How to use sessions — Django documentation
How to use sessions
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).
Enabling sessions
Sessions are implemented via a piece of middleware.
To enable session functionality, do the following:
- Edit the :setting:`MIDDLEWARE` setting and make sure it contains
'django.contrib.sessions.middleware.SessionMiddleware'
. The defaultsettings.py
created bydjango-admin startproject
hasSessionMiddleware
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 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.
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 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. 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.
Using sessions in views
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.
- class backends.base.SessionBase
This is the base class for all session objects. It has the following standard dictionary methods:
- __getitem__(key)
Example:
fav_color = request.session['fav_color']
- __setitem__(key, value)
Example:
request.session['fav_color'] = 'blue'
- __delitem__(key)
Example:
del request.session['fav_color']
. This raisesKeyError
if the givenkey
isn’t already in the session.
- __contains__(key)
Example:
'fav_color' in request.session
- get(key, default=None)
Example:
fav_color = request.session.get('fav_color', 'red')
- pop(key, default=__not_given)
Example:
fav_color = request.session.pop('fav_color', 'blue')
- keys()
- items()
- setdefault()
- clear()
It also has these methods:
- 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 django.contrib.auth.logout() function calls it).
- 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.
- test_cookie_worked()
Returns either
True
orFalse
, depending on whether the user’s browser accepted the test cookie. Due to the way cookies work, you’ll have to callset_test_cookie()
on a previous, separate page request. See Setting test cookies below for more information.
- delete_test_cookie()
Deletes the test cookie. Use this to clean up after yourself.
- 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, callingrequest.session.set_expiry(300)
would make the session expire in 5 minutes.If
value
is adatetime
ortimedelta
object, the session will expire at that specific date/time. Note thatdatetime
andtimedelta
values are only serializable if you are using the PickleSerializer.If
value
is0
, the user’s session cookie will expire when the user’s Web browser is closed.If
value
isNone
, 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.
- 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 adatetime
object. Defaults to the current time.expiry
: expiry information for the session, as adatetime
object, anint
(in seconds), orNone
. Defaults to the value stored in the session by set_expiry(), if there is one, orNone
.
- 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 get_expiry_age().
- get_expire_at_browser_close()
Returns either
True
orFalse
, depending on whether the user’s session cookie will expire when the user’s Web browser is closed.
- clear_expired()
Removes expired sessions from the session store. This class method is called by :djadmin:`clearsessions`.
- cycle_key()
Creates a new session key while retaining the current session data. django.contrib.auth.login() calls this method to mitigate against session fixation.
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 Write your own serializer, 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 pickle
to serialize session data. If you’re using the signed 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 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 raisesUnicodeDecodeError
), can’t be stored.See the Write your own serializer 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.
Write your own serializer
Note that unlike PickleSerializer, the 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 fairly straightforward (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 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.
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 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.
The session is not saved if the response’s status code is 500.
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 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
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 Django settings 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`
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
json
serializable value when using JSONSerializer or any picklable Python object when using PickleSerializer. See thepickle
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 SessionBase and implement data manipulation methods, namely:
exists()
create()
save()
delete()
load()
clear_expired()
In order to build a custom session engine or to customize an existing one, you may create a new class inheriting from SessionBase or any other existing SessionStore
class.
Extending most of the session engines is quite straightforward, but doing so with database-backed session engines generally requires some extra effort (see the next section for details).
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 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.
- 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).
- session_data
A string containing an encoded and serialized session dictionary.
- 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.
- get_decoded()
Returns decoded session data.
Decoding is performed by the session store class.
You can also customize the model manager by subclassing BaseSessionManager:
- class base_session.BaseSessionManager
- 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.
- 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.
- 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.
- 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.