testbed/pallets__flask/docs/deploying/mod_wsgi.rst

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

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

.. 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 mod_wsgi.

.. _Apache: https://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-py3

If you are using a yum based distribution (Fedora, OpenSUSE, etc..) you
can install it as follows:

.. sourcecode:: text

    $ yum install mod_wsgi

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

.. sourcecode:: text

    $ pkg install ap24-py37-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 :file:`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 a factory function is used in a :file:`__init__.py` file, then the function should be imported::

    from yourapplication import create_app
    application = create_app()

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.:
:file:`/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.  Keep in mind that you will have to actually install your
application into the virtualenv as well.  Alternatively there is the
option to just patch the path in the ``.wsgi`` file before the import::

    import sys
    sys.path.insert(0, '/path/to/the/application')

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>

Note: WSGIDaemonProcess isn't implemented in Windows and Apache will
refuse to run with the above configuration. On a Windows system, eliminate those lines:

.. sourcecode:: apache

    <VirtualHost *>
        ServerName example.com
        WSGIScriptAlias / C:\yourdir\yourapp.wsgi
        <Directory C:\yourdir>
            Order deny,allow
            Allow from all
        </Directory>
    </VirtualHost>

Note: There have been some changes in access control configuration
for `Apache 2.4`_.

.. _Apache 2.4: https://httpd.apache.org/docs/trunk/upgrading.html

Most notably, the syntax for directory permissions has changed from httpd 2.2

.. sourcecode:: apache

    Order allow,deny
    Allow from all

to httpd 2.4 syntax

.. sourcecode:: apache

    Require all granted


For more information consult the `mod_wsgi documentation`_.

.. _mod_wsgi: https://github.com/GrahamDumpleton/mod_wsgi
.. _installation instructions: https://modwsgi.readthedocs.io/en/develop/installation.html
.. _virtual python: https://pypi.org/project/virtualenv/
.. _mod_wsgi documentation: https://modwsgi.readthedocs.io/en/develop/index.html

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

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

**Problem:** application does not run, errorlog shows SystemExit ignored
    You have an ``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 :file:`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'
    with open(activate_this) as file_:
        exec(file_.read(), 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.