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. Components are registered with the middleware kwarg when instantiating Falcon’s API class.

Falcon’s middleware interface is defined as follows:

class ExampleComponent(object):
    def process_request(self, req, resp):
        """Process the request before routing it.

        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.
        """

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.API. 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 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

If one of the process_request middleware methods raises an error, it will be processed according to the error 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, in which case the framework will use the latter exception to update the resp object. 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, mob3, and routing>
    mob2.process_response
mob1.process_response

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.