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.. _startup_chapter:
Startup
=======
When you cause :mod:`repoze.bfg` to start up in a console window,
you'll see something much like this show up on the console:
.. code-block:: text
$ paster serve myproject/MyProject.ini
Starting server in PID 16601.
serving on 0.0.0.0:6543 view at http://127.0.0.1:6543
This chapter explains what happens between the time you press the
"Return" key on your keyboard after typing ``paster serve
myproject/MyProject.ini`` and the time the line ``serving on
0.0.0.0:6543 ...`` is output to your console.
.. index::
single: startup process
The Startup Process
-------------------
The easiest and best-documented way to start and serve a
:mod:`repoze.bfg` application is to use the ``paster serve`` command
against a :term:`PasteDeploy` ``.ini`` file. This uses the ``.ini``
file to infer settings and starts a server listening on a port. For
the purposes of this discussion, we'll assume that you are using this
command to run your :mod:`repoze.bfg` application.
Here's a high-level time-ordered overview of what happens when you
press ``return`` after running ``paster serve MyProject.ini``.
#. The :term:`PasteDeploy` ``paster`` command is invoked under your
shell with the arguments ``serve`` and ``MyProject.ini``. As a
result, the :term:`PasteDeploy` framework recognizes that it is
meant to begin to run and serve an application using the
information contained within the ``MyProject.ini`` file.
#. The PasteDeploy framework finds a section named either
``[app:main]``, ``[pipeline:main]``, or ``[composite::main]`` in
the ``.ini`` file. This section represents the configuration of a
:term:`WSGI` application that will be served. If you're using a
simple application (e.g. an ``[app:main]`` section of a
default-generated :mod:`repoze.bfg` project), the application
:term:`entry point` or :term:`dotted Python name` will be named on
the ``use=`` line within the section's configuration. If, instead
of a simple application, you're using a WSGI :term:`pipeline`
(e.g. a ``[pipeline:main]`` section), the application named on the
"last" element will refer to your :mod:`repoze.bfg` application.
If instead of a simple application or a pipeline, you're using a
Paste "composite" (e.g. ``[composite:main]``), refer to the
documentation for that particular composite to understand how to
make it refer to your :mod:`repoze.bfg` application.
#. The application's *constructor* (named by the entry point reference
or dotted Python name on the ``use=`` line) is passed the key/value
parameters mentioned within the section in which it's defined. The
constructor is meant to return a :term:`router` instance, which is
a :term:`WSGI` application.
For :mod:`repoze.bfg` applications, the constructor will be a
function named ``app`` in the ``run.py`` file within the
:term:`package` in which your application lives. If this function
succeeds, it will return a :mod:`repoze.bfg` :term:`router`
instance. Here's the contents of an example ``run.py`` module:
.. literalinclude:: MyProject/myproject/run.py
:linenos:
Note that the constructor function accepts a ``global_config``
argument (which is a dictionary of key/value pairs mentioned in the
``[DEFAULT]`` section of the configuration file). It also accepts
a ``**settings`` argument, which collects another set of arbitrary
key/value pairs. The arbitrary key/value pairs received by this
function in ``**settings`` will be composed of all the key/value
pairs that are present in the ``[app:main]`` section (except for
the ``use=`` setting) when this function is called by the
:term:`PasteDeploy` framework when you run ``paster serve``.
Our generated ``MyProject.ini`` file looks like so:
.. literalinclude:: MyProject/MyProject.ini
:linenos:
In this case, the ``myproject.run:app`` function referred to by the
entry point URI ``egg:MyProject#app`` (see :ref:`MyProject_ini` for
more information about entry point URIs, and how they relate to
callables), will receive the key/value pairs
``{'reload_templates':'true', 'debug_authorization':'false',
'debug_notfound':'false'}``.
#. The PasteDeploy application constructor itself is invoked. It is
represented by the ``app`` function in ``run.py``. A typical
:mod:`repoze.bfg` ``app`` function will look like the below.
.. literalinclude:: MyProject/myproject/run.py
:linenos:
Note that the ``app`` function imports the ``get_root`` :term:`root
factory` function from the ``myproject.models`` Python module.
#. The ``app`` function first constructs a
:class:`repoze.bfg.configuration.Configurator` instance, passing
``get_root`` to it as its ``root_factory`` argument, and
``settings`` dictionary captured via the ``**settings`` kwarg as
its ``settings`` argument.
``get_root`` is a root factory callable that is invoked on every
request to retrieve the application root. It is not called during
startup, only when a request is handled.
``settings`` dictionary contains all the options in the
``[app:main]`` section of our .ini file except the "use" option
(which is internal to paste). In this case, ``**settings`` will be
something like ``{'reload_templates':'true',
'debug_authorization':'false', 'debug_notfound':'false'}``.
#. The ``app`` function then calls the
:meth:`repoze.bfg.configuration.Configurator.load_zcml` method,
passing in a ``zcml_file`` value. ``zcml_file`` is the value of
the ``configure_zcml`` setting or a default of ``configure.zcml``.
This filename is relative to the run.py file that the ``app``
function lives in. The ``load_zcml`` function processes each
:term:`ZCML declaration` in the ZCML file implied by the
``zcml_file`` argument. If ``load_zcml`` fails to parse the ZCML
file (or any file which is included by the ZCML file), a
``XMLConfigurationError`` is raised and processing ends. If it
succeeds, an :term:`application registry` is populated using all
the :term:`ZCML declaration` statements present in the file.
#. The :meth:`repoze.bfg.configuration.Configurator.make_wsgi_app`
method is called. The result is a :term:`router` instance. The
router is associated with the :term:`application registry` implied
by the configurator previously populated by ZCML. The router is a
WSGI application.
#. A :class:`repoze.bfg.interfaces.WSGIApplicationCreatedEvent` event
is emitted (see :ref:`events_chapter` for more information about
events).
#. Assuming there were no errors, the ``app`` function in
``myproject`` returns the router instance created by
``make_wsgi_app`` back to PasteDeploy. As far as PasteDeploy is
concerned, it is "just another WSGI application".
#. PasteDeploy starts the WSGI *server* defined within the
``[server:main]`` section. In our case, this is the ``Paste#http``
server (``use = egg:Paste#http``), and it will listen on all
interfaces (``host = 0.0.0.0``), on port number 6543 (``port =
6543``). The server code itself is what prints ``serving on
0.0.0.0:6543 view at http://127.0.0.1:6543``. The server serves
the application, and the application is running, waiting to receive
requests.
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