=====================
Web Server with Flask
=====================

Basic Flask Server to serve static html files
=============================================

My use case is simple: Using my documention built with Sphinx as a content for my Web server.

Creating a basic Flask Application
----------------------------------

This Application has one function that uses the User input to display some custom text.

.. code-block:: python

    from flask import Flask

    app = Flask(__name__)

    @app.route('/user/<name>')
    def user(name):
        return '<h1>Hello, {}!</h1>'.format(name)

    if __name__ == '__main__':
        app.run(debug=True, host='0.0.0.0', port=80)

.. note:: 
    
    To access it from any device on your local network, set host='0.0.0.0'

Serving Static files
--------------------

The method send_static_files is used to output the build of our sphinx project.

.. code-block:: python

    from flask import Flask

    app = Flask(__name__, static_url_path='/', static_folder='_build/')

    @app.route('/')
    @app.route('/<path:path>')
    def serve_sphinx_docs(path='index.html'):
        return app.send_static_file(path)

    if __name__ == '__main__':
        app.run(debug=True, host='0.0.0.0', port=80)


Run a production server
=======================

When running publicly rather than in development, you should not use the built-in development server (flask run). The development server is provided by Werkzeug for convenience, but is not designed to be particularly efficient, stable, or secure.

Instead, use a production WSGI server. For example, to use Waitress, first install it in the virtual environment:

.. code-block:: bash

    $ pip install waitress

Creat another script to import waitress and your Flask App and use the serve method:

.. code-block:: python

    from waitress import serve
    import flask_app

    serve(flask_app.app, host='0.0.0.0', port=8080, url_scheme='https')

.. note:: 

    Specifying the proper host, port and url_scheme is crucial.

.. tip::

    To check which services are running use: `sudo service --status-all`

Create a service with systemd to run the server at startup
==========================================================

In order to automatically run the Web server at each startup, we can create a service with systemd:

source: https://linuxconfig.org/how-to-autostart-python-script-on-raspberry-pi

.. note:: 

    All service files are located in `/etc/systemd/system`

.. code-block:: bash

    user@pi:/etc/systemd/system $ ls
    bluetooth.target.wants                      graphical.target.wants
    cloudflared.service                         halt.target.wants
    cloudflared-update.service                  jenkins.service.d
    cloudflared-update.timer                    multi-user.target.wants
    dbus-fi.w1.wpa_supplicant1.service          network-online.target.wants
    dbus-org.bluez.service                      poweroff.target.wants
    dbus-org.freedesktop.Avahi.service          printer.target.wants
    dbus-org.freedesktop.ModemManager1.service  reboot.target.wants
    dbus-org.freedesktop.nm-dispatcher.service  remote-fs.target.wants
    dbus-org.freedesktop.timesync1.service      runwaitress.service
    default.target                              sockets.target.wants
    dev-serial1.device.wants                    sshd.service
    display-manager.service                     sysinit.target.wants
    getty.target.wants                          timers.target.wants
    getty@tty1.service.d                        wayvnc.service.wants


.. code-block:: bash

    user@pi:/etc/systemd/system $ cat runwaitress.service
    [Unit]
    Description=Run production server with Waitress
    After=multi-user.target

    [Service]
    Type=oneshot
    ExecStart=/home/user/virtEnv/Website/bin/python /home/user/myApp/waitress_server.py --serve-in-foreground


    [Install]
    WantedBy=multi-user.target

Apply the appropriate permissions to the file so that systemd can execute it. Afterwards, we will apply the proper permissions to the systemd timer file. 

.. code-block:: bash

    $ sudo chmod 744 myApp/waitress_server.py
    $ sudo chmod 664 /etc/systemd/system/runwaitress.service

Next, enable the service unit: 

.. code-block:: bash

    $ sudo systemctl daemon-reload
    $ sudo systemctl enable runwaitress.service

Common pitfalls
===============

Issues encountered and how they have been fixed.

Python OSError: [Errno 98] Address already in use
-------------------------------------------------

The Python error "OSError: [Errno 98] Address already in use" occurs when you have another process running on the specified port.

You can resolve the error by using the SO_REUSEADDR attribute or by identifying and stopping the process before you start your server.

To get the process ID of the process that is running on the specified port, use the `lsof` command.

.. code-block:: bash

    $ sudo lsof -i :8081
    COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
    python  1931 root    6u  IPv4   9077      0t0  TCP *:tproxy (LISTEN)


The PID column contains the ID of the process that is running on the specified port.

You can stop the process by issuing the kill command.

.. code-block:: bash

    $ sudo kill -9 <YOUR_PID>