bundled/flask/docs/deploying/mod_wsgi.rst @ 627b1d4e0fa3 issue-93

.. _mod_wsgi-deployment:

mod_wsgi (Apache)
=================

If you are using the `Apache`_ webserver you should consider using `mod_wsgi`_.

.. admonition:: Watch Out

   Please make sure in advance that your ``app.run()`` call you might
   have in your application file, is inside an ``if __name__ ==
   '__main__':`` 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 mod_wsgi.

.. _Apache: http://httpd.apache.org/

Installing `mod_wsgi`
---------------------

If you don't have `mod_wsgi` installed yet you have to either install it using
a package manager or compile it yourself.

The mod_wsgi `installation instructions`_ cover source installations on UNIX
systems.

If you are using Ubuntu/Debian you can apt-get it and activate it as follows:

.. sourcecode:: text

    # apt-get install libapache2-mod-wsgi

On FreeBSD install `mod_wsgi` by compiling the `www/mod_wsgi` port or by using
pkg_add:

.. sourcecode:: text

    # pkg_add -r mod_wsgi

If you are using pkgsrc you can install `mod_wsgi` by compiling the
`www/ap2-wsgi` package.

If you encounter segfaulting child processes after the first apache reload you
can safely ignore them.  Just restart the server.

Creating a `.wsgi` file
-----------------------

To run your application you need a `yourapplication.wsgi` file.  This file
contains the code `mod_wsgi` is executing on startup to get the application
object.  The object called `application` in that file is then used as
application.

For most applications the following file should be sufficient::

    from yourapplication import app as application

If you don't have a factory function for application creation but a singleton
instance you can directly import that one as `application`.

Store that file somewhere that you will find it again (e.g.:
`/var/www/yourapplication`) and make sure that `yourapplication` and all
the libraries that are in use are on the python load path.  If you don't
want to install it system wide consider using a `virtual python`_ instance.

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

The last thing you have to do is to create an Apache configuration file for
your application.  In this example we are telling `mod_wsgi` to execute the
application under a different user for security reasons:

.. sourcecode:: apache

    <VirtualHost *>
        ServerName example.com

        WSGIDaemonProcess yourapplication user=user1 group=group1 threads=5
        WSGIScriptAlias / /var/www/yourapplication/yourapplication.wsgi

        <Directory /var/www/yourapplication>
            WSGIProcessGroup yourapplication
            WSGIApplicationGroup %{GLOBAL}
            Order deny,allow
            Allow from all
        </Directory>
    </VirtualHost>

For more information consult the `mod_wsgi wiki`_.

.. _mod_wsgi: http://code.google.com/p/modwsgi/
.. _installation instructions: http://code.google.com/p/modwsgi/wiki/QuickInstallationGuide
.. _virtual python: http://pypi.python.org/pypi/virtualenv
.. _mod_wsgi wiki: http://code.google.com/p/modwsgi/wiki/

Troubleshooting
---------------

If your application does not run, follow this guide to troubleshoot:

**Problem:** application does not run, errorlog shows SystemExit ignored
    You have a ``app.run()`` call in your application file that is not
    guarded by an ``if __name__ == '__main__':`` condition.  Either remove
    that :meth:`~flask.Flask.run` call from the file and move it into a
    separate `run.py` file or put it into such an if block.

**Problem:** application gives permission errors
    Probably caused by your application running as the wrong user.  Make
    sure the folders the application needs access to have the proper
    privileges set and the application runs as the correct user (``user``
    and ``group`` parameter to the `WSGIDaemonProcess` directive)

**Problem:** application dies with an error on print
    Keep in mind that mod_wsgi disallows doing anything with
    :data:`sys.stdout` and :data:`sys.stderr`.  You can disable this
    protection from the config by setting the `WSGIRestrictStdout` to
    ``off``:

    .. sourcecode:: apache

        WSGIRestrictStdout Off

    Alternatively you can also replace the standard out in the .wsgi file
    with a different stream::

        import sys
        sys.stdout = sys.stderr

**Problem:** accessing resources gives IO errors
    Your application probably is a single .py file you symlinked into the
    site-packages folder.  Please be aware that this does not work,
    instead you either have to put the folder into the pythonpath the file
    is stored in, or convert your application into a package.

    The reason for this is that for non-installed packages, the module
    filename is used to locate the resources and for symlinks the wrong
    filename is picked up.

Support for Automatic Reloading
-------------------------------

To help deployment tools you can activate support for automatic reloading.
Whenever something changes the `.wsgi` file, `mod_wsgi` will reload all
the daemon processes for us.

For that, just add the following directive to your `Directory` section:

.. sourcecode:: apache

   WSGIScriptReloading On

Working with Virtual Environments
---------------------------------

Virtual environments have the advantage that they never install the
required dependencies system wide so you have a better control over what
is used where.  If you want to use a virtual environment with mod_wsgi you
have to modify your `.wsgi` file slightly.

Add the following lines to the top of your `.wsgi` file::

    activate_this = '/path/to/env/bin/activate_this.py'
    execfile(activate_this, dict(__file__=activate_this))

This sets up the load paths according to the settings of the virtual
environment.  Keep in mind that the path has to be absolute.