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.. index::
single: request processing
single: request
.. _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 a 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`.
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