flask是python web开发比较主流的框架之一，也是我在工作中使用的主要开发框架。一直对其是如何保证线程安全的问题比较好奇，所以简单的探究了一番，由于只是简单查看了源码，并未深入细致研究，因此以下内容仅为个人理解，不保证正确性。
首先是很多文章都说flask会为每一个request启动一个线程，每个request都在单独线程中处理，因此保证了线程安全。于是就做了一个简单的测试。首先是写一个简单的flask程序（只需要有最简单的功能用于测试即可），然后我们知道一个flask应用启动之后实际上是作为一个 WSGI application的，之后所有接收到的请求都会经由flask的wsgi_app(self, environ, start_response)方法去处理，所以就来看一下这个方法（注释已去掉）。
def wsgi_app(self, environ, start_response): ctx = self.request_context(environ) ctx.push() error = None try: try: response = self.full_dispatch_request() except Exception as e: error = e response = self.handle_exception(e) return response(environ, start_response) finally: if self.should_ignore_error(error): error = None ctx.auto_pop(error)
那么这个request_context又是什么东西呢？它是一个RequestContext对象，文档是这么说的：
The request context contains all request relevant information. It is created at the beginning of the request and pushed to the `_request_ctx_stack` and removed at the end of it. It will create the URL adapter and request object for the WSGI environment provided.
说的很清楚，这个对象的上下文包含着request相关的信息。也就是说每一个请求到来之后，flask都会为它新建一个RequestContext对象，并且将这个对象push进全局变量_request_ctx_stack中，在push前还要检查_app_ctx_stack，如果_app_ctx_stack的栈顶元素不存在或是与当前的应用不一致，则首先push appcontext 到_app_ctx_stack中，再push requestcontext。源码如下:
def push(self): top = _request_ctx_stack.top if top is not None and top.preserved: top.pop(top._preserved_exc) # Before we push the request context we have to ensure that there # is an application context. app_ctx = _app_ctx_stack.top if app_ctx is None or app_ctx.app != self.app: app_ctx = self.app.app_context() app_ctx.push() self._implicit_app_ctx_stack.append(app_ctx) else: self._implicit_app_ctx_stack.append(None) if hasattr(sys, 'exc_clear'): sys.exc_clear() _request_ctx_stack.push(self) # Open the session at the moment that the request context is # available. This allows a custom open_session method to use the # request context (e.g. code that access database information # stored on `g` instead of the appcontext). self.session = self.app.open_session(self.request) if self.session is None: self.session = self.app.make_null_session()
通过上面的两步，每一个请求的应用上下文和请求上下文就被push到了全局变量_request_ctx_stack和_app_ctx_stack中。
现在我们知道了_request_ctx_stack和_app_ctx_stack是何时被push的，每一个请求到来都会导致新的RequestContext和AppContext被建立并push，一旦请求处理完毕就被pop出去。而无论是_app_ctx_stack还是_request_ctx_stack都是一个LocalStack对象，这是werkzeug中的一个对象，看看它里边有什么：
class LocalStack(object): def __init__(self): self._local = Local() def __release_local__(self): self._local.__release_local__() def _get__ident_func__(self): return self._local.__ident_func__ def _set__ident_func__(self, value): object.__setattr__(self._local, '__ident_func__', value) __ident_func__ = property(_get__ident_func__, _set__ident_func__) del _get__ident_func__, _set__ident_func__ def __call__(self): def _lookup(): rv = self.top if rv is None: raise RuntimeError('object unbound') return rv return LocalProxy(_lookup) def push(self, obj): """Pushes a new item to the stack""" rv = getattr(self._local, 'stack', None) if rv is None: self._local.stack = rv = [] rv.append(obj) return rv def pop(self): """Removes the topmost item from the stack, will return the old value or `None` if the stack was already empty. """ stack = getattr(self._local, 'stack', None) if stack is None: return None elif len(stack) == 1: release_local(self._local) return stack[-1] else: return stack.pop() @property def top(self): """The topmost item on the stack. If the stack is empty, `None` is returned. """ try: return self._local.stack[-1] except (AttributeError, IndexError): return None
可以看到，这个对象的几乎所有重要属性在_local这一属性中，它是一个Local对象，很有意思，如果看一下Local的构造器，会发现其中包含有重要属性__ident_func__，
def __init__(self):
object.__setattr__(self, '__storage__', {})
object.__setattr__(self, '__ident_func__', get_ident)
这一属性由get_ident方法提供，这个方法的作用是提供当前线程的id，用于区别同时存在的多个线程Return a non-zero integer that uniquely identifiamongst other threads that exist simultaneously.
到此为止，可见作为一个全局变量_request_ctx_stack和_app_ctx_stack应该都是只有一个线程去处理，没有发现哪里有可以为每个请求都开启一个线程的代码，实际测试一下，可以发现确实所有的请求都只运行在一个线程上（使用pycharm的debug模式可以看到当前程序启动
的所有线程，在当前这种情型下除了主线程外只有一个Thread-6，无论多少请求都一样)
这下就有趣了，传说中的每个请求一个线程果然没有出现，那么flask的线程安全是如何保证的呢？如果把每次请求到来时附带的environ（wsgi_app方法参数中的environ）打印看看的话就会发现，每个environ都携带了请求相关的全部上下文信息，在请求到来的时候通过附带的
environ重建context，并push到栈中，然后立刻处理该请求，处理完毕后将其pop出去。
那么很多文章说的每个请求一个线程到底是在哪里建立的呢？这就要去仔细看一下flask.app的run方法了:
def run(self, host=None, port=None, debug=None, **options): from werkzeug.serving import run_simple if host is None: host = '127.0.0.1' if port is None: server_name = self.config['SERVER_NAME'] if server_name and ':' in server_name: port = int(server_name.rsplit(':', 1)[1]) else: port = 5000 if debug is not None: self.debug = bool(debug) options.setdefault('use_reloader', self.debug) options.setdefault('use_debugger', self.debug) try: run_simple(host, port, self, **options) finally: # reset the first request information if the development server # reset normally. This makes it possible to restart the server # without reloader and that stuff from an interactive shell. self._got_first_request = False
这个方法实际上是对werkzeug的run_simple方法的简单包装。而run_simple方法则有趣的多（这一段把注释也贴上）
def run_simple(hostname, port, application, use_reloader=False, use_debugger=False, use_evalex=True, extra_files=None, reloader_interval=1, reloader_type='auto', threaded=False, processes=1, request_handler=None, static_files=None, passthrough_errors=False, ssl_context=None): """Start a WSGI application. Optional features include a reloader, multithreading and fork support. This function has a command-line interface too:: python -m werkzeug.serving --help .. versionadded:: 0.5 `static_files` was added to simplify serving of static files as well as `passthrough_errors`. .. versionadded:: 0.6 support for SSL was added. .. versionadded:: 0.8 Added support for automatically loading a SSL context from certificate file and private key. .. versionadded:: 0.9 Added command-line interface. .. versionadded:: 0.10 Improved the reloader and added support for changing the backend through the `reloader_type` parameter. See :ref:`reloader` for more information. :param hostname: The host for the application. eg: ``'localhost'`` :param port: The port for the server. eg: ``8080`` :param application: the WSGI application to execute :param use_reloader: should the server automatically restart the python process if modules were changed? :param use_debugger: should the werkzeug debugging system be used? :param use_evalex: should the exception evaluation feature be enabled? :param extra_files: a list of files the reloader should watch additionally to the modules. For example configuration files. :param reloader_interval: the interval for the reloader in seconds. :param reloader_type: the type of reloader to use. The default is auto detection. Valid values are ``'stat'`` and ``'watchdog'``. See :ref:`reloader` for more information. :param threaded: should the process handle each request in a separate thread? :param processes: if greater than 1 then handle each request in a new process up to this maximum number of concurrent processes. :param request_handler: optional parameter that can be used to replace the default one. You can use this to replace it with a different :class:`~BaseHTTPServer.BaseHTTPRequestHandler` subclass. :param static_files: a dict of paths for static files. This works exactly like :class:`SharedDataMiddleware`, it's actually just wrapping the application in that middleware before serving. :param passthrough_errors: set this to `True` to disable the error catching. This means that the server will die on errors but it can be useful to hook debuggers in (pdb etc.) :param ssl_context: an SSL context for the connection. Either an :class:`ssl.SSLContext`, a tuple in the form ``(cert_file, pkey_file)``, the string ``'adhoc'`` if the server should automatically create one, or ``None`` to disable SSL (which is the default). """ if use_debugger: from werkzeug.debug import DebuggedApplication application = DebuggedApplication(application, use_evalex) if static_files: from werkzeug.wsgi import SharedDataMiddleware application = SharedDataMiddleware(application, static_files) def log_startup(sock): display_hostname = hostname not in ('', '*') and hostname or 'localhost' if ':' in display_hostname: display_hostname = '[%s]' % display_hostname quit_msg = '(Press CTRL+C to quit)' port = sock.getsockname()[1] _log('info', ' * Running on %s://%s:%d/ %s', ssl_context is None and 'http' or 'https', display_hostname, port, quit_msg) def inner(): try: fd = int(os.environ['WERKZEUG_SERVER_FD']) except (LookupError, ValueError): fd = None srv = make_server(hostname, port, application, threaded, processes, request_handler, passthrough_errors, ssl_context, fd=fd) if fd is None: log_startup(srv.socket) srv.serve_forever() if use_reloader: # If we're not running already in the subprocess that is the # reloader we want to open up a socket early to make sure the # port is actually available. if os.environ.get('WERKZEUG_RUN_MAIN') != 'true': if port == 0 and not can_open_by_fd: raise ValueError('Cannot bind to a random port with enabled ' 'reloader if the Python interpreter does ' 'not support socket opening by fd.') # Create and destroy a socket so that any exceptions are # raised before we spawn a separate Python interpreter and # lose this ability. address_family = select_ip_version(hostname, port) s = socket.socket(address_family, socket.SOCK_STREAM) s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) s.bind((hostname, port)) if hasattr(s, 'set_inheritable'): s.set_inheritable(True) # If we can open the socket by file descriptor, then we can just # reuse this one and our socket will survive the restarts. if can_open_by_fd: os.environ['WERKZEUG_SERVER_FD'] = str(s.fileno()) s.listen(LISTEN_QUEUE) log_startup(s) else: s.close() from ._reloader import run_with_reloader run_with_reloader(inner, extra_files, reloader_interval, reloader_type) else: inner()
默认情况下会执行inner方法，inner方法创建了一个server并启动，这样一个flask应用算是真正的启动了。那么秘密就在make_server里了
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)
好了，这一下我们一直以来的疑问就找到答案了，原来一个flask应用的server并非只有一种类型，它是可以设定的，默认情况下创建的是一个 BaseWSGIServer，如果指定了threaded参数就启动一个ThreadedWSGIServer，如果设定的processes>1则启动一个ForkingWSGIServer。
事已至此，后面的事情就是追本溯源了：
class ThreadedWSGIServer(ThreadingMixIn, BaseWSGIServer): """A WSGI server that does threading.""" multithread = True
ThreadedWSGIServer是ThreadingMixIn和BaseWSGIServer的子类，
class ThreadingMixIn: """Mix-in class to handle each request in a new thread.""" # Decides how threads will act upon termination of the # main process daemon_threads = False def process_request_thread(self, request, client_address): """Same as in BaseServer but as a thread. In addition, exception handling is done here. """ try: self.finish_request(request, client_address) self.shutdown_request(request) except: self.handle_error(request, client_address) self.shutdown_request(request) def process_request(self, request, client_address): """Start a new thread to process the request.""" t = threading.Thread(target = self.process_request_thread, args = (request, client_address)) t.daemon = self.daemon_threads t.start()
源码写的太明白了，原来是ThreadingMixIn的实例以多线程的方式去处理每一个请求，这样对开发者来说，只有在启动app时将threaded参数设定为True,flask才会真正以多线程的方式去处理每一个请求。
实际去测试一下，发现将threaded设置为True后，果然每一个请求都会开启一个单独的线程去处理。
