flask/docs/deploying/fastcgi.rst

.. _deploying-fastcgi:

FastCGI
=======

FastCGI is a deployment option on servers like `nginx`_, `lighttpd`_, and
`cherokee`_; see :doc:`uwsgi` and :doc:`wsgi-standalone` for other options. To
use your WSGI application with any of them you will need a FastCGI server first.
The most popular one is `flup`_ which we will use for this guide. Make sure to
have it installed to follow along.

.. admonition:: Watch Out

   Please make sure in advance that any ``app.run()`` calls you might
   have in your application file are inside an ``if __name__ ==
   '__main__':`` block or moved to a separate file.  Just make sure it's
   not called because this will always start a local WSGI server which
   we do not want if we deploy that application to FastCGI.

Creating a `.fcgi` file
-----------------------

First you need to create the FastCGI server file.  Let's call it
`yourapplication.fcgi`::

    #!/usr/bin/python
    from flup.server.fcgi import WSGIServer
    from yourapplication import app

    if __name__ == '__main__':
        WSGIServer(app).run()

This is enough for Apache to work, however nginx and older versions of
lighttpd need a socket to be explicitly passed to communicate with the
FastCGI server.  For that to work you need to pass the path to the
socket to the :class:`~flup.server.fcgi.WSGIServer`::

    WSGIServer(application, bindAddress='/path/to/fcgi.sock').run()

The path has to be the exact same path you define in the server
config.

Save the :file:`yourapplication.fcgi` file somewhere you will find it again.
It makes sense to have that in :file:`/var/www/yourapplication` or something
similar.

Make sure to set the executable bit on that file so that the servers
can execute it:

.. sourcecode:: text

    # chmod +x /var/www/yourapplication/yourapplication.fcgi

Configuring Apache
------------------

The example above is good enough for a basic Apache deployment but your
`.fcgi` file will appear in your application URL e.g.
``example.com/yourapplication.fcgi/news/``. There are few ways to configure
your application so that yourapplication.fcgi does not appear in the URL.
A preferable way is to use the ScriptAlias and SetHandler configuration
directives to route requests to the FastCGI server. The following example
uses FastCgiServer to start 5 instances of the application which will
handle all incoming requests::

    LoadModule fastcgi_module /usr/lib64/httpd/modules/mod_fastcgi.so

    FastCgiServer /var/www/html/yourapplication/app.fcgi -idle-timeout 300 -processes 5

    <VirtualHost *>
        ServerName webapp1.mydomain.com
        DocumentRoot /var/www/html/yourapplication

        AddHandler fastcgi-script fcgi
        ScriptAlias / /var/www/html/yourapplication/app.fcgi/

        <Location />
            SetHandler fastcgi-script
        </Location>
    </VirtualHost>

These processes will be managed by Apache. If you're using a standalone
FastCGI server, you can use the FastCgiExternalServer directive instead.
Note that in the following the path is not real, it's simply used as an
identifier to other
directives such as AliasMatch::

    FastCgiServer /var/www/html/yourapplication -host 127.0.0.1:3000

If you cannot set ScriptAlias, for example on a shared web host, you can use
WSGI middleware to remove yourapplication.fcgi from the URLs. Set .htaccess::

    <IfModule mod_fcgid.c>
       AddHandler fcgid-script .fcgi
       <Files ~ (\.fcgi)>
           SetHandler fcgid-script
           Options +FollowSymLinks +ExecCGI
       </Files>
    </IfModule>

    <IfModule mod_rewrite.c>
       Options +FollowSymlinks
       RewriteEngine On
       RewriteBase /
       RewriteCond %{REQUEST_FILENAME} !-f
       RewriteRule ^(.*)$ yourapplication.fcgi/$1 [QSA,L]
    </IfModule>

Set yourapplication.fcgi::

    #!/usr/bin/python
    #: optional path to your local python site-packages folder
    import sys
    sys.path.insert(0, '<your_local_path>/lib/python<your_python_version>/site-packages')

    from flup.server.fcgi import WSGIServer
    from yourapplication import app

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

       def __call__(self, environ, start_response):
           environ['SCRIPT_NAME'] = ''
           return self.app(environ, start_response)

    app = ScriptNameStripper(app)

    if __name__ == '__main__':
        WSGIServer(app).run()

Configuring lighttpd
--------------------

A basic FastCGI configuration for lighttpd looks like that::

    fastcgi.server = ("/yourapplication.fcgi" =>
        ((
            "socket" => "/tmp/yourapplication-fcgi.sock",
            "bin-path" => "/var/www/yourapplication/yourapplication.fcgi",
            "check-local" => "disable",
            "max-procs" => 1
        ))
    )

    alias.url = (
        "/static/" => "/path/to/your/static/"
    )

    url.rewrite-once = (
        "^(/static($|/.*))$" => "$1",
        "^(/.*)$" => "/yourapplication.fcgi$1"
    )

Remember to enable the FastCGI, alias and rewrite modules. This configuration
binds the application to ``/yourapplication``.  If you want the application to
work in the URL root you have to work around a lighttpd bug with the
:class:`~werkzeug.contrib.fixers.LighttpdCGIRootFix` middleware.

Make sure to apply it only if you are mounting the application the URL
root. Also, see the Lighty docs for more information on `FastCGI and Python
<https://redmine.lighttpd.net/projects/lighttpd/wiki/Docs_ModFastCGI>`_ (note that
explicitly passing a socket to run() is no longer necessary).

Configuring nginx
-----------------

Installing FastCGI applications on nginx is a bit different because by
default no FastCGI parameters are forwarded.

A basic Flask FastCGI configuration for nginx looks like this::

    location = /yourapplication { rewrite ^ /yourapplication/ last; }
    location /yourapplication { try_files $uri @yourapplication; }
    location @yourapplication {
        include fastcgi_params;
        fastcgi_split_path_info ^(/yourapplication)(.*)$;
        fastcgi_param PATH_INFO $fastcgi_path_info;
        fastcgi_param SCRIPT_NAME $fastcgi_script_name;
        fastcgi_pass unix:/tmp/yourapplication-fcgi.sock;
    }

This configuration binds the application to ``/yourapplication``.  If you
want to have it in the URL root it's a bit simpler because you don't
have to figure out how to calculate ``PATH_INFO`` and ``SCRIPT_NAME``::

    location / { try_files $uri @yourapplication; }
    location @yourapplication {
        include fastcgi_params;
        fastcgi_param PATH_INFO $fastcgi_script_name;
        fastcgi_param SCRIPT_NAME "";
        fastcgi_pass unix:/tmp/yourapplication-fcgi.sock;
    }

Running FastCGI Processes
-------------------------

Since nginx and others do not load FastCGI apps, you have to do it by
yourself.  `Supervisor can manage FastCGI processes.
<http://supervisord.org/configuration.html#fcgi-program-x-section-settings>`_
You can look around for other FastCGI process managers or write a script
to run your `.fcgi` file at boot, e.g. using a SysV ``init.d`` script.
For a temporary solution, you can always run the ``.fcgi`` script inside
GNU screen.  See ``man screen`` for details, and note that this is a
manual solution which does not persist across system restart::

    $ screen
    $ /var/www/yourapplication/yourapplication.fcgi

Debugging
---------

FastCGI deployments tend to be hard to debug on most web servers.  Very
often the only thing the server log tells you is something along the
lines of "premature end of headers".  In order to debug the application
the only thing that can really give you ideas why it breaks is switching
to the correct user and executing the application by hand.

This example assumes your application is called `application.fcgi` and
that your web server user is `www-data`::

    $ su www-data
    $ cd /var/www/yourapplication
    $ python application.fcgi
    Traceback (most recent call last):
      File "yourapplication.fcgi", line 4, in <module>
    ImportError: No module named yourapplication

In this case the error seems to be "yourapplication" not being on the
python path.  Common problems are:

-   Relative paths being used.  Don't rely on the current working directory.
-   The code depending on environment variables that are not set by the
    web server.
-   Different python interpreters being used.

.. _nginx: https://nginx.org/
.. _lighttpd: https://www.lighttpd.net/
.. _cherokee: http://cherokee-project.com/
.. _flup: https://pypi.org/project/flup/