Middleware¶
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 App class. A middleware component is simply a class that implements one or more of the event handler methods defined below.
Falcon’s middleware interface is defined as follows:
class ExampleComponent:
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.
"""
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.
class ExampleComponent:
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.
"""
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:
class ExampleComponent:
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
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
falcon.App
or 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
<route to resource responder method>
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
<route to responder method>
_
mob2.process_response
mob1.process_response
Short-Circuiting¶
A process_request or process_resource middleware method may short-circuit
further request processing by setting 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 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 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
<skip mob3.process_request>
<skip mob1/mob2/mob3.process_resource>
<skip route to resource responder method>
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 HTTPError
or 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
HTTPError
and one for HTTPStatus
. These can
be overridden via 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
<skip mob1/mob2 process_resource>
<skip mob3.process_request>
<skip mob3.process_resource>
<skip route to resource responder method>
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 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.