictionary values have been assigned or
deleted::

    # Session is modified.
    request.session['foo'] = 'bar'

    # Session is modified.
    del request.session['foo']

    # Session is modified.
    request.session['foo'] = {}

    # Gotcha: Session is NOT modified, because this alters
    # request.session['foo'] instead of request.session.
    request.session['foo']['bar'] = 'baz'

In the last case of the above example, we can tell the session object
explicitly that it has been modified by setting the ``modified`` attribute on
the session object::

    request.session.modified = True

To change this default behavior, set the :setting:`SESSION_SAVE_EVERY_REQUEST`
setting to ``True``. When set to ``True``, Django will save the session to the
database on every single request.

Note that the session cookie is only sent when a session has been created or
modified. If :setting:`SESSION_SAVE_EVERY_REQUEST` is ``True``, the session
cookie will be sent on every request.

Similarly, the ``expires`` part of a session cookie is updated each time the
session cookie is sent.

The session is not saved if the response's status code is 500.

.. _browser-length-vs-persistent-sessions:

Browser-length sessions vs. persistent sessions
===============================================

You can control whether the session framework uses browser-length sessions vs.
persistent sessions with the :setting:`SESSION_EXPIRE_AT_BROWSER_CLOSE`
setting.

By default, :setting:`SESSION_EXPIRE_AT_BROWSER_CLOSE` is set to ``False``,
which means session cookies will be stored in users' browsers for as long as
:setting:`SESSION_COOKIE_AGE`. Use this if you don't want people to have to
log in every time they open a browser.

If :setting:`SESSION_EXPIRE_AT_BROWSER_CLOSE` is set to ``True``, Django will
use browser-length cookies -- cookies that expire as soon as the user closes
their browser. Use this if you want people to have to log in every time they
open a browser.

This setting is a global default and can be overwritten at a per-session level
by explicitly calling the :meth:`~backends.base.SessionBase.set_expiry` method
of ``request.session`` as described above in `using sessions in views`_.

.. note::

    Some browsers (Chrome, for example) provide settings that allow users to
    continue browsing sessions after closing and re-opening the browser. In
    some cases, this can interfere with the
    :setting:`SESSION_EXPIRE_AT_BROWSER_CLOSE` setting and prevent sessions
    from expiring on browser close. Please be aware of this while testing
    Django applications which have the
    :setting:`SESSION_EXPIRE_AT_BROWSER_CLOSE` setting enabled.

.. _clearing-the-session-store:

Clearing the session store
==========================

As users create new sessions on your website, session data can accumulate in
your session store. If you're using the database backend, the
``django_session`` database table will grow. If you're using the file backend,
your temporary directory will contain an increasing number of files.

To understand this problem, consider what happens with the database backend.
When a user logs in, Django adds a row to the ``django_session`` database
table. Django updates this row each time the session data changes. If the user
logs out manually, Django deletes the row. But if the user does *not* log out,
the row never gets deleted. A similar process happens with the file backend.

Django does *not* provide automatic purging of expired sessions. Therefore,
it's your job to purge expired sessions on a regular basis. Django provides a
clean-up management command for this purpose: :djadmin:`clearsessions`. It's
recommended to call this command on a regular basis, for example as a daily
cron job.

Note that the cache backend isn't vulnerable to this problem, because caches
automatically delete stale data. Neither is the cookie backend, because the
session data is stored by the users' browsers.

Settings
========

A few :ref:`Django settings <settings-sessions>` give you control over session
behavior:

* :setting:`SESSION_CACHE_ALIAS`
* :setting:`SESSION_COOKIE_AGE`
* :setting:`SESSION_COOKIE_DOMAIN`
* :setting:`SESSION_COOKIE_HTTPONLY`
* :setting:`SESSION_COOKIE_NAME`
* :setting:`SESSION_COOKIE_PATH`
* :setting:`SESSION_COOKIE_SAMESITE`
* :setting:`SESSION_COOKIE_SECURE`
* :setting:`SESSION_ENGINE`
* :setting:`SESSION_EXPIRE_AT_BROWSER_CLOSE`
* :setting:`SESSION_FILE_PATH`
* :setting:`SESSION_SAVE_EVERY_REQUEST`
* :setting:`SESSION_SERIALIZER`

.. _topics-session-security:

Session security
================

Subdomains within a site are able to set cookies on the client for the whole
domain. This makes session fixation possible if cookies are permitted from
subdomains not controlled by trusted users.

For example, an attacker could log into ``good.example.com`` and get a valid
session for their account. If the attacker has control over ``bad.example.com``,
they can use it to send their session key to you since a subdomain is permitted
to set cookies on ``*.example.com``. When you visit ``good.example.com``,
you'll be logged in as the attacker and might inadvertently enter your
sensitive personal data (e.g. credit card info) into the attacker's account.

Another possible attack would be if ``good.example.com`` sets its
:setting:`SESSION_COOKIE_DOMAIN` to ``"example.com"`` which would cause
session cookies from that site to be sent to ``bad.example.com``.

Technical details
=================

* The session dictionary accepts any :mod:`json` serializable value when using
  :class:`~django.contrib.sessions.serializers.JSONSerializer` or any
  picklable Python object when using
  :class:`~django.contrib.sessions.serializers.PickleSerializer`. See the
  :mod:`pickle` module for more information.

* Session data is stored in a database table named ``django_session`` .

* Django only sends a cookie if it needs to. If you don't set any session
  data, it won't send a session cookie.

The ``SessionStore`` object
---------------------------

When working with sessions internally, Django uses a session store object from
the corresponding session engine. By convention, the session store object class
is named ``SessionStore`` and is located in the module designated by
:setting:`SESSION_ENGINE`.

All ``SessionStore`` classes available in Django inherit from
:class:`~backends.base.SessionBase` and implement data manipulation methods,
namely:

* ``exists()``
* ``create()``
* ``save()``
* ``delete()``
* ``load()``
* :meth:`~backends.base.SessionBase.clear_expired`

In order to build a custom session engine or to customize an existing one, you
may create a new class inheriting from :class:`~backends.base.SessionBase` or
any other existing ``SessionStore`` class.

You can extend the session engines, but doing so with database-backed session
engines generally requires some extra effort (see the next section for
details).

.. _extending-database-backed-session-engines:

Extending database-backed session engines
=========================================

Creating a custom database-backed session engine built upon those included in
Django (namely ``db`` and ``cached_db``) may be done by inheriting
:class:`~base_session.AbstractBaseSession` and either ``SessionStore`` class.

``AbstractBaseSession`` and ``BaseSessionManager`` are importable from
``django.contrib.sessions.base_session`` so that they can be imported without
including ``django.contrib.sessions`` in :setting:`INSTALLED_APPS`.

.. class:: base_session.AbstractBaseSession


    The abstract base session model.

    .. attribute:: session_key

        Primary key. The field itself may contain up to 40 characters. The
        current implementation generates a 32-character string (a random
        sequence of digits and lowercase ASCII letters).

    .. attribute:: session_data

        A string containing an encoded and serialized session dictionary.

    .. attribute:: expire_date

        A datetime designating when the session expires.

        Expired sessions are not available to a user, however, they may still
        be stored in the database until the :djadmin:`clearsessions` management
        command is run.

    .. classmethod:: get_session_store_class()

        Returns a session store class to be used with this session model.

    .. method:: get_decoded()

        Returns decoded session data.

        Decoding is performed by the session store class.

You can also customize the model manager by subclassing
:class:`~django.contrib.sessions.base_session.BaseSessionManager`:

.. class:: base_session.BaseSessionManager

    .. method:: encode(session_dict)

        Returns the given session dictionary serialized and encoded as a string.

        Encoding is performed by the session store class tied to a model class.

    .. method:: save(session_key, session_dict, expire_date)

        Saves session data for a provided session key, or deletes the session
        in case the data is empty.

Customization of ``SessionStore`` classes is achieved by overriding methods
and properties described below:

.. class:: backends.db.SessionStore

    Implements database-backed session store.

    .. classmethod:: get_model_class()

        Override this method to return a custom session model if you need one.

    .. method:: create_model_instance(data)

        Returns a new instance of the session model object, which represents
        the current session state.

        Overriding this method provides the ability to modify session model
        data before it's saved to database.

.. class:: backends.cached_db.SessionStore

    Implements cached database-backed session store.

    .. attribute:: cache_key_prefix

        A prefix added to a session key to build a cache key string.

Example
-------

The example below shows a custom database-backed session engine that includes
an additional database column to store an account ID (thus providing an option
to query the database for all active sessions for an account)::

    from django.contrib.sessions.backends.db import SessionStore as DBStore
    from django.contrib.sessions.base_session import AbstractBaseSession
    from django.db import models

    class CustomSession(AbstractBaseSession):
        account_id = models.IntegerField(null=True, db_index=True)

        @classmethod
        def get_session_store_class(cls):
            return SessionStore

    class SessionStore(DBStore):
        @classmethod
        def get_model_class(cls):
            return CustomSession

        def create_model_instance(self, data):
            obj = super().create_model_instance(data)
            try:
                account_id = int(data.get('_auth_user_id'))
            except (ValueError, TypeError):
                account_id = None
            obj.account_id = account_id
            return obj

If you are migrating from the Django's built-in ``cached_db`` session store to
a custom one based on ``cached_db``, you should override the cache key prefix
in order to prevent a namespace clash::

    class SessionStore(CachedDBStore):
        cache_key_prefix = 'mysessions.custom_cached_db_backend'

        # ...

Session IDs in URLs
===================

The Django sessions framework is entirely, and solely, cookie-based. It does
not fall back to putting session IDs in URLs as a last resort, as PHP does.
This is an intentional design decision. Not only does that behavior make URLs
ugly, it makes your site vulnerable to session-ID theft via the "Referer"
header.