.. index::
   single: request processing
   single: request
   single: router

.. _router_chapter:

Request Processing
==================

Once a :mod:`repoze.bfg` application is up and running, it is ready to
accept requests and return responses.

What happens from the time a :term:`WSGI` request enters a
:mod:`repoze.bfg` application through to the point that
:mod:`repoze.bfg` hands off a response back to WSGI for upstream
processing?

#. A user initiates a request from his browser to the hostname and
   port number of the WSGI server used by the :mod:`repoze.bfg`
   application.

#. The WSGI server used by the :mod:`repoze.bfg` application passes
   the WSGI environment to the ``__call__`` method of the
   :mod:`repoze.bfg` :term:`router` object.

#. A :term:`request` object is created based on the WSGI environment.

#. A :class:`repoze.bfg.interfaces.INewRequest` :term:`event` is sent
   to any subscribers.

#. If any :term:`route` has been defined within application
   configuration, the :mod:`repoze.bfg` :term:`router` calls a
   :term:`URL dispatch` "route mapper."  The job of the mapper is to
   examine the ``PATH_INFO`` implied by the request to determine
   whether any user-defined :term:`route` matches the current WSGI
   environment.  The :term:`router` passes the request as an argument
   to the mapper.

#. If any route matches, the WSGI environment is mutated; a
   ``bfg.routes.route`` key and a ``bfg.routes.matchdict`` are added
   to the WSGI environment, and an attribute named ``matchdict`` is
   added to the request.  A root object associated with the route
   found is also generated.  If the :term:`route configuration`
   which matched has an associated a ``factory`` argument, this
   factory is used to generate the root object, otherwise a default
   :term:`root factory` is used.

#. If a route match was *not* found, and a ``root_factory`` argument
   was passed to the :term:`Configurator` constructor, that callable
   is used to generate the root object.  If the ``root_factory``
   argument passed to the Configurator constructor is ``None``, a
   default root factory is used to generate a root object.

#. The :mod:`repoze.bfg` router calls a "traverser" function with the
   root object and the request.  The traverser function attempts to
   traverse the root object (using any existing ``__getitem__`` on the
   root object and subobjects) to find a :term:`context`.  If the root
   object has no ``__getitem__`` method, the root itself is assumed to
   be the context.  The exact traversal algorithm is described in
   :ref:`traversal_chapter`. The traverser function returns a
   dictionary, which contains a :term:`context` and a :term:`view
   name` as well as other ancillary information.

#. The request is decorated with various names returned from the
   traverser (such as ``context``, ``view_name``, and so forth), so
   they can be accessed via e.g. ``request.context`` within
   :term:`view` code.

#. A :class:`repoze.bfg.interfaces.IAfterTraversal` :term:`event` is
   sent to any subscribers.

#. :mod:`repoze.bfg` looks up a :term:`view` callable using the
   context, the request, and the view name.  If a view callable
   doesn't exist for this combination of objects (based on the type of
   the context, the type of the request, and the value of the view
   name, and any :term:`predicate` attributes applied to the view
   configuration), :mod:`repoze.bfg` uses a "not found" view callable
   to generate a response, and returns that response.

#. If a view callable was found, :mod:`repoze.bfg` calls the view
   function.

#. If an :term:`authorization policy` is in use, and the view was
   protected by a :term:`permission`, :mod:`repoze.bfg` passes the
   context, the request, and the view_name to a function which
   determines whether the view being asked for can be executed by the
   requesting user, based on credential information in the request and
   security information attached to the context.  If it returns
   ``True``, :mod:`repoze.bfg` calls the view callable to obtain a
   response.  If it returns ``False``, it uses a :term:`forbidden
   view` callable to generate a response.

#. A :class:`repoze.bfg.interfaces.INewResponse` :term:`event` is sent
   to any subscribers.

#. The response object's ``app_iter``, ``status``, and ``headerlist``
   attributes are used to generate a WSGI response.  The response is
   sent back to the upstream WSGI server.

.. image:: router.png

This is a very high-level overview that leaves out various details.
For more detail about subsystems invoked by the BFG router such as
traversal, URL dispatch, views, and event processing, see
:ref:`contextfinding_chapter`, :ref:`views_chapter`, and
:ref:`events_chapter`.