為什么引入web框架

   web應用的本質

• 瀏覽器發送一個HTTP請求；
• 服務器收到請求，生成一個HTML文檔；
• 服務器把HTML文檔作為HTTP響應的Body發送給瀏覽器；
• 瀏覽器收到HTTP響應，從HTTP Body取出HTML文檔并顯示；

   涉及的問題  

• 解析http請求
• 找到對應的處理函數
• 生成并發送http響應

web框架工作流程

中間件

• 中間件是請求或者應用開始和結束時注入代碼的機制
• 常見的web中間件： 鑒權、打印log、session、統計信息、處理數據庫連接  等等

golang http 服務端編程

• golang 的net/http包提供了http編程的相關接口，封裝了內部TCP連接和報文解析的復雜瑣碎的細節 
  go c.serve(ctx)最終會啟動goroutine處理請求
• 使用者只需要和http.request 和 http.ResponseWriter 兩個對象交互就行(也就是一個struct 實現net/http包中的Handler interface中的 ServeHttp方法 )

//E:\Go\src\net\http\server.go +82 
type Handler interface {
	ServeHTTP(ResponseWriter, *Request)
}

//純 net.http包的server方法
package main

import (
    "io"
    "net/http"
)

type helloHandler struct{}

func (h *helloHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Hello, world!"))
}

func main() {
    http.Handle("/", &helloHandler{})
    http.ListenAndServe(":12345", nil)
}

//////////////////////////////////////////////////////////////////

import (
    "net/http"
)

type Handle struct{}

func (h Handle) ServeHTTP(response http.ResponseWriter, request *http.Request) {
    switch request.URL.Path {
    case "/info":
        response.Write([]byte("info"))
    default:

    }
}

func main() {
    http.ListenAndServe(":8888", Handle{})
}

• net/http 的另外一個重要的概念  ，ServeMux （多路傳輸）：ServeMux 可以注冊多了 URL 和 handler 的對應關系，并自動把請求轉發到對應的 handler 進行處理

• // 下面看一個帶路由的http server
  package main
  
  import (
  	"io"
  	"net/http"
  )
  
  func helloHandler(w http.ResponseWriter, r *http.Request) {
  	io.WriteString(w, "Hello, world!\n")
  }
  
  func echoHandler(w http.ResponseWriter, r *http.Request) {
  	io.WriteString(w, r.URL.Path)
  }
  
  func main() {
  	mux := http.NewServeMux()
  	mux.HandleFunc("/hello", helloHandler)
  	mux.HandleFunc("/", echoHandler)
  
  	http.ListenAndServe(":12345", mux)
  }
  
  // 其實ServeMux 也實現了Handler的ServeHTTP方法所以也是Handler接口
  //E:\Go\src\net\http\server.go 2382
  func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
  	if r.RequestURI == "*" {
  		if r.ProtoAtLeast(1, 1) {
  			w.Header().Set("Connection", "close")
  		}
  		w.WriteHeader(StatusBadRequest)
  		return
  	}
  	h, _ := mux.Handler(r)
  	h.ServeHTTP(w, r)
  }
  
  
  // E:\Go\src\net\http\server.go +2219可以看到 net/http包中的 基于map 路由查找
  // Find a handler on a handler map given a path string.
  // Most-specific (longest) pattern wins.
  func (mux *ServeMux) match(path string) (h Handler, pattern string) {
  	// Check for exact match first.
  	v, ok := mux.m[path]
  	if ok {
  		return v.h, v.pattern
  	}
  
  	// Check for longest valid match.
  	var n = 0
  	for k, v := range mux.m {
  		if !pathMatch(k, path) {
  			continue
  		}
  		if h == nil || len(k) > n {
  			n = len(k)
  			h = v.h
  			pattern = v.pattern
  		}
  	}
  	return
  }
  

   

golang web框架 GIN golang gin web框架

//gin框架初始化的流程
1.初始化engine 
2.注冊中間件
3.注冊路由

//響應流程
1.路由，找到handle
2.將請求和響應用Context包裝起來供業務代碼使用
3.依次調用中間件和處理函數
4.輸出結果

//gin 里面最重要的兩個數據結構
//1.Context在中間件中傳遞本次請求的各種數據、管理流程，進行響應
//2.Engine gin 引擎，是框架的實例，它包含多路復用器，中間件和配置設置




// 下面看下open-falcon-api中的實際應用
//open-falcon-api里面注冊路由和中間件
//E:\go_path\src\github.com\open-falcon\falcon-plus\modules\api\app\controller\graph\graph_routes.go
// 首先注冊/api/v1/開頭的path的路由組
// 然后Use 一個auth的中間件 ，作用是檢查token
// 這個組后面的所有path 都使用這個中間件 
// 也就是訪問 /graph/endpoint 時會過 3個中間件(log recovery  auth )+一個EndpointRegexpQuery處理函數
// 
func Routes(r *gin.Engine) {

	db = config.Con()
	authapi := r.Group("/api/v1")
	authapi.Use(utils.AuthSessionMidd)
	authapi.GET("/graph/endpointobj", EndpointObjGet)
	authapi.GET("/graph/endpoint", EndpointRegexpQuery)
	authapi.GET("/graph/endpoint_counter", EndpointCounterRegexpQuery)
	authapi.POST("/graph/history", QueryGraphDrawData)
	authapi.POST("/graph/lastpoint", QueryGraphLastPoint)
	authapi.POST("/graph/info", QueryGraphItemPosition)
	authapi.DELETE("/graph/endpoint", DeleteGraphEndpoint)
	authapi.DELETE("/graph/counter", DeleteGraphCounter)

	grfanaapi := r.Group("/api")
	grfanaapi.GET("/v1/grafana", GrafanaMainQuery)
	grfanaapi.GET("/v1/grafana/metrics/find", GrafanaMainQuery)
	grfanaapi.POST("/v1/grafana/render", GrafanaRender)
	grfanaapi.GET("/v1/grafana/render", GrafanaRender)

}


func AuthSessionMidd(c *gin.Context) {
	auth, err := h.SessionChecking(c)
	if !viper.GetBool("skip_auth") {
		if err != nil || auth != true {
			log.Debugf("error: %v, auth: %v", err, auth)
			c.Set("auth", auth)
			h.JSONR(c, http.StatusUnauthorized, err)
			c.Abort()
			return
		}
	}
	c.Set("auth", auth)
}


// E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\context.go +715 最后會調用這里的Render方法
// Render writes the response headers and calls render.Render to render data.
func (c *Context) Render(code int, r render.Render) {
	c.Status(code)

	if !bodyAllowedForStatus(code) {
		r.WriteContentType(c.Writer)
		c.Writer.WriteHeaderNow()
		return
	}

	if err := r.Render(c.Writer); err != nil {
		panic(err)
	}
}

// 可以看到這里的bind 是在gin在解析請求payload是否和函數中要求的struct一致
// E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\context.go +504
// bind會根據請求中的Content-Type header判斷是json 還是xml
// 并且根據struct中的tag通過反射解析payload

// Bind checks the Content-Type to select a binding engine automatically,
// Depending the "Content-Type" header different bindings are used:
//     "application/json" --> JSON binding
//     "application/xml"  --> XML binding
// otherwise --> returns an error.
// It parses the request's body as JSON if Content-Type == "application/json" using JSON or XML as a JSON input.
// It decodes the json payload into the struct specified as a pointer.
// It writes a 400 error and sets Content-Type header "text/plain" in the response if input is not valid.
func (c *Context) Bind(obj interface{}) error {
	b := binding.Default(c.Request.Method, c.ContentType())
	return c.MustBindWith(obj, b)
}


type APIEndpointObjGetInputs struct {
	Endpoints []string `json:"endpoints" form:"endpoints"`
	Deadline  int64    `json:"deadline" form:"deadline"`
}


func EndpointObjGet(c *gin.Context) {
	inputs := APIEndpointObjGetInputs{
		Deadline: 0,
	}
	if err := c.Bind(&inputs); err != nil {
		h.JSONR(c, badstatus, err)
		return
	}
	if len(inputs.Endpoints) == 0 {
		h.JSONR(c, http.StatusBadRequest, "endpoints missing")
		return
	}

	var result []m.Endpoint = []m.Endpoint{}
	dt := db.Graph.Table("endpoint").
		Where("endpoint in (?) and ts >= ?", inputs.Endpoints, inputs.Deadline).
		Scan(&result)
	if dt.Error != nil {
		h.JSONR(c, http.StatusBadRequest, dt.Error)
		return
	}

	endpoints := []map[string]interface{}{}
	for _, r := range result {
		endpoints = append(endpoints, map[string]interface{}{"id": r.ID, "endpoint": r.Endpoint, "ts": r.Ts})
	}

	h.JSONR(c, endpoints)
}

//E:\go_path\src\github.com\open-falcon\falcon-plus\modules\api\main.go  +78
//初始化gin
routes := gin.Default()

//E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\gin.go +148
// Default returns an Engine instance with the Logger and Recovery middleware already attached.
func Default() *Engine {
	debugPrintWARNINGDefault()
	engine := New()
	engine.Use(Logger(), Recovery())
	return engine
}

//E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\gin.go +119 
// new方法 返回一個不帶中間件的 單例engine ，并且把context 放在池中
func New() *Engine {
	debugPrintWARNINGNew()
	engine := &Engine{
		RouterGroup: RouterGroup{
			Handlers: nil,
			basePath: "/",
			root:     true,
		},
		FuncMap:                template.FuncMap{},
		RedirectTrailingSlash:  true,
		RedirectFixedPath:      false,
		HandleMethodNotAllowed: false,
		ForwardedByClientIP:    true,
		AppEngine:              defaultAppEngine,
		UseRawPath:             false,
		UnescapePathValues:     true,
		MaxMultipartMemory:     defaultMultipartMemory,
		trees:                  make(methodTrees, 0, 9),
		delims:                 render.Delims{Left: "{{", Right: "}}"},
		secureJsonPrefix:       "while(1);",
	}
	engine.RouterGroup.engine = engine
	engine.pool.New = func() interface{} {
		return engine.allocateContext()
	}
	return engine
}


//E:\go_path\src\github.com\open-falcon\falcon-plus\modules\api\app\controller\routes.go
//r.Run(port) 最后調用的是  net.http.ListenAndServe
func (engine *Engine) Run(addr ...string) (err error) {
	defer func() { debugPrintError(err) }()

	address := resolveAddress(addr)
	debugPrint("Listening and serving HTTP on %s\n", address)
	err = http.ListenAndServe(address, engine)
	return
}




//E:\Go\src\net\http\server.go +2686
func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
	handler := sh.srv.Handler
	if handler == nil {
		handler = DefaultServeMux
	}
	if req.RequestURI == "*" && req.Method == "OPTIONS" {
		handler = globalOptionsHandler{}
	}
	handler.ServeHTTP(rw, req)
}

//E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\gin.go +321
//我們可以看到 在gin中實現了ServeHTTP方法  net.http.Handler

// ServeHTTP conforms to the http.Handler interface. 
// 這里使用sync.pool cache context數據結構，避免頻繁GC，每次使用都初始化
//一個struct實現了 interface中的方法 就說明這個struct是這個類型
func (engine *Engine) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	c := engine.pool.Get().(*Context)
	c.writermem.reset(w)
	c.Request = req
	c.reset()

	engine.handleHTTPRequest(c)
    
	engine.pool.Put(c)
}



// gin 里面處理請求的核心方法
// 根據一些配置去 壓縮前綴樹 radix.tree中找到對應的handleChain 然后執行
// 注意這句handlers, params, tsr := root.getValue(path, c.Params, unescape)
// 路由查找的過程是 從基數樹的根節點一直匹配到和請求一致的節點找到對應的handlerchain
// 注冊路由 E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\gin.go +243 
// 從下面的addRoute方法中可以看到gin 為每個HttpMethod  GET POST PUT DELETE都注冊了一顆tree
// 并且有priority 即最長的路徑優先匹配
/*
func (engine *Engine) addRoute(method, path string, handlers HandlersChain) {
	assert1(path[0] == '/', "path must begin with '/'")
	assert1(method != "", "HTTP method can not be empty")
	assert1(len(handlers) > 0, "there must be at least one handler")

	debugPrintRoute(method, path, handlers)
	root := engine.trees.get(method)
	if root == nil {
		root = new(node)
		engine.trees = append(engine.trees, methodTree{method: method, root: root})
	}
	root.addRoute(path, handlers)
}
*/


func (engine *Engine) handleHTTPRequest(c *Context) {
	httpMethod := c.Request.Method
	path := c.Request.URL.Path
	unescape := false
	if engine.UseRawPath && len(c.Request.URL.RawPath) > 0 {
		path = c.Request.URL.RawPath
		unescape = engine.UnescapePathValues
	}

	// Find root of the tree for the given HTTP method
	t := engine.trees
	for i, tl := 0, len(t); i < tl; i++ {
		if t[i].method != httpMethod {
			continue
		}
		root := t[i].root
		// Find route in tree
		handlers, params, tsr := root.getValue(path, c.Params, unescape)
		if handlers != nil {
			c.handlers = handlers
			c.Params = params
			c.Next()
			c.writermem.WriteHeaderNow()
			return
		}
		if httpMethod != "CONNECT" && path != "/" {
			if tsr && engine.RedirectTrailingSlash {
				redirectTrailingSlash(c)
				return
			}
			if engine.RedirectFixedPath && redirectFixedPath(c, root, engine.RedirectFixedPath) {
				return
			}
		}
		break
	}

	if engine.HandleMethodNotAllowed {
		for _, tree := range engine.trees {
			if tree.method == httpMethod {
				continue
			}
			if handlers, _, _ := tree.root.getValue(path, nil, unescape); handlers != nil {
				c.handlers = engine.allNoMethod
				serveError(c, http.StatusMethodNotAllowed, default405Body)
				return
			}
		}
	}
	c.handlers = engine.allNoRoute
	serveError(c, http.StatusNotFound, default404Body)
}

python django  （django框架復雜，先簡單看一下）

# 入口文件 
def execute_from_command_line(argv=None):
    """
    A simple method that runs a ManagementUtility.
    """
    utility = ManagementUtility(argv)
    utility.execute()


def execute(self):
        """
        Given the command-line arguments, figure out which subcommand is being
        run, create a parser appropriate to that command, and run it.
        """
        try:
            subcommand = self.argv[1]
        except IndexError:
            subcommand = 'help'  # Display help if no arguments were given.

        # Preprocess options to extract --settings and --pythonpath.
        # These options could affect the commands that are available, so they
        # must be processed early.
        parser = CommandParser(usage='%(prog)s subcommand [options] [args]', add_help=False, allow_abbrev=False)
        parser.add_argument('--settings')
        parser.add_argument('--pythonpath')
        parser.add_argument('args', nargs='*')  # catch-all
        try:
            options, args = parser.parse_known_args(self.argv[2:])
            handle_default_options(options)
        except CommandError:
            pass  # Ignore any option errors at this point.

        try:
            settings.INSTALLED_APPS
        except ImproperlyConfigured as exc:
            self.settings_exception = exc
        except ImportError as exc:
            self.settings_exception = exc

        if settings.configured:
            # Start the auto-reloading dev server even if the code is broken.
            # The hardcoded condition is a code smell but we can't rely on a
            # flag on the command class because we haven't located it yet.
            if subcommand == 'runserver' and '--noreload' not in self.argv:
                try:
                    autoreload.check_errors(django.setup)()
                except Exception:
                    # The exception will be raised later in the child process
                    # started by the autoreloader. Pretend it didn't happen by
                    # loading an empty list of applications.
                    apps.all_models = defaultdict(OrderedDict)
                    apps.app_configs = OrderedDict()
                    apps.apps_ready = apps.models_ready = apps.ready = True

                    # Remove options not compatible with the built-in runserver
                    # (e.g. options for the contrib.staticfiles' runserver).
                    # Changes here require manually testing as described in
                    # #27522.
                    _parser = self.fetch_command('runserver').create_parser('django', 'runserver')
                    _options, _args = _parser.parse_known_args(self.argv[2:])
                    for _arg in _args:
                        self.argv.remove(_arg)

            # In all other cases, django.setup() is required to succeed.
            else:
                django.setup()

        self.autocomplete()

        if subcommand == 'help':
            if '--commands' in args:
                sys.stdout.write(self.main_help_text(commands_only=True) + '\n')
            elif not options.args:
                sys.stdout.write(self.main_help_text() + '\n')
            else:
                self.fetch_command(options.args[0]).print_help(self.prog_name, options.args[0])
        # Special-cases: We want 'django-admin --version' and
        # 'django-admin --help' to work, for backwards compatibility.
        elif subcommand == 'version' or self.argv[1:] == ['--version']:
            sys.stdout.write(django.get_version() + '\n')
        elif self.argv[1:] in (['--help'], ['-h']):
            sys.stdout.write(self.main_help_text() + '\n')
        else:
            self.fetch_command(subcommand).run_from_argv(self.argv)

#C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\management\__init__.py +301
'''
#1.fetch_command 最終會調用C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\management\__init__.py 的find_commands()
最終會找到 django\core\management\commands 下面的所有的command
	check
	compilemessages
	createcachetable
	dbshell
	diffsettings
	dumpdata
	flush
	inspectdb
	loaddata
	makemessages
	makemigrations
	migrate
	runserver
	sendtestemail
	shell
	showmigrations
	sqlflush
	sqlmigrate
	sqlsequencereset
	squashmigrations
	startapp
	startproject
	test
	testserver

2. run_from_argv 調 execute() 再調用handle()
'''

			
# 最常用的runserver
#C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\management\commands\runserver.py + 
# execute()-->handle()-->run()-->inner_run()-->get_wsgi_application() #WSGIHandler 在這里加載中間件
# C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\handlers\wsgi.py
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)

        response._handler_class = self.__class__

        status = '%d %s' % (response.status_code, response.reason_phrase)
        response_headers = [
            *response.items(),
            *(('Set-Cookie', c.output(header='')) for c in response.cookies.values()),
        ]
        start_response(status, response_headers)
        if getattr(response, 'file_to_stream', None) is not None and environ.get('wsgi.file_wrapper'):
            response = environ['wsgi.file_wrapper'](response.file_to_stream)
        return response


    def inner_run(self, *args, **options):
        # If an exception was silenced in ManagementUtility.execute in order
        # to be raised in the child process, raise it now.
        autoreload.raise_last_exception()

        threading = options['use_threading']
        # 'shutdown_message' is a stealth option.
        shutdown_message = options.get('shutdown_message', '')
        quit_command = 'CTRL-BREAK' if sys.platform == 'win32' else 'CONTROL-C'

        self.stdout.write("Performing system checks...\n\n")
        self.check(display_num_errors=True)
        # Need to check migrations here, so can't use the
        # requires_migrations_check attribute.
        self.check_migrations()
        now = datetime.now().strftime('%B %d, %Y - %X')
        self.stdout.write(now)
        self.stdout.write((
            "Django version %(version)s, using settings %(settings)r\n"
            "Starting development server at %(protocol)s://%(addr)s:%(port)s/\n"
            "Quit the server with %(quit_command)s.\n"
        ) % {
            "version": self.get_version(),
            "settings": settings.SETTINGS_MODULE,
            "protocol": self.protocol,
            "addr": '[%s]' % self.addr if self._raw_ipv6 else self.addr,
            "port": self.port,
            "quit_command": quit_command,
        })

        try:
            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:
            # Use helpful error messages instead of ugly tracebacks.
            ERRORS = {
                errno.EACCES: "You don't have permission to access that port.",
                errno.EADDRINUSE: "That port is already in use.",
                errno.EADDRNOTAVAIL: "That IP address can't be assigned to.",
            }
            try:
                error_text = ERRORS[e.errno]
            except KeyError:
                error_text = e
            self.stderr.write("Error: %s" % error_text)
            # Need to use an OS exit because sys.exit doesn't work in a thread
            os._exit(1)
        except KeyboardInterrupt:
            if shutdown_message:
                self.stdout.write(shutdown_message)
            sys.exit(0)
			

# C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\handlers\base.py
class BaseHandler:
    _view_middleware = None
    _template_response_middleware = None
    _exception_middleware = None
    _middleware_chain = None

    def load_middleware(self):
        """
        Populate middleware lists from settings.MIDDLEWARE.

        Must be called after the environment is fixed (see __call__ in subclasses).
        """
        self._view_middleware = []
        self._template_response_middleware = []
        self._exception_middleware = []

        handler = convert_exception_to_response(self._get_response)
        for middleware_path in reversed(settings.MIDDLEWARE):
            middleware = import_string(middleware_path)
            try:
                mw_instance = middleware(handler)
            except MiddlewareNotUsed as exc:
                if settings.DEBUG:
                    if str(exc):
                        logger.debug('MiddlewareNotUsed(%r): %s', middleware_path, exc)
                    else:
                        logger.debug('MiddlewareNotUsed: %r', middleware_path)
                continue

            if mw_instance is None:
                raise ImproperlyConfigured(
                    'Middleware factory %s returned None.' % middleware_path
                )

            if hasattr(mw_instance, 'process_view'):
                self._view_middleware.insert(0, mw_instance.process_view)
            if hasattr(mw_instance, 'process_template_response'):
                self._template_response_middleware.append(mw_instance.process_template_response)
            if hasattr(mw_instance, 'process_exception'):
                self._exception_middleware.append(mw_instance.process_exception)

            handler = convert_exception_to_response(mw_instance)

        # We only assign to this when initialization is complete as it is used
        # as a flag for initialization being complete.
        self._middleware_chain = handler
		
#get_handler 函數最終會返回一個 WSGIHandler 的實例。WSGIHandler 類只實現了 def __call__(self, environ, start_response) , 使它本身能夠成為 WSGI 中的應用程序, 并且實現 __call__ 能讓類的行為跟函數一樣。		
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)

        response._handler_class = self.__class__

        status = '%d %s' % (response.status_code, response.reason_phrase)
        response_headers = [
            *response.items(),
            *(('Set-Cookie', c.output(header='')) for c in response.cookies.values()),
        ]
        start_response(status, response_headers)
        if getattr(response, 'file_to_stream', None) is not None and environ.get('wsgi.file_wrapper'):
            response = environ['wsgi.file_wrapper'](response.file_to_stream)
        return response
		
# C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\socketserver.py + 215
    def serve_forever(self, poll_interval=0.5):
        """
        處理一個http請求直到關閉
        """
        self.__is_shut_down.clear()
        try:
            # XXX: Consider using another file descriptor or connecting to the
            # socket to wake this up instead of polling. Polling reduces our
            # responsiveness to a shutdown request and wastes cpu at all other
            # times.
            with _ServerSelector() as selector:
                selector.register(self, selectors.EVENT_READ)

                while not self.__shutdown_request:
                    ready = selector.select(poll_interval)
                    if ready:
					    #如果 fd可用調用處理方法
                        self._handle_request_noblock()

                    self.service_actions()
        finally:
            self.__shutdown_request = False
            self.__is_shut_down.set()



    def _handle_request_noblock(self):
        """Handle one request, without blocking.

        I assume that selector.select() has returned that the socket is
        readable before this function was called, so there should be no risk of
        blocking in get_request().
        """
        try:
            request, client_address = self.get_request()
        except OSError:
            return
        if self.verify_request(request, client_address):
            try:
			    #這里是真正處理請求的地方
                self.process_request(request, client_address)
            except Exception:
                self.handle_error(request, client_address)
                self.shutdown_request(request)
            except:
                self.shutdown_request(request)
                raise
        else:
            self.shutdown_request(request)
			
    
    def process_request(self, request, client_address):
        """Call finish_request.

        Overridden by ForkingMixIn and ThreadingMixIn.

        """
        self.finish_request(request, client_address)
        self.shutdown_request(request)
	

	#finish_request 最后調用這個BaseRequestHandler
	class BaseRequestHandler:
    '''
    
    '''

    def __init__(self, request, client_address, server):
        self.request = request
        self.client_address = client_address
        self.server = server
        self.setup()
        try:
            self.handle()
        finally:
            self.finish()
			
			
	# C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\servers\basehttp.py +156		
	def handle(self):
	'''
	這里對請求長度做限制
	parse_request對http解包
	'''

	self.raw_requestline = self.rfile.readline(65537)
	if len(self.raw_requestline) > 65536:
		self.requestline = ''
		self.request_version = ''
		self.command = ''
		self.send_error(414)
		return

	if not self.parse_request():  # An error code has been sent, just exit
		return

	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())
	
	#get_app 返回之前裝配的WSGIAPP最終
	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)

        response._handler_class = self.__class__

        status = '%d %s' % (response.status_code, response.reason_phrase)
        response_headers = [
            *response.items(),
            *(('Set-Cookie', c.output(header='')) for c in response.cookies.values()),
        ]
        start_response(status, response_headers)
        if getattr(response, 'file_to_stream', None) is not None and environ.get('wsgi.file_wrapper'):
            response = environ['wsgi.file_wrapper'](response.file_to_stream)
        return response