Django Launch and Request Flow

Basic information

Django version: 2.1

Django launch flow

PEP 3333

PEP 3333
- 制定Web Server與Python web applications/frameworks的介面標準
角色
- Web Server/Gatway
  - 呼叫Application/Framework的callable實例，並傳入environ, start_response兩個參數
    - environ:
      - Type: Dict
      - 當前環境的上下文管理變數，為Dict type
    - start_response:
      - Type: Callable
      - 接收两个参数, 分别是响应的状态码
      - 返回Response Body
- Application/Framework
  - 需要實作一個callble的實例，包含函數、方法、類，或帶有__call__的方法
- Middleware
  - 對上級的Web Server或Middleware扮演Application的角色
  - 對下級的Application或Middleware扮演Server的角色

Interaction Diagram

Explanation

manage.py runserver
- 此為每個Django Application的入口，在建立Django Project時，會自動產生
- 重點步驟
  - 指定DJANGO_SETTINGS_MODUL，此為重要配置，Django Project相關配置皆使用該文件設定
  - 使用django.core.management的execute_from_command_line將參數傳遞至Django內部
management @django/core/management/init.py
- 初始化ManagementUtility，並呼叫其execute方法
utility: ManagementUtility @django/core/management/init.py
- execute方法
  - 判斷argv中的輸入，執行對應的動作，現在參數為”runserver”
  - self.fetch_command(subcommand).run_from_argv(self.argv)
    - fetch_command(subcommand)取得Command

Command @django/core/management/commands/runserver.py

run_from_argv(self.argv)
- run_from_argv在Command的父類 @django/core/management/base.py #BaseCommand
execute(self, args, *options) #BaseCommand
handle(self, args, *options) #Command
run(self, **options) #Command

inner_run(self, args, *options) #Command

def inner_run(self, *args, **options):
    # ...
    try:
        # 取得WSGIHandler @django/core/handlers/wsgi.py
        handler = self.get_handler(*args, **options)
        run(self.addr, int(self.port), handler,
            ipv6=self.use_ipv6, threading=threading, server_cls=self.server_cls)
    except socket.error as e:
        # ...

run @django/core/servers/basehttp.py

def run(addr, port, wsgi_handler, ipv6=False, threading=False, server_cls=WSGIServer):
    # ...
    # httpd_cls是WSGIServer @django/core/servers/basehttp.py
    # WSGIRequestHandler @django/core/servers/basehttp.py
    # WSGIRequestHandler最中會指定給WSGIServer的父類BaseServer的self.RequestHandlerClass變數
    httpd = httpd_cls(server_address, WSGIRequestHandler, ipv6=ipv6)
    # ...
    # wsgi_handler由上一步傳入的WSGIHandler @django/core/handlers/wsgi.py
    httpd.set_app(wsgi_handler)
    httpd.serve_forever()

httpd.set_app

1 2	def set_app(self,application): self.application = application

application會在之後處理HTTP request時候使用

httpd.serve_forever
- 父類 BaseServer中的方法
- 不斷接收與處理HTTP request，直到接收到shutdown信號

Djagno with PEP3333

Web Server/Gatway <-> WSGIServer
Application/Framework <-> WSGIHandler

Django Request and Response Flow

Interaction Diagram

Explanation

HTTP request
- 當有HTTP/HTTPS request到達時，WSGIServer的serve_forever方法中的which loop會透過_handle_request_noblock處理request，之後調用內部的process_request，finish_request方法
- 上述方法皆在WSGIServer的父類BaseServer中
  1
  2
  3
  def finish_request(self, request, client_address):
  """Finish one request by instantiating RequestHandlerClass."""
  self.RequestHandlerClass(request, client_address, self)

RequestHandlerClass

在finish_request方法中調用了self.RequestHandlerClass
RequestHandlerClass變量是run @django/core/servers/basehttp.py時傳入，Class為WSGIRequestHandler @django/core/servers/basehttp.py

WSGIRequestHandler在其父類BaseRequestHandler的int方法中呼叫handle方法

class BaseRequestHandler:
    def __init__(self, request, client_address, server):
        # ...
        try:
            self.handle()
        finally:
            # ...

handle方法實作在WSGIRequestHandler內，並呼叫自身的handle_one_request

class WSGIRequestHandler(simple_server.WSGIRequestHandler):
    def handle(self):
        # ...
        handler = ServerHandler(
            self.rfile, self.wfile, self.get_stderr(), self.get_environ()
        )
        handler.request_handler = self      # backpointer for logging
        handler.run(self.server.get_app())

ServerHandler

run方法在其父類BaseHandler中

class BaseHandler:
    def run(self, application):
        # ...
        try:
            self.setup_environ()
            self.result = application(self.environ, self.start_response)
            self.finish_response()
        except:
            # ...
    def finish_response(self):
        try:
            if not self.result_is_file() or not self.sendfile():
                for data in self.result:
                    self.write(data)
                self.finish_content()
        finally:
            self.close()

application(self.environ, self.start_response)
- application為run方法的參數，使用WSGIRequestHandler中的self.server.get_app()取得
self.server.get_app()
- self.server是在WSGIRequestHandler初始化時，其父類的參數
- 參數是在WSGIServer中，使用self傳入，故self.server就是WSGIServer object
- get_app方法對應WSGIServer的set_app方法，是在run @django/core/servers/basehttp.py 傳入，對應的是WSGIHandler
- 故self.server.get_app()回傳的就是WSGIHandler class

WSGIHandler @django/core/handlers/wsgi.py

在application(self.environ, self.start_response)會呼叫init
- 載入middleware

在finish_response方法中的self.result會呼叫WSGIHandler的call

開始處理request，根據順序走訪middleware與開發者定義View，最終回傳response
call是根據PEP3333的Application規定實作，讓WSGIHandler成為callable，並且接收environ, start_response 2個參數

class WSGIHandler(base.BaseHandler):
    request_class = WSGIRequest
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.load_middleware()
    def __call__(self, environ, start_response):
        set_script_prefix(get_script_name(environ))
        signals.request_started.send(sender=self.__class__, environ=environ)
        request = self.request_class(environ)
        response = self.get_response(request)
        # ...
        return response
class BaseHandler:
    def get_response(self, request):
    """Return an HttpResponse object for the given HttpRequest."""
    # Setup default url resolver for this thread
    set_urlconf(settings.ROOT_URLCONF)
    response = self._middleware_chain(request)
    response._closable_objects.append(request)
    if response.status_code >= 400:
        log_response(
            '%s: %s', response.reason_phrase, request.path,
            response=response,
            request=request,
        )
    return response