flask 源码浅析(flask 如何处理请求(多线程，多进程，IO多路复用))

刺骨的言语ヽ痛彻心扉 2022-05-28 11:27 94阅读 0赞

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][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][https_blog.csdn.net_qq546770908_article_details_53082870]

[http_www.360doc.com_content_13_0522_18_1542811_287328391.shtml]: http://www.360doc.com/content/13/0522/18/1542811_287328391.shtml
[https_blog.csdn.net_qq546770908_article_details_53082870]: https://blog.csdn.net/qq546770908/article/details/53082870