Django中的Session和Cookie

简单来说，Cookie和Session都是为了记录用户相关信息的方式，最大的区别就是Cookie在客户端记录而Session在服务端记录内容。

那么Cookie和Session之间的联系是怎么建立的呢？换言之，当服务器接收到一个请求时候，根据什么来判断读取哪个Session的呢？

对于Django默认情况来说，当用户登录后就可以发现Cookie里有一个sessionid的字段，根据这个key就可以取得在服务器端记录的详细内容。如果将这个字段删除，刷新页面就会发现变成未登录状态了。

对于Session的处理主要在源码django/contrib/sessions/middleware.py中，如下所示：

import time
from importlib import import_module

from django.conf import settings
from django.contrib.sessions.backends.base import UpdateError
from django.core.exceptions import SuspiciousOperation
from django.utils.cache import patch_vary_headers
from django.utils.deprecation import MiddlewareMixin
from django.utils.http import cookie_date


class SessionMiddleware(MiddlewareMixin):
    def __init__(self, get_response=None):
        self.get_response = get_response
        engine = import_module(settings.SESSION_ENGINE)
        self.SessionStore = engine.SessionStore

    def process_request(self, request):
        session_key = request.COOKIES.get(settings.SESSION_COOKIE_NAME)
        request.session = self.SessionStore(session_key)

    def process_response(self, request, response):
        """
        If request.session was modified, or if the configuration is to save the
        session every time, save the changes and set a session cookie or delete
        the session cookie if the session has been emptied.
        """
        try:
            accessed = request.session.accessed
            modified = request.session.modified
            empty = request.session.is_empty()
        except AttributeError:
            pass
        else:
            # First check if we need to delete this cookie.
            # The session should be deleted only if the session is entirely empty
            if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
                response.delete_cookie(
                    settings.SESSION_COOKIE_NAME,
                    path=settings.SESSION_COOKIE_PATH,
                    domain=settings.SESSION_COOKIE_DOMAIN,
                )
            else:
                if accessed:
                    patch_vary_headers(response, ('Cookie',))
                if (modified or settings.SESSION_SAVE_EVERY_REQUEST) and not empty:
                    if request.session.get_expire_at_browser_close():
                        max_age = None
                        expires = None
                    else:
                        max_age = request.session.get_expiry_age()
                        expires_time = time.time() + max_age
                        expires = cookie_date(expires_time)
                    # Save the session data and refresh the client cookie.
                    # Skip session save for 500 responses, refs #3881.
                    if response.status_code != 500:
                        try:
                            request.session.save()
                        except UpdateError:
                            raise SuspiciousOperation(
                                "The request's session was deleted before the "
                                "request completed. The user may have logged "
                                "out in a concurrent request, for example."
                            )
                        response.set_cookie(
                            settings.SESSION_COOKIE_NAME,
                            request.session.session_key, max_age=max_age,
                            expires=expires, domain=settings.SESSION_COOKIE_DOMAIN,
                            path=settings.SESSION_COOKIE_PATH,
                            secure=settings.SESSION_COOKIE_SECURE or None,
                            httponly=settings.SESSION_COOKIE_HTTPONLY or None,
                        )
        return response

当接收到一个请求时候，先在Cookie里取出key，然后根据key创建Session对象，在response时候判断是否要删除或者修改sessionid。

也就是说，Django中如果客户把浏览器Cookie禁用后，用户相关的功能就全都失效了，因为服务端根本没法知道当前用户是谁。

对于这种情况，关键点就是如何把sessionid不使用Cookie传递给客户端，常见的比如放在URL中，也就是URL重写技术。想实现这点可以自己写Middleware。不过django并不建议这么做：

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.