.. _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::

  $ 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 the time the line ``serving on
0.0.0:6543 ...`` is output to your console.

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.

.. sidebar:: Using :mod:`repoze.bfg` Without ``paster``

   ``paster serve`` is by no means the only way to start up and serve
   a :mod:`repoze.bfg` application.  Any :term:`WSGI` server is
   capable of running a :mod:`repoze.bfg` application, and some WSGI
   servers (such as :term:`mod_wsgi`) don't require the
   :term:`PasteDeploy` framework's ``paster serve`` command to do
   server process management.  Each :term:`WSGI` server has its own
   documentation about how it creates a process to run an application,
   and there are many of them, so we cannot provide the details for
   each here.  But the concepts are largely the same, whatever server
   you happen to use.

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]`` or ``[pipeline: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.

#. 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.

   For ``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 constructor itself is invoked.  A generated :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.  It
   then also imports the "bare" ``myproject`` package, and passes
   ``get_root``, ``myproject``, and the ``settings`` keyword as the
   ``app`` function's extra keyword arguments to the ``make_app``
   function of the ``repoze.bfg.router`` module.  ``**settings`` here
   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'}``.

   ``get_root`` is the first argument to ``make_app``, and it 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.

   We pass in the bare ``myproject`` package so that the ``make_app``
   callback knows where to look for the :term:`application registry`
   file (conventionally named ``configure.zcml``).  ``make_app`` will
   use the package's path and look for ``configure.zcml`` within that
   package's filesystem directory.

   If you for some reason need or want to load a different application
   registry filename for your application, you can pass an optional
   ``filename=`` parameter to make_app (e.g. ``make_app(get_root,
   myproject, filename='meta.zcml', settings=settings``).  If the
   filename is absolute, the ``package`` argument is ignored.

#. The ``make_app`` function does its work.  It finds and parses the
   ZCML represented by the application registry file.  If it fails to
   parse one or more ZCML files, a ``XMLConfigurationError`` is raised
   (or possibly another error if the ZCML file just doesn't exist).
   If it succeeds, an :term:`application registry` is created,
   representing the view registrations (and other registrations) for
   your application.  A :term:`router` instance is created, and the
   router is associated with the application registry.  The router
   represents your application; the settings in the application
   registry that is created will be used for your application.

#. A ``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_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.