为什么引入web框架

web应用的本质

• 浏览器发送一个HTTP请求；
• 服务器收到请求，生成一个HTML文档；
• 服务器把HTML文档作为HTTP响应的Body发送给浏览器；
• 浏览器收到HTTP响应，从HTTP Body取出HTML文档并显示；

涉及的问题

• 解析http请求
• 找到对应的处理函数
• 生成并发送http响应

web框架工作流程

image

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​

中间件

• 中间件是请求或者应用开始和结束时注入代码的机制
• 常见的web中间件： 鉴权、打印log、session、统计信息、处理数据库连接 等等

image

image.gif

​

golang http 服务端编程

• golang 的net/http包提供了http编程的相关接口，封装了内部TCP连接和报文解析的复杂琐碎的细节
  go c.serve(ctx)最终会启动goroutine处理请求
• 使用者只需要和http.request 和 http.ResponseWriter 两个对象交互就行(也就是一个struct 实现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)
}

//纯 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{})
}

image.gif

• net/http 的另外一个重要的概念 ，ServeMux （多路传输）：ServeMux 可以注册多了 URL 和 handler 的对应关系，并自动把请求转发到对应的 handler 进行处理

• // 下面看一个带路由的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)
  }
  
  // 其实ServeMux 也实现了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
  }
  
  

  

  image.gif

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.注册中间件
3.注册路由

//响应流程
1.路由，找到handle
2.将请求和响应用Context包装起来供业务代码使用
3.依次调用中间件和处理函数
4.输出结果

//gin 里面最重要的两个数据结构
//1.Context在中间件中传递本次请求的各种数据、管理流程，进行响应
//2.Engine gin 引擎，是框架的实例，它包含多路复用器，中间件和配置设置

// 下面看下open-falcon-api中的实际应用
//open-falcon-api里面注册路由和中间件
//E:\go_path\src\github.com\open-falcon\falcon-plus\modules\api\app\controller\graph\graph_routes.go
// 首先注册/api/v1/开头的path的路由组
// 然后Use 一个auth的中间件 ，作用是检查token
// 这个组后面的所有path 都使用这个中间件 
// 也就是访问 /graph/endpoint 时会过 3个中间件(log recovery  auth )+一个EndpointRegexpQuery处理函数
// 
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 最后会调用这里的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)
    }
}

// 可以看到这里的bind 是在gin在解析请求payload是否和函数中要求的struct一致
// E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\context.go +504
// bind会根据请求中的Content-Type header判断是json 还是xml
// 并且根据struct中的tag通过反射解析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 返回一个不带中间件的 单例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) 最后调用的是  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
//我们可以看到 在gin中实现了ServeHTTP方法  net.http.Handler

// ServeHTTP conforms to the http.Handler interface. 
// 这里使用sync.pool cache context数据结构，避免频繁GC，每次使用都初始化
//一个struct实现了 interface中的方法 就说明这个struct是这个类型
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 里面处理请求的核心方法
// 根据一些配置去 压缩前缀树 radix.tree中找到对应的handleChain 然后执行
// 注意这句handlers, params, tsr := root.getValue(path, c.Params, unescape)
// 路由查找的过程是 从基数树的根节点一直匹配到和请求一致的节点找到对应的handlerchain
// 注册路由 E:\go_path\src\github.com\open-falcon\falcon-plus\vendor\github.com\gin-gonic\gin\gin.go +243 
// 从下面的addRoute方法中可以看到gin 为每个HttpMethod  GET POST PUT DELETE都注册了一颗tree
// 并且有priority 即最长的路径优先匹配
/*
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)
}

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python django （django框架复杂，先简单看一下）

# 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 最终会调用C:\Users\Administrator\AppData\Local\Programs\Python\Python37\Lib\site-packages\django\core\management\__init__.py 的find_commands()
最终会找到 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 调 execute() 再调用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 在这里加载中间件
# 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 函数最终会返回一个 WSGIHandler 的实例。WSGIHandler 类只实现了 def __call__(self, environ, start_response) , 使它本身能够成为 WSGI 中的应用程序, 并且实现 __call__ 能让类的行为跟函数一样。     
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):
        """
        处理一个http请求直到关闭
        """
        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可用调用处理方法
                        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:
                #这里是真正处理请求的地方
                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 最后调用这个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):
    '''
    这里对请求长度做限制
    parse_request对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 返回之前装配的WSGIAPP最终
    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

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