Hello Coders,

This article explains how to deploy a Flask application sandboxed with a virtualenv and served by Apache HTTP server using the mod_wsgi module.

For newcomers, Flask is a lightweight web application framework written in Python. Sometimes classified as a microframework, Flask provides a lightweight codebase that can be easily extended to become an API, a simple web app, or a complex eCommerce platform.

Dependencies

Apache / httpd (on CentOS) server
mod_wsgi
Flask web framework
Python3
Virtualenv

Prepare the environment

Install Apache server

$ sudo yum install httpd
$ 
$ # by default the server is down.
$ sudo systemctl start httpd

Install mod_wsgi

$ sudo yum install mod_wsgi
$
$ # restart apache
$ sudo systemctl restart httpd

Test if the `mod_wsgi` module is loaded

$ sudo httpd -M | grep wsgi
wsgi_module (shared) # <-- the OK response

Install Virtualenv

Virtual environments will sandbox the app to run isolated from the global server environment

$ sudo pip install virtualenv

Code the Flask App

We will use a simple Flask application that serves a simple Hello World message for all routes.
As mentioned before, this setup is executed on CentOs. The steps are:

Go to /var/www - the www_root of Apache server, and create the project directory.

$ cd /var/www
$ mkdir hitme

To have a runnable Flask app, we need to create two files: run.py and app.py inside the app folder. The files are structured as below:

/var/www/hitme
          | - run.py
          | - app/__init__.py

Where run.py is responsible to bootstrap the Flask app defined in the app directory.

app/init.py file contents


from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello world!"

run.py file contents


import os
from app import app

#----------------------------------------
# launch
#----------------------------------------

if __name__ == "__main__":
    port = int(os.environ.get("PORT", 5000))
    app.run(host='0.0.0.0', port=port, debug=True)

Test the Flask App

We have the test application, now let's start it to see something on the screen. First, we need to create and activate the virtual environment.

$ cd /var/www
$ virtualenv --python=python3 hitme # the venv is created inside the app folder
$ cd /var/www/hitme
$ source bin/activate

At this point, we will run the next commands inside the VENV. Let's install Flask

$ pip install flask

To run, a Flask application require FLASK_APP environment variable.

$ export FLASK_APP=run.py # please notice the name
$ flask run # start the app
$ # our app is running on port 5000

Apache Configuration

To execute a Flask application under the Apache HTTP server we need to bridge our application to the Apache engine using the mod_wsgi module. For this we need to create a new file wsgi.py inside our project folder:

/var/www/hitme
          | - wsgi.py
          | - run.py
          | - app/__init__.py

wsgi.py file contents

#!/usr/bin/env python

import sys
import site

site.addsitedir('/var/www/hitme/lib/python3.6/site-packages')

sys.path.insert(0, '/var/www/hitme')

from app import app as application

The next step is to configure Apache to serve the app and use this wsgi loader. The following settings should be added to the httpd.conf. On CentOS the file location is /etc/httpd/conf/httpd.conf

<VirtualHost *:80>

     ServerName localhost

     WSGIDaemonProcess hitme user=apache group=apache threads=2

     WSGIScriptAlias / /var/www/hitme/wsgi.py

     <Directory /var/www/hitme>
         Require all granted
     </Directory>

</VirtualHost>

Close and save the file and restart Apache to load the new settings.

$ # restart apache
$ sudo systemctl restart httpd

Our Flask app should be served by the Apache HTTP server. We can test the deploy by using lynx command:

$ lynx localhost # lynx

Thanks for reading! For more resources, feel free to access:

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Flask Deploy with Apache on CentOS - Minimal Setup