Django middleware analysis

SessionMiddleware

 browser sends a Cookie comprising SESSION_COOKIE_NAME 

middleware django_session from the withdrawn deposit according SESSION_COOKIE_NAME session, 
set the session attribute of the request (SessionStore), comprising _auth_user_id, _auth_user_backend, _auth_user_hash other 
synchronization session and return response back to the browser the cookie status 

Source:

class SessionMiddleware(MiddlewareMixin):
    def __init__(self, get_response=None):
        #初始化session，django启动时执行一次
        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) #从cookie中读取sessionkey
        request.session = self.SessionStore(session_key) #根据sessionkey从django_session中读取session

    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
            # 如果cookie有sessionid并且请求中session是空的，将cookie中的session置空
            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 you changed session and the session is not empty in the request, save session data, and update the cookie returned (except the return status 500) 
                IF (Modified or settings.SESSION_SAVE_EVERY_REQUEST) and  not empty:
                     IF request.session.get_expire_at_browser_close ( ): 
                        the max_age = None 
                        Expires = None
                     the else : 
                        the max_age = request.session.get_expiry_age () 
                        expires_time = the time.time () + the max_age 
                        Expires = http_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,
                            samesite=settings.SESSION_COOKIE_SAMESITE,
                        )
        return response