.. _middleware: Middleware ========== .. contents:: :local: Middleware components provide a way to execute logic before the framework routes each request, after each request is routed but before the target responder is called, or just before the response is returned for each request. .. Note:: Unlike hooks, middleware methods apply globally to the entire App. Components are registered with the `middleware` kwarg when instantiating Falcon's :ref:`App class `. A middleware component is simply a class that implements one or more of the event handler methods defined below. .. tabs:: .. tab:: WSGI Falcon's middleware interface is defined as follows: .. code:: python class ExampleMiddleware: def process_request(self, req, resp): """Process the request before routing it. Note: Because Falcon routes each request based on req.path, a request can be effectively re-routed by setting that attribute to a new value from within process_request(). Args: req: Request object that will eventually be routed to an on_* responder method. resp: Response object that will be routed to the on_* responder. """ def process_resource(self, req, resp, resource, params): """Process the request after routing. Note: This method is only called when the request matches a route to a resource. Args: req: Request object that will be passed to the routed responder. resp: Response object that will be passed to the responder. resource: Resource object to which the request was routed. params: A dict-like object representing any additional params derived from the route's URI template fields, that will be passed to the resource's responder method as keyword arguments. """ def process_response(self, req, resp, resource, req_succeeded): """Post-processing of the response (after routing). Args: req: Request object. resp: Response object. resource: Resource object to which the request was routed. May be None if no route was found for the request. req_succeeded: True if no exceptions were raised while the framework processed and routed the request; otherwise False. """ # A middleware instance can then be added to the Falcon app init from falcon import App app = App(middleware=[ExampleMiddleware()]) .. tab:: ASGI The ASGI middleware interface is similar to WSGI, but also supports the standard ASGI lifespan events. However, because lifespan events are an optional part of the ASGI specification, they may or may not fire depending on your ASGI server. .. code:: python class ExampleMiddleware: async def process_startup(self, scope, event): """Process the ASGI lifespan startup event. Invoked when the server is ready to start up and receive connections, but before it has started to do so. To halt startup processing and signal to the server that it should terminate, simply raise an exception and the framework will convert it to a "lifespan.startup.failed" event for the server. Args: scope (dict): The ASGI scope dictionary for the lifespan protocol. The lifespan scope exists for the duration of the event loop. event (dict): The ASGI event dictionary for the startup event. """ async def process_shutdown(self, scope, event): """Process the ASGI lifespan shutdown event. Invoked when the server has stopped accepting connections and closed all active connections. To halt shutdown processing and signal to the server that it should immediately terminate, simply raise an exception and the framework will convert it to a "lifespan.shutdown.failed" event for the server. Args: scope (dict): The ASGI scope dictionary for the lifespan protocol. The lifespan scope exists for the duration of the event loop. event (dict): The ASGI event dictionary for the shutdown event. """ async def process_request(self, req, resp): """Process the request before routing it. Note: Because Falcon routes each request based on req.path, a request can be effectively re-routed by setting that attribute to a new value from within process_request(). Args: req: Request object that will eventually be routed to an on_* responder method. resp: Response object that will be routed to the on_* responder. """ async def process_resource(self, req, resp, resource, params): """Process the request after routing. Note: This method is only called when the request matches a route to a resource. Args: req: Request object that will be passed to the routed responder. resp: Response object that will be passed to the responder. resource: Resource object to which the request was routed. params: A dict-like object representing any additional params derived from the route's URI template fields, that will be passed to the resource's responder method as keyword arguments. """ async def process_response(self, req, resp, resource, req_succeeded): """Post-processing of the response (after routing). Args: req: Request object. resp: Response object. resource: Resource object to which the request was routed. May be None if no route was found for the request. req_succeeded: True if no exceptions were raised while the framework processed and routed the request; otherwise False. """ async def process_request_ws(self, req, ws): """Process a WebSocket handshake request before routing it. Note: Because Falcon routes each request based on req.path, a request can be effectively re-routed by setting that attribute to a new value from within process_request(). Args: req: Request object that will eventually be passed into an on_websocket() responder method. ws: The WebSocket object that will be passed into on_websocket() after routing. """ async def process_resource_ws(self, req, ws, resource, params): """Process a WebSocket handshake request after routing. Note: This method is only called when the request matches a route to a resource. Args: req: Request object that will be passed to the routed responder. ws: WebSocket object that will be passed to the routed responder. resource: Resource object to which the request was routed. params: A dict-like object representing any additional params derived from the route's URI template fields, that will be passed to the resource's responder method as keyword arguments. """ # A middleware instance can then be added to the Falcon app init from falcon.asgi import App app = App(middleware=[ExampleMiddleware()]) It is also possible to implement a middleware component that is compatible with both ASGI and WSGI apps. This is done by applying an `*_async` postfix to distinguish the two different versions of each middleware method, as in the following example: .. code:: python class ExampleMiddleware: def process_request(self, req, resp): """Process WSGI request using synchronous logic. Note that req and resp are instances of falcon.Request and falcon.Response, respectively. """ async def process_request_async(self, req, resp): """Process ASGI request using asynchronous logic. Note that req and resp are instances of falcon.asgi.Request and falcon.asgi.Response, respectively. """ .. Tip:: Because *process_request* executes before routing has occurred, if a component modifies ``req.path`` in its *process_request* method, the framework will use the modified value to route the request. For example:: # Route requests based on the host header. req.path = '/' + req.host + req.path .. Tip:: The *process_resource* method is only called when the request matches a route to a resource. To take action when a route is not found, a :py:meth:`sink ` may be used instead. .. Tip:: In order to pass data from a middleware function to a resource function use the ``req.context`` and ``resp.context`` objects. These context objects are intended to hold request and response data specific to your app as it passes through the framework. Each component's *process_request*, *process_resource*, and *process_response* methods are executed hierarchically, as a stack, following the ordering of the list passed via the `middleware` kwarg of :class:`falcon.App` or :class:`falcon.asgi.App`. For example, if a list of middleware objects are passed as ``[mob1, mob2, mob3]``, the order of execution is as follows:: mob1.process_request mob2.process_request mob3.process_request mob1.process_resource mob2.process_resource mob3.process_resource mob3.process_response mob2.process_response mob1.process_response Note that each component need not implement all `process_*` methods; in the case that one of the three methods is missing, it is treated as a noop in the stack. For example, if ``mob2`` did not implement *process_request* and ``mob3`` did not implement *process_response*, the execution order would look like this:: mob1.process_request _ mob3.process_request mob1.process_resource mob2.process_resource mob3.process_resource _ mob2.process_response mob1.process_response Short-Circuiting ---------------- A *process_request* or *process_resource* middleware method may short-circuit further request processing by setting :attr:`falcon.Response.complete` to ``True``, e.g.:: resp.complete = True After the method returns, setting this flag will cause the framework to skip any remaining *process_request* and *process_resource* methods, as well as the responder method that the request would have been routed to. However, any *process_response* middleware methods will still be called. In a similar manner, setting :attr:`falcon.Response.complete` to ``True`` from within a *process_resource* method will short-circuit further request processing at that point. In the example below, you can see how request processing will be short-circuited once :attr:`falcon.Response.complete` has been set to ``True``, i.e., the framework will prevent ``mob3.process_request``, all *process_resource* methods, as well as the routed responder method from processing the request. However, all *process_response* methods will still be called:: mob1.process_request mob2.process_request # resp.complete = True mob3.process_response mob2.process_response mob1.process_response This feature affords use cases in which the response may be pre-constructed, such as in the case of caching. Exception Handling ------------------ If one of the *process_request* middleware methods raises an exception, it will be processed according to the exception type. If the type matches a registered error handler, that handler will be invoked and then the framework will begin to unwind the stack, skipping any lower layers. The error handler may itself raise an instance of :class:`~.HTTPError` or :class:`~.HTTPStatus`, in which case the framework will use the latter exception to update the *resp* object. .. Note:: By default, the framework installs two handlers, one for :class:`~.HTTPError` and one for :class:`~.HTTPStatus`. These can be overridden via :meth:`~.falcon.App.add_error_handler`. Regardless, the framework will continue unwinding the middleware stack. For example, if *mob2.process_request* were to raise an error, the framework would execute the stack as follows:: mob1.process_request mob2.process_request mob3.process_response mob2.process_response mob1.process_response As illustrated above, by default, all *process_response* methods will be executed, even when a *process_request*, *process_resource*, or *on_\** resource responder raises an error. This behavior is controlled by the :ref:`App class's ` `independent_middleware` keyword argument. Finally, if one of the *process_response* methods raises an error, or the routed ``on_*`` responder method itself raises an error, the exception will be handled in a similar manner as above. Then, the framework will execute any remaining middleware on the stack.