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 cookie based backend).
Sessions are implemented via a piece of middleware.
To enable session functionality, do the following:
If you don’t want to use sessions, you might as well remove the SessionMiddleware line from MIDDLEWARE_CLASSES and 'django.contrib.sessions' from your INSTALLED_APPS. It’ll save you a small bit of overhead.
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.
If you want to use a database-backed session, you need to add 'django.contrib.sessions' to your INSTALLED_APPS setting.
Once you have configured your installation, run manage.py syncdb to install the single database table that stores session data.
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 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.
If you have multiple caches defined in CACHES, Django will use the default cache. To use another cache, set 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:
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.
To use file-based sessions, set the SESSION_ENGINE setting to "django.contrib.sessions.backends.file".
You might also want to set the 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.
To use cookies-based sessions, set the SESSION_ENGINE setting to "django.contrib.sessions.backends.signed_cookies". The session data will be stored using Django’s tools for cryptographic signing and the SECRET_KEY setting.
Note
It’s recommended to leave the SESSION_COOKIE_HTTPONLY setting 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 SESSION_COOKIE_AGE.
Performance
Finally, the size of a cookie can have an impact on the speed of your site.
When SessionMiddleware is activated, each 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.
This is the base class for all session objects. It has the following standard dictionary methods:
Example: fav_color = request.session['fav_color']
Example: request.session['fav_color'] = 'blue'
Example: del request.session['fav_color']. This raises KeyError if the given key isn’t already in the session.
Example: 'fav_color' in request.session
Example: fav_color = request.session.get('fav_color', 'red')
Example: fav_color = request.session.pop('fav_color')
It also has these methods:
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 django.contrib.auth.logout() function calls it).
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.
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.
Deletes the test cookie. Use this to clean up after yourself.
Sets the expiration time for the session. You can pass a number of different values:
Reading a session is not considered activity for expiration purposes. Session expiration is computed from the last time the session was modified.
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 SESSION_COOKIE_AGE.
This function accepts two optional keyword arguments:
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 SESSION_COOKIE_AGE seconds from now.
This function accepts the same keyword argumets as get_expiry_age().
Returns either True or False, depending on whether the user’s session cookie will expire when the user’s Web browser is closed.
Removes expired sessions from the session store. This class method is called by clearsessions.
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 django.contrib.auth.logout() function actually does a bit more than this to prevent inadvertent data leakage. It calls the 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.
As a convenience, Django provides an easy way to test whether the user’s browser accepts cookies. Just call the set_test_cookie() method of request.session in a view, and call 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 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')
An API is available to manipulate session data outside of a view:
>>> from django.contrib.sessions.backends.db import SessionStore
>>> import datetime
>>> s = SessionStore()
>>> s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10)
>>> s.save()
>>> s.session_key
'2b1189a188b44ad18c35e113ac6ceead'
>>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')
>>> s['last_login']
datetime.datetime(2005, 8, 20, 13, 35, 0)
In order to prevent session fixation attacks, sessions keys that don’t exist are regenerated:
>>> from django.contrib.sessions.backends.db import SessionStore
>>> s = SessionStore(session_key='no-such-session-here')
>>> s.save()
>>> s.session_key
'ff882814010ccbc3c870523934fee5a2'
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}
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 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 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.
You can control whether the session framework uses browser-length sessions vs. persistent sessions with the SESSION_EXPIRE_AT_BROWSER_CLOSE setting.
By default, SESSION_EXPIRE_AT_BROWSER_CLOSE is set to False, which means session cookies will be stored in users’ browsers for as long as SESSION_COOKIE_AGE. Use this if you don’t want people to have to log in every time they open a browser.
If 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 set_expiry() method of request.session as described above in using sessions in views.
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: 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.
A few Django settings give you control over session behavior:
Default: django.contrib.sessions.backends.db
Controls where Django stores session data. Valid values are:
See configuring the session engine for more details.
Default: /tmp/
If you’re using file-based session storage, this sets the directory in which Django will store session data.
Default: None
The domain to use for session cookies. Set this to a string such as ".example.com" (note the leading dot!) for cross-domain cookies, or use None for a standard domain cookie.
Default: True
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.
Default: 'sessionid'
The name of the cookie to use for sessions. This can be whatever you want.
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.
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.
Default: False
Whether to expire the session when the user closes his or her browser. See “Browser-length sessions vs. persistent sessions” above.
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.
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.
Dec 23, 2012