=================== 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`). Enabling sessions ================= Sessions are implemented via a piece of :doc:`middleware `. To enable session functionality, do the following: * Edit the :setting:`MIDDLEWARE_CLASSES` setting and make sure it contains ``'django.contrib.sessions.middleware.SessionMiddleware'``. The default ``settings.py`` created by ``django-admin.py startproject`` has ``SessionMiddleware`` activated. If you don't want to use sessions, you might as well remove the ``SessionMiddleware`` line from :setting:`MIDDLEWARE_CLASSES` and ``'django.contrib.sessions'`` from your :setting:`INSTALLED_APPS`. It'll save you a small bit of overhead. 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 syncdb`` to install the single database table that stores session data. 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 ` 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. 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 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 --------------------------- .. versionadded:: 1.4 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 ` and the :setting:`SECRET_KEY` setting. .. note:: It's recommended to set the :setting:`SESSION_COOKIE_HTTPONLY` setting to ``True`` to prevent tampering of the stored data from JavaScript. .. warning:: **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`_. 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`: http://tools.ietf.org/html/rfc2965#section-5.3 .. _`replay attacks`: http://en.wikipedia.org/wiki/Replay_attack .. _`speed of your site`: http://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) Example: ``fav_color = request.session.pop('fav_color')`` .. method:: keys .. method:: items .. method:: setdefault .. method:: clear It also has these methods: .. method:: flush Delete the current session data from the session and regenerate the session key value that is sent back to the user in the 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:: 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`. .. 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. .. 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. 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 an easy way to test whether the user's browser accepts cookies. Just 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:: 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_to_response('foo/login_form.html') Using sessions out of views =========================== An API is available to manipulate session data outside of a view:: >>> from django.contrib.sessions.backends.db import SessionStore >>> import datetime >>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead') >>> s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10) >>> s['last_login'] datetime.datetime(2005, 8, 20, 13, 35, 0) >>> s.save() If ``session_key`` isn't provided, one will be generated automatically:: >>> from django.contrib.sessions.backends.db import SessionStore >>> s = SessionStore() >>> s.save() >>> s.session_key '2b1189a188b44ad18c35e113ac6ceead' If you're using the ``django.contrib.sessions.backends.db`` backend, each session is just 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 ``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. 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 his or her 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`_. Clearing the session table ========================== If you're using the database backend, note that session data can accumulate in the ``django_session`` database table and Django does *not* provide automatic purging. Therefore, it's your job to purge expired sessions on a regular basis. To understand this problem, consider what happens when a user uses a session. 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. Django provides a sample clean-up script: ``django-admin.py cleanup``. That script deletes any session in the session table whose ``expire_date`` is in the past -- but your application may have different requirements. Settings ======== A few :doc:`Django settings ` give you control over session behavior: SESSION_ENGINE -------------- Default: ``django.contrib.sessions.backends.db`` Controls where Django stores session data. Valid values are: * ``'django.contrib.sessions.backends.db'`` * ``'django.contrib.sessions.backends.file'`` * ``'django.contrib.sessions.backends.cache'`` * ``'django.contrib.sessions.backends.cached_db'`` * ``'django.contrib.sessions.backends.signed_cookies'`` See `configuring the session engine`_ for more details. SESSION_FILE_PATH ----------------- Default: ``/tmp/`` If you're using file-based session storage, this sets the directory in which Django will store session data. SESSION_COOKIE_AGE ------------------ Default: ``1209600`` (2 weeks, in seconds) The age of session cookies, in seconds. SESSION_COOKIE_DOMAIN --------------------- Default: ``None`` The domain to use for session cookies. Set this to a string such as ``".lawrence.com"`` (note the leading dot!) for cross-domain cookies, or use ``None`` for a standard domain cookie. SESSION_COOKIE_HTTPONLY ----------------------- Default: ``False`` Whether to use HTTPOnly flag on the session cookie. If this is set to ``True``, client-side JavaScript will not to be able to access the session cookie. HTTPOnly_ is a flag included in a Set-Cookie HTTP response header. It is not part of the :rfc:`2109` standard for cookies, and it isn't honored consistently by all browsers. However, when it is honored, it can be a useful way to mitigate the risk of client side script accessing the protected cookie data. .. _HTTPOnly: http://www.owasp.org/index.php/HTTPOnly SESSION_COOKIE_NAME ------------------- Default: ``'sessionid'`` The name of the cookie to use for sessions. This can be whatever you want. SESSION_COOKIE_PATH ------------------- Default: ``'/'`` The path set on the session cookie. This should either match the URL path of your Django installation or be parent of that path. This is useful if you have multiple Django instances running under the same hostname. They can use different cookie paths, and each instance will only see its own session cookie. SESSION_COOKIE_SECURE --------------------- Default: ``False`` Whether to use a secure cookie for the session cookie. If this is set to ``True``, the cookie will be marked as "secure," which means browsers may ensure that the cookie is only sent under an HTTPS connection. SESSION_EXPIRE_AT_BROWSER_CLOSE ------------------------------- Default: ``False`` Whether to expire the session when the user closes his or her browser. See "Browser-length sessions vs. persistent sessions" above. SESSION_SAVE_EVERY_REQUEST -------------------------- Default: ``False`` Whether to save the session data on every request. If this is ``False`` (default), then the session data will only be saved if it has been modified -- that is, if any of its dictionary values have been assigned or deleted. .. _Django settings: ../settings/ Technical details ================= * The session dictionary should accept any pickleable Python object. 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. 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.