===================

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 or

    Redis 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 have to choose between a database-backed

cache or a non-persistent cache.

The cached database backend (``cached_db``) uses a write-through cache --

session writes are applied to both the cache and the database. Session reads

use the cache, or the database if the data has been evicted from the cache. To

use this backend, set :setting:`SESSION_ENGINE` to

``"django.contrib.sessions.backends.cached_db"``, and follow the configuration

instructions for the `using database-backed sessions`_.

The cache backend (``cache``) stores session data only in your cache. This is

faster because it avoids database persistence, but you will have to consider

what happens when cache data is evicted. Eviction can occur if the cache fills

up or the cache server is restarted, and it will mean session data is lost,

including logging out users. To use this backend, set :setting:`SESSION_ENGINE`

to ``"django.contrib.sessions.backends.cache"``.

The cache backend can be made persistent by using a persistent cache, such as

Redis with appropriate configuration. But unless your cache is definitely

configured for sufficient persistence, opt for the cached database backend.

This avoids edge cases caused by unreliable data storage in production.

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`` **or** ``SECRET_KEY_FALLBACKS`` **are not kept

    secret and you are using the**

    ``django.contrib.sessions.serializers.PickleSerializer``, **this can lead

    to arbitrary remote code execution.**

    An attacker in possession of the :setting:`SECRET_KEY` or

    :setting:`SECRET_KEY_FALLBACKS` 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 :rfc:`common limit of 4096 bytes <2965#section-5.3>`

    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.

.. _`replay attacks`: https://en.wikipedia.org/wiki/Replay_attack

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()

      Returns the value of the setting :setting:`SESSION_COOKIE_AGE`. This can

      be overridden in a custom session backend.

    .. 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.

      * 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``.

      .. note::

        This method is used by session backends to determine the session expiry

        age in seconds when saving the session. It is not really intended for

        usage outside of that context.

        In particular, while it is **possible** to determine the remaining

        lifetime of a session **just when** you have the correct

        ``modification`` value **and** the ``expiry`` is set as a ``datetime``

        object, where you do have the ``modification`` value, it is more

        straight-forward to calculate the expiry by-hand::

            expires_at = modification + timedelta(seconds=settings.SESSION_COOKIE_AGE)

    .. 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`, and similar notes on usage apply.

    .. 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` (or any key of

:setting:`SECRET_KEY_FALLBACKS`) 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` or any key of

    :setting:`SECRET_KEY_FALLBACKS` becomes known by an attacker.

    .. deprecated:: 4.1

        Due to the risk of remote code execution, this serializer is deprecated

        and will be removed in Django 5.0.

.. _custom-serializers:

Write your own serializer

~~~~~~~~~~~~~~~~~~~~~~~~~

Note that 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.check_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 reopening 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`.

* 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.