之前有阅读过tornado 底层的实现，tornado 为了解决C10K 问题(没听说过C10K问题的请查看: http://www.360doc.com/content/13/0522/18/1542811_287328391.shtml)，在Linux 平台下是使用了epoll（python2.6 开始支持epoll），unix 平台下 tornado 使用了kque , 由于flask 之前没有看过底层的实现，因此趁着清明假期看了一下flask，到底是来一个请求使用一个线程呢，还是进程呢，还是IO多路复用。

涉及到的源码文件

site-packages/flask/app.py, site-packages/werkzeug/serving.py, Lib/socketserver.py

首先肯定要先看入口函数啦

app.py 里面的 run 函数

def run(self, host=None, port=None, debug=None, **options):

该函数通过 run_simple(host, port, self, **options) 启动了socket 服务器(无论是哪个web框架，其实底层都是使用socketserver 监听在某个套接字上来处理请求的)

run_simple 然后到调用 serving.py 里面的make_server

make_server 源码定义如下:

def make_server(host=None, port=None, app=None, threaded=False, processes=1,
                request_handler=None, passthrough_errors=False,
                ssl_context=None, fd=None):
    """Create a new server instance that is either threaded, or forks
    or just processes one request after another.
    """
    if threaded and processes > 1:
        raise ValueError("cannot have a multithreaded and "
                         "multi process server.")
    elif threaded:
        return ThreadedWSGIServer(host, port, app, request_handler,
                                  passthrough_errors, ssl_context, fd=fd)
    elif processes > 1:
        return ForkingWSGIServer(host, port, app, processes, request_handler,
                                 passthrough_errors, ssl_context, fd=fd)
    else:
        return BaseWSGIServer(host, port, app, request_handler,
                              passthrough_errors, ssl_context, fd=fd)

可以看到flask 为我们提供了三种方式来处理请求
1 使用多线程来进行处理
2 使用多进程来进行处理
3 使用poll 或者 select IO多路复用的方式进行处理

BaseWSGIServer 这个类是使用IO 多路复用的
下面有个方法 start_forever

    def serve_forever(self):
        self.shutdown_signal = False
        try:
            HTTPServer.serve_forever(self)
        except KeyboardInterrupt:
            pass
        finally:
            self.server_close()

我们主要来看HttpServer.serve_forever方法

 def serve_forever(self, poll_interval=0.5):
        """Handle one request at a time until shutdown.

        Polls for shutdown every poll_interval seconds. Ignores
        self.timeout. If you need to do periodic tasks, do them in
        another thread.
        """
        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:
                        self._handle_request_noblock()

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

_ServerSelector() 定义了 到底该使用select 还是 poll

# poll/select have the advantage of not requiring any extra file descriptor,
# contrarily to epoll/kqueue (also, they require a single syscall).
if hasattr(selectors, 'PollSelector'):
    _ServerSelector = selectors.PollSelector
else:
    _ServerSelector = selectors.SelectSelector

对于IO多路复用不熟悉的，推荐查看该文章: https://blog.csdn.net/qq546770908/article/details/53082870