Paste Deployment

:author: Ian Bicking ianb@colorstudy.com

.. contents::

Documents:

.. toctree::
:maxdepth: 1

news
modules/loadwsgi
modules/config
modules/converters
license

.. comment:
The names used in sections should be more concrete, and it should
be clearer that they are just arbitrary names.

Introduction

Paste Deployment is a system for finding and configuring WSGI
applications and servers. For WSGI application consumers it provides
a single, simple function (loadapp) for loading a WSGI application
from a configuration file or a Python Egg. For WSGI application
providers it only asks for a single, simple entry point to your
application, so that application users don't need to be exposed to the
implementation details of your application.

The result is something a system administrator can install and manage
without knowing any Python, or the details of the WSGI application or
its container.

Paste Deployment currently does not require other parts of Paste <http://pythonpaste.org>_, and is distributed as a separate package.

To see updates that have been made to Paste Deploy see the news file <news.html>_.

Paste Deploy is released under the MIT license <http://www.opensource.org/licenses/mit-license.php>_.

Status

Paste Deploy has passed version 1.0. Paste Script is an actively
maintained project. As of 1.0, we'll make a strong effort to maintain
backward compatibility (this actually started happening long before
1.0, but now it is explicit). This will include deprecation warnings
when necessary. Major changes will take place under new functions or
with new entry points.

Note that the most key aspect of Paste Deploy is the entry points it
defines (such as paste.app_factory). Paste Deploy is not the only
consumer of these entry points, and many extensions can best take
place by utilizing the entry points instead of using Paste Deploy
directly. The entry points will not change; if changes are necessary,
new entry points will be defined.

Installation

First make sure you have either
setuptools <http://peak.telecommunity.com/DevCenter/setuptools>_ or its
modern replacement
distribute <http://pypi.python.org/pypi/distribute>_ installed.
For Python 3.x you need distribute as setuptools does not work on it.

Then you can install Paste Deployment using pip <http://www.pip-installer.org/en/latest/installing.html>_ by running::

$ sudo pip install PasteDeploy

If you want to track development, do::

$ hg clone http://bitbucket.org/ianb/pastedeploy
$ cd pastedeploy
$ sudo python setup.py develop

This will install the package globally, but will load the files in the
checkout. You can also simply install PasteDeploy==dev.

For downloads and other information see the Cheese Shop PasteDeploy page <http://cheeseshop.python.org/pypi/PasteDeploy>_.

A complimentary package is Paste Script </script/>_. To install
that, use pip install PasteScript (or pip install PasteScript==dev).

From the User Perspective

In the following sections, the Python API for using Paste Deploy is
given. This isn't what users will be using (but it is useful for
Python developers and useful for setting up tests fixtures).

The primary interaction with Paste Deploy is through its configuration
files. The primary thing you want to do with a configuration file is
serve it. To learn about serving configuration files, see the ``paster serve`` command <http://pythonpaste.org/script/#paster-serve>_.

The Config File

A config file has different sections.  The only sections Paste Deploy
cares about have prefixes, like ``app:main`` or ``filter:errors`` --
the part after the ``:`` is the "name" of the section, and the part
before gives the "type".  Other sections are ignored.  

The format is a simple `INI format
<http://en.wikipedia.org/wiki/INI_file>`_: ``name = value``.  You can
extend the value by indenting subsequent lines.  ``#`` is a comment.

Typically you have one or two sections, named "main": an application
section (``[app:main]``) and a server section (``[server:main]``).
``[composite:...]`` signifies something that dispatches to multiple
applications (example below).

Here's a typical configuration file that also shows off mounting
multiple applications using `paste.urlmap
<http://pythonpaste.org/module-paste.urlmap.html>`_::

    [composite:main]
    use = egg:Paste#urlmap
    / = home
    /blog = blog
    /wiki = wiki
    /cms = config:cms.ini

    [app:home]
    use = egg:Paste#static
    document_root = %(here)s/htdocs

    [filter-app:blog]
    use = egg:Authentication#auth
    next = blogapp    
    roles = admin
    htpasswd = /home/me/users.htpasswd

    [app:blogapp]
    use = egg:BlogApp
    database = sqlite:/home/me/blog.db

    [app:wiki]
    use = call:mywiki.main:application
    database = sqlite:/home/me/wiki.db

I'll explain each section in detail now::

    [composite:main]
    use = egg:Paste#urlmap
    / = home
    /blog = blog
    /cms = config:cms.ini

That this is a ``composite`` section means it dispatches the request
to other applications.  ``use = egg:Paste#urlmap`` means to use the
composite application named ``urlmap`` from the ``Paste`` package.
``urlmap`` is a particularly common composite application -- it uses a
path prefix to map your request to another application.  These are
the applications like "home", "blog", "wiki" and "config:cms.ini".  The last
one just refers to another file ``cms.ini`` in the same directory.

Next up::

    [app:home]
    use = egg:Paste#static
    document_root = %(here)s/htdocs

``egg:Paste#static`` is another simple application, in this case it
just serves up non-dynamic files.  It takes one bit of configuration:
``document_root``.  You can use variable substitution, which will pull
variables from the section ``[DEFAULT]`` (case sensitive!) with
markers like ``%(var_name)s``.  The special variable ``%(here)s`` is
the directory containing the configuration file; you should use that
in lieu of relative filenames (which depend on the current directory,
which can change depending how the server is run).

Then::

    [filter-app:blog]
    use = egg:Authentication#auth
    next = blogapp    
    roles = admin
    htpasswd = /home/me/users.htpasswd

    [app:blogapp]
    use = egg:BlogApp
    database = sqlite:/home/me/blog.db

The ``[filter-app:blog]`` section means that you want an application
with a filter applied.  The application being filtered is indicated
with ``next`` (which refers to the next section).  The
``egg:Authentication#auth`` filter doesn't actually exist, but one
could imagine it logs people in and checks permissions.

That last section is just a reference to an application that you
probably installed with ``pip install BlogApp``, and one bit of
configuration you passed to it (``database``).

Lastly::

    [app:wiki]
    use = call:mywiki.main:application
    database = sqlite:/home/me/wiki.db

This section is similar to the previous one, with one important difference.
Instead of an entry point in an egg, it refers directly to the ``application``
variable in the ``mywiki.main`` module. The reference consist of two parts,
separated by a colon. The left part is the full name of the module and the
right part is the path to the variable, as a Python expression relative to the
containing module.

So, that's most of the features you'll use.

Basic Usage
-----------

The basic way you'll use Paste Deployment is to load `WSGI
<http://www.python.org/peps/pep-3333.html>`_ applications.  Many
Python frameworks now support WSGI, so applications written for these
frameworks should be usable.

The primary function is ``paste.deploy.loadapp``.  This loads an
application given a URI.  You can use it like::

    from paste.deploy import loadapp
    wsgi_app = loadapp('config:/path/to/config.ini')

There's two URI formats currently supported: ``config:`` and ``egg:``.

``config:`` URIs
----------------

URIs that being with ``config:`` refer to configuration files.  These
filenames can be relative if you pass the ``relative_to`` keyword
argument to ``loadapp()``.  

.. note:: 
   
   Filenames are never considered relative to the current working
   directory, as that is a unpredictable location.  Generally when
   a URI has a context it will be seen as relative to that context;
   for example, if you have a ``config:`` URI inside another
   configuration file, the path is considered relative to the
   directory that contains that configuration file.

Config Format
~~~~~~~~~~~~~

Configuration files are in the INI format.  This is a simple format
that looks like::

    [section_name]
    key = value
    another key = a long value
        that extends over multiple lines

All values are strings (no quoting is necessary).  The keys and
section names are case-sensitive, and may contain punctuation and
spaces (though both keys and values are stripped of leading and
trailing whitespace).  Lines can be continued with leading whitespace.

Lines beginning with ``#`` (preferred) or ``;`` are considered
comments.

Applications
~~~~~~~~~~~~

You can define multiple applications in a single file; each
application goes in its own section.  Even if you have just one
application, you must put it in a section.

Each section name defining an application should be prefixed with
``app:``.  The "main" section (when just defining one application)
would go in ``[app:main]`` or just ``[app]``.

There's two ways to indicate the Python code for the application.  The
first is to refer to another URI or name::

    [app:myapp]
    use = config:another_config_file.ini#app_name

    # or any URI:
    [app:myotherapp]
    use = egg:MyApp

    # or a callable from a module:
    [app:mythirdapp]
    use = call:my.project:myapplication

    # or even another section:
    [app:mylastapp]
    use = myotherapp

It would seem at first that this was pointless; just a way to point to
another location.  However, in addition to loading the application
from that location, you can also add or change the configuration.

The other way to define an application is to point exactly to some
Python code::

    [app:myapp]
    paste.app_factory = myapp.modulename:app_factory

You must give an explicit *protocol* (in this case
``paste.app_factory``), and the value is something to import.  In
this case the module ``myapp.modulename`` is loaded, and the
``app_factory`` object retrieved from it.

See `Defining Factories`_ for more about the protocols.

Configuration
~~~~~~~~~~~~~

Configuration is done through keys besides ``use`` (or the protocol
names).  Any other keys found in the section will be passed as keyword
arguments to the factory.  This might look like::

    [app:blog]
    use = egg:MyBlog
    database = mysql://localhost/blogdb
    blogname = This Is My Blog!

You can override these in other sections, like::

    [app:otherblog]
    use = blog
    blogname = The other face of my blog

This way some settings could be defined in a generic configuration
file (if you have ``use = config:other_config_file``) or you can
publish multiple (more specialized) applications just by adding a
section.

Global Configuration

Often many applications share the same configuration. While you can
do that a bit by using other config sections and overriding values,
often you want that done for a bunch of disparate configuration
values. And typically applications can't take "extra" configuration
parameters; with global configuration you do something equivalent to
"if this application wants to know the admin email, this is it".

Applications are passed the global configuration separately, so they
must specifically pull values out of it; typically the global
configuration serves as the basis for defaults when no local
configuration is passed in.

Global configuration to apply to every application defined in a file
should go in a special section named [DEFAULT]. You can override
global configuration locally like::

[DEFAULT]
admin_email = webmaster@example.com

[app:main]
use = ...
set admin_email = bob@example.com

That is, by using set in front of the key.

Composite Applications

"Composite" applications are things that act like applications, but
are made up of other applications.  One example would be a URL mapper,
where you mount applications at different URL paths.  This might look
like::

    [composite:main]
    use = egg:Paste#urlmap
    / = mainapp
    /files = staticapp

    [app:mainapp]
    use = egg:MyApp

    [app:staticapp]
    use = egg:Paste#static
    document_root = /path/to/docroot

The composite application "main" is just like any other application
from the outside (you load it with ``loadapp`` for instance), but it
has access to other applications defined in the configuration file.

Other Objects
~~~~~~~~~~~~~

In addition to sections with ``app:``, you can define filters and
servers in a configuration file, with ``server:`` and ``filter:``
prefixes.  You load these with ``loadserver`` and ``loadfilter``.  The
configuration works just the same; you just get back different kinds
of objects.

Filter Composition
~~~~~~~~~~~~~~~~~~

There are several ways to apply filters to applications.  It mostly
depends on how many filters, and in what order you want to apply them.

The first way is to use the ``filter-with`` setting, like::

    [app:main]
    use = egg:MyEgg
    filter-with = printdebug

    [filter:printdebug]
    use = egg:Paste#printdebug
    # and you could have another filter-with here, and so on...

Also, two special section types exist to apply filters to your
applications: ``[filter-app:...]`` and ``[pipeline:...]``.  Both of
these sections define applications, and so can be used wherever an
application is needed.

``filter-app`` defines a filter (just like you would in a
``[filter:...]`` section), and then a special key ``next`` which
points to the application to apply the filter to.

``pipeline:`` is used when you need apply a number of filters.  It
takes *one* configuration key ``pipeline`` (plus any global
configuration overrides you want).  ``pipeline`` is a list of filters
ended by an application, like::

    [pipeline:main]
    pipeline = filter1 egg:FilterEgg#filter2 filter3 app

    [filter:filter1]
    ...

Getting Configuration
~~~~~~~~~~~~~~~~~~~~~

If you want to get the configuration without creating the application,
you can use the ``appconfig(uri)`` function, which is just like the
``loadapp()`` function except it returns the configuration that would
be used, as a dictionary.  Both global and local configuration is
combined into a single dictionary, but you can look at just one or the
other with the attributes ``.local_conf`` and ``.global_conf``.

``egg:`` URIs
-------------

`Python Eggs <http://peak.telecommunity.com/DevCenter/PythonEggs>`_
are a distribution and installation format produced by `setuptools
<http://peak.telecommunity.com/DevCenter/setuptools>`_ and
`distribute <http://packages.python.org/distribute/>`_ that adds metadata to a
normal Python package (among other things).

You don't need to understand a whole lot about Eggs to use them.  If
you have a `distutils
<http://python.org/doc/current/lib/module-distutils.html>`_
``setup.py`` script, just change::

    from distutils.core import setup

to::

    from setuptools import setup

Now when you install the package it will be installed as an egg.

The first important part about an Egg is that it has a
*specification*.  This is formed from the name of your distribution
(the ``name`` keyword argument to ``setup()``), and you can specify a
specific version.  So you can have an egg named ``MyApp``, or
``MyApp==0.1`` to specify a specific version.

The second is *entry points*.  These are references to Python objects
in your packages that are named and have a specific protocol.
"Protocol" here is just a way of saying that we will call them with
certain arguments, and expect a specific return value.  We'll talk
more about the protocols later_.

.. _later: `Defining Factories`_

The important part here is how we define entry points.  You'll add an
argument to ``setup()`` like::

    setup(
        name='MyApp',
        ...
        entry_points={
            'paste.app_factory': [
                'main=myapp.mymodule:app_factory',
                'ob2=myapp.mymodule:ob_factory'],
            },
        )

This defines two applications named ``main`` and ``ob2``.  You can
then refer to these by ``egg:MyApp#main`` (or just ``egg:MyApp``,
since ``main`` is the default) and ``egg:MyApp#ob2``.

The values are instructions for importing the objects.  ``main`` is
located in the ``myapp.mymodule`` module, in an object named
``app_factory``.

There's no way to add configuration to objects imported as Eggs.

Defining Factories
------------------

This lets you point to factories (that obey the specific protocols we
mentioned).  But that's not much use unless you can create factories
for your applications.

There's a few protocols: ``paste.app_factory``,
``paste.composite_factory``, ``paste.filter_factory``, and lastly
``paste.server_factory``.  Each of these expects a callable (like a
function, method, or class).

``paste.app_factory``

The application is the most common. You define one like::

def app_factory(global_config, **local_conf):
    return wsgi_app

The global_config is a dictionary, and local configuration is
passed as keyword arguments. The function returns a WSGI application.

paste.composite_factory

Composites are just slightly more complex::

    def composite_factory(loader, global_config, **local_conf):
        return wsgi_app

The ``loader`` argument is an object that has a couple interesting
methods.  ``get_app(name_or_uri, global_conf=None)`` return a WSGI
application with the given name.  ``get_filter`` and ``get_server``
work the same way.

A more interesting example might be a composite factory that does
something.  For instance, consider a "pipeline" application::

    def pipeline_factory(loader, global_config, pipeline):
        # space-separated list of filter and app names:
        pipeline = pipeline.split()
        filters = [loader.get_filter(n) for n in pipeline[:-1]]
        app = loader.get_app(pipeline[-1])
        filters.reverse() # apply in reverse order!
        for filter in filters:
            app = filter(app)
        return app

Then we use it like::

    [composite:main]
    use = <pipeline_factory_uri>
    pipeline = egg:Paste#printdebug session myapp

    [filter:session]
    use = egg:Paste#session
    store = memory

    [app:myapp]
    use = egg:MyApp

``paste.filter_factory``
~~~~~~~~~~~~~~~~~~~~~~~~~

Filter factories are just like app factories (same signature), except
they return filters.  Filters are callables that take a WSGI
application as the only argument, and return a "filtered" version of
that application.

Here's an example of a filter that checks that the ``REMOTE_USER`` CGI
variable is set, creating a really simple authentication filter::

    def auth_filter_factory(global_conf, req_usernames):
        # space-separated list of usernames:
        req_usernames = req_usernames.split()
        def filter(app):
            return AuthFilter(app, req_usernames)
        return filter

    class AuthFilter(object):
        def __init__(self, app, req_usernames):
            self.app = app
            self.req_usernames = req_usernames

        def __call__(self, environ, start_response):
            if environ.get('REMOTE_USER') in self.req_usernames:
                return self.app(environ, start_response)
            start_response(
                '403 Forbidden', [('Content-type', 'text/html')])
            return ['You are forbidden to view this resource']

``paste.filter_app_factory``

This is very similar to paste.filter_factory, except that it also
takes a wsgi_app argument, and returns a WSGI application. So if
you changed the above example to::

class AuthFilter(object):
    def __init__(self, app, global_conf, req_usernames):
        ....

Then AuthFilter would serve as a filter_app_factory
(req_usernames is a required local configuration key in this
case).

paste.server_factory

This takes the same signature as applications and filters, but returns
a server.

A server is a callable that takes a single argument, a WSGI
application.  It then serves the application.

An example might look like::

    def server_factory(global_conf, host, port):
        port = int(port)
        def serve(app):
            s = Server(app, host=host, port=port)
            s.serve_forever()
        return serve

The implementation of ``Server`` is left to the user.

``paste.server_runner``
~~~~~~~~~~~~~~~~~~~~~~~~

Like ``paste.server_factory``, except ``wsgi_app`` is passed as the
first argument, and the server should run immediately.

Outstanding Issues
------------------

* Should there be a "default" protocol for each type of object?  Since
  there's currently only one protocol, it seems like it makes sense
  (in the future there could be multiple).  Except that
  ``paste.app_factory`` and ``paste.composite_factory`` overlap
  considerably.

* ConfigParser's INI parsing is kind of annoying.  I'd like it both
  more constrained and less constrained.  Some parts are sloppy (like
  the way it interprets ``[DEFAULT]``).

* ``config:`` URLs should be potentially relative to other locations,
  e.g., ``config:$docroot/...``.  Maybe using variables from
  ``global_conf``?

* Should other variables have access to ``global_conf``?

* Should objects be Python-syntax, instead of always strings?  Lots of
  code isn't usable with Python strings without a thin wrapper to
  translate objects into their proper types.

* Some short-form for a filter/app, where the filter refers to the
  "next app".  Maybe like::

    [app-filter:app_name]
    use = egg:...
    next = next_app

    [app:next_app]
    ...