# Τεκμηρίωση του Django - 1.10.7

# 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 `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 `MIDDLEWARE` and `'django.contrib.sessions'` from your `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 `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.

Προειδοποίηση

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 `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:

- Set `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 `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 `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.

### Using cookie-based sessions

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.

Σημείωση

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

Προειδοποίηση

If the SECRET_KEY is not kept secret and you are using the `PickleSerializer`, this can lead to arbitrary remote code execution.

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

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

The session data is signed but not encrypted

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

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

No freshness guarantee

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

Performance

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

## 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 raises `KeyError` if the given `key` 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` 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.

  `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, calling `request.session.set_expiry(300)` would make the session expire in 5 minutes.
    - If `value` is a `datetime` or `timedelta` object, the session will expire at that specific date/time. Note that `datetime` and `timedelta` values are only serializable if you are using the `PickleSerializer`.
    - If `value` is `0`, the user’s session cookie will expire when the user’s Web browser is closed.
    - If `value` is `None`, the session reverts to using the global session expiry policy.

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

  `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 `SESSION_COOKIE_AGE`.

    This function accepts two optional keyword arguments:

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

## Setting test cookies

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:

    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

Σημείωση

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 `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 `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.

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 `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 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.

Σημείωση

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 `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 `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: `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:

- `SESSION_CACHE_ALIAS`
- `SESSION_COOKIE_AGE`
- `SESSION_COOKIE_DOMAIN`
- `SESSION_COOKIE_HTTPONLY`
- `SESSION_COOKIE_NAME`
- `SESSION_COOKIE_PATH`
- `SESSION_COOKIE_SECURE`
- `SESSION_ENGINE`
- `SESSION_EXPIRE_AT_BROWSER_CLOSE`
- `SESSION_FILE_PATH`
- `SESSION_SAVE_EVERY_REQUEST`
- `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 attackers account.

Another possible attack would be if `good.example.com` sets its `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 the `pickle` module for more information.
- Session data is stored in a database table named `django_session` .
- Django only sends a cookie if it needs to. If you don’t set any session data, it won’t send a session cookie.

### The `SessionStore` object

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