django.session-middleware
下面是django使用session的例子，参考djangoBook：
def login(request):
if request.method != 'POST':
raise Http404('Only POSTs are allowed')
try:
m = Member.objects.get(username=request.POST['username'])
if m.password == request.POST['password']:
request.session['member_id'] = m.id
return HttpResponseRedirect('/you-are-logged-in/')
except Member.DoesNotExist:
return HttpResponse("Your username and password didn't match.")
通过POST表单，提交username和password。
然后在Member模型中，查找是否匹配。若匹配，则对request.session看作字典赋值，就会建立起session。
下图是django session的处理流程：
我们主要看SessionMiddleware的实现：
class SessionMiddleware(object): def __init__(self): engine = import_module(settings.SESSION_ENGINE) self.SessionStore = engine.SessionStore
这是SessionMiddleware的初始化。从settings模块中读取SESSION_ENGINE的值，默认是'django.contrib.sessions.backends.db'，采用数据库存储。还可以设置别的值，比如'django.contrib.sessions.backends.file'，'django.contrib.sessions.backends.cache'等等。
下面是处理客户端请求：
def process_request(self, request): session_key = request.COOKIES.get(settings.SESSION_COOKIE_NAME, None) request.session = self.SessionStore(session_key)
cookie里面会存settings.SESSION_COOKIE_NAME（默认为'sessionid'）的值。
取出cookie中的值，然后根据session_key获取相应的session_data值。
SessionSrore类是一个包装成字典的类，负责session_data的存储和修改。
在看看如何在response更新session的：
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)
else:
if accessed:
patch_vary_headers(response, ('Cookie',))
if modified or settings.SESSION_SAVE_EVERY_REQUEST:
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:
request.session.save()
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
这里分几种情况讨论：
如果session被清除了，那么客户端的cookie也应该要清除。通过response.delete_cookie( )方法清除。
如果session被修改，并且没有500error， 就会跟新客户端的cookie，通过response.set_cookie( )方法。