// Tutorial //

How To Serve Flask Applications with Gunicorn and Nginx on Ubuntu 22.04

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Ubuntu 22.04

A previous version of this tutorial was written by Kathleen Juell.

Introduction

In this guide, you will build a Python application using the Flask microframework on Ubuntu 22.04. The majority of this tutorial will be about how to set up the Gunicorn application server and how to launch the application and configure Nginx to act as a front-end reverse proxy.

Prerequisites

Before starting this guide, you should have:

• A server with Ubuntu 22.04 installed and a non-root user with sudo privileges. Follow our initial server setup guide for guidance.

• Nginx installed, following Steps 1 and 2 of How To Install Nginx on Ubuntu 22.04.

• A domain name configured to point to your server. You can purchase one on Namecheap or get one for free on Freenom. You can learn how to point domains to DigitalOcean by following the relevant documentation on domains and DNS. Be sure to create the following DNS records:

  • An A record with your_domain pointing to your server’s public IP address.
  • An A record with www.your_domain pointing to your server’s public IP address.

• Familiarity with the WSGI specification, which the Gunicorn server will use to communicate with your Flask application. This discussion covers WSGI in more detail.

Step 1 — Installing the Components from the Ubuntu Repositories

The first step will be to install all of required pieces from the Ubuntu repositories. This includes pip, the Python package manager, which will manage the Python components. You will also get the Python development files necessary to build some of the Gunicorn components.

First,update the local package index and install the packages that will allow you to build your Python environment. These will include python3-pip, along with a few more packages and development tools necessary for a robust programming environment:

With these packages in place, the next step is to create a virtual environment for your project.

Step 2 — Creating a Python Virtual Environment

Next, you’ll set up a virtual environment in order to isolate the Flask application from the other Python files on your system.

Start by installing the python3-venv package, which will install the venv module:

Next, make a parent directory for your Flask project. Move into the directory with the cd command after you create it:

Create a virtual environment to store your Flask project’s Python requirements by typing:

This will install a local copy of Python and pip into a directory called myprojectenv within your project directory.

Before installing applications within the virtual environment, you need to activate it. Do so by typing:

Your prompt will change to indicate that you are now operating within the virtual environment. It will look something like this: (myprojectenv)user@host:~/myproject$.

Step 3 — Setting Up a Flask Application

Now that you are in your virtual environment, you can install Flask and Gunicorn and get started on designing your application.

First, install wheel with the local instance of pip to ensure that your packages will install even if they are missing wheel archives:

Next, install Flask and Gunicorn:

Creating a Sample App

Now that you have Flask available, you can create a simple application. Flask is a microframework. It does not include many of the tools that more full-featured frameworks might, and exists mainly as a module that you can import into your projects to assist you in initializing a web application.

While your application might be more complex, we’ll create our Flask app in a single file, called myproject.py:

The application code will live in this file. It will import Flask and instantiate a Flask object. You can use this to define the functions that should be run when a specific route is requested:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "<h1 style='color:blue'>Hello There!</h1>"

if __name__ == "__main__":
    app.run(host='0.0.0.0')

This basically defines what content to present when the root domain is accessed. Save and close the file when you’re finished.

If you followed the initial server setup guide, you should have a UFW firewall enabled. To test the application, you need to allow access to port 5000:

Now you can test your Flask app by typing:

You will see output like the following, including a helpful warning reminding you not to use this server setup in production:

Visit your server’s IP address followed by :5000 in your web browser:

http://your_server_ip:5000

You should see something like this:

When you are finished, hit CTRL-C in your terminal window to stop the Flask development server.

Creating the WSGI Entry Point

Next, create a file that will serve as the entry point for your application. This will tell the Gunicorn server how to interact with the application.

Call the file wsgi.py:

In this file, import the Flask instance from our application and then run it:

from myproject import app

if __name__ == "__main__":
    app.run()

Save and close the file when you are finished.

Step 4 — Configuring Gunicorn

Your application is now written with an entry point established. You can now move on to configuring Gunicorn.

Before moving on, check that Gunicorn can serve the application correctly.

You can do this by passing it the name of the application’s entry point. This is constructed as the name of the module (minus the .py extension), plus the name of the callable within the application. In this case, it is wsgi:app.

Also specify the interface and port to bind to using the 0.0.0.0:5000 argument so that the application will be started on a publicly available interface:

You should see output like the following:

Visit your server’s IP address with :5000 appended to the end in your web browser again:

http://your_server_ip:5000

You should see your application’s output:

When you have confirmed that it’s functioning properly, press CTRL-C in your terminal window.

When you are done using the virtual environment, you can deactivate it:

Any Python commands will now use the system’s Python environment again.

Next, create the systemd service unit file. Creating a systemd unit file will allow Ubuntu’s init system to automatically start Gunicorn and serve the Flask application whenever the server boots.

Create a unit file ending in .service within the /etc/systemd/system directory to begin:

Inside, you’ll start with the [Unit] section, which is used to specify metadata and dependencies. Add a description of your service here and tell the init system to only start this after the networking target has been reached:

[Unit]
Description=Gunicorn instance to serve myproject
After=network.target

Next, add a [Service] section. This will specify the user and group that you want the process to run under. Give your regular user account ownership of the process since it owns all of the relevant files. Also give group ownership to the www-data group so that Nginx can communicate easily with the Gunicorn processes. Remember to replace the username here with your username:

[Unit]
Description=Gunicorn instance to serve myproject
After=network.target

[Service]
User=sammy
Group=www-data

Next, map out the working directory and set the PATH environmental variable so that the init system knows that the executables for the process are located within our virtual environment. Also specify the command to start the service. This command will do the following:

• Start 3 worker processes (though you should adjust this as necessary)
• Create and bind to a Unix socket file, myproject.sock, within our project directory. We’ll set an umask value of 007 so that the socket file is created giving access to the owner and group, while restricting other access
• Specify the WSGI entry point file name, along with the Python callable within that file (wsgi:app)

Systemd requires that you give the full path to the Gunicorn executable, which is installed within your virtual environment.

Remember to replace the username and project paths with your own information:

[Unit]
Description=Gunicorn instance to serve myproject
After=network.target

[Service]
User=sammy
Group=www-data
WorkingDirectory=/home/sammy/myproject
Environment="PATH=/home/sammy/myproject/myprojectenv/bin"
ExecStart=/home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 wsgi:app

Finally, add an [Install] section. This will tell systemd what to link this service to if you enable it to start at boot. You’ll want this service to start when the regular multi-user system is up and running:

[Unit]
Description=Gunicorn instance to serve myproject
After=network.target

[Service]
User=sammy
Group=www-data
WorkingDirectory=/home/sammy/myproject
Environment="PATH=/home/sammy/myproject/myprojectenv/bin"
ExecStart=/home/sammy/myproject/myprojectenv/bin/gunicorn --workers 3 --bind unix:myproject.sock -m 007 wsgi:app

[Install]
WantedBy=multi-user.target

With that, your systemd service file is complete. Save and close it now.

You can now start the Gunicorn service that you created and enable it so that it starts at boot:

Let’s check the status:

You should see output like this:

If you see any errors, be sure to resolve them before continuing with the tutorial.

Step 5 — Configuring Nginx to Proxy Requests

Your Gunicorn application server should now be up and running, waiting for requests on the socket file in the project directory. Now you can configure Nginx to pass web requests to that socket by making some small additions to its configuration file.

Begin by creating a new server block configuration file in Nginx’s sites-available directory. Call this myproject to keep in line with the rest of the guide:

Open up a server block and tell Nginx to listen on the default port 80. Also tell it to use this block for requests for our server’s domain name:

server {
    listen 80;
    server_name your_domain www.your_domain;
}

Next, add a location block that matches every request. Within this block, you’ll include the proxy_params file that specifies some general proxying parameters that need to be set. You’ll then pass the requests to the socket you defined using the proxy_pass directive:

server {
    listen 80;
    server_name your_domain www.your_domain;

    location / {
        include proxy_params;
        proxy_pass http://unix:/home/sammy/myproject/myproject.sock;
    }
}

Save and close the file when you’re finished.

To enable the Nginx server block configuration you’ve just created, link the file to the sites-enabled directory:

With the file in that directory, you can test for syntax errors:

If this returns without indicating any issues, restart the Nginx process to read the new configuration:

Finally, adjust the firewall again. You no longer need access through port 5000, so you can remove that rule. You can then allow full access to the Nginx server:

You should now be able to navigate to your server’s domain name in your web browser:

http://your_domain

You should see your application’s output:

If you encounter any errors, trying checking the following:

• sudo less /var/log/nginx/error.log: checks the Nginx error logs.
• sudo less /var/log/nginx/access.log: checks the Nginx access logs.
• sudo journalctl -u nginx: checks the Nginx process logs.
• sudo journalctl -u myproject: checks your Flask app’s Gunicorn logs.

Step 6 — Securing the Application

To ensure that traffic to your server remains secure, let’s get an SSL certificate for your domain. There are multiple ways to do this, including getting a free certificate from Let’s Encrypt, generating a self-signed certificate, or buying one from another provider and configuring Nginx to use it by following Steps 2 through 6 of How to Create a Self-signed SSL Certificate for Nginx in Ubuntu 22.04. We will use with option one (Let’s Encrypt) for the sake of expediency.

Install Certbot’s Nginx package with apt:

Certbot provides a variety of ways to obtain SSL certificates through plugins. The Nginx plugin will take care of reconfiguring Nginx and reloading the config whenever necessary. To use this plugin, type the following:

This runs certbot with the --nginx plugin, using -d to specify the names we’d like the certificate to be valid for.

If this is your first time running certbot, you will be prompted to enter an email address and agree to the terms of service. After doing so, certbot will communicate with the Let’s Encrypt server, then run a challenge to verify that you control the domain you’re requesting a certificate for.

If that’s successful, certbot will ask how you’d like to configure your HTTPS settings:

Select your choice then hit ENTER. The configuration will be updated, and Nginx will reload to pick up the new settings. certbot will wrap up with a message telling you the process was successful and where your certificates are stored:

If you followed the Nginx installation instructions in the prerequisites, you will no longer need the redundant HTTP profile allowance:

To verify the configuration, navigate once again to your domain, using https://:

https://your_domain

You should see your application output once again, along with your browser’s security indicator, which should indicate that the site is secured.

Conclusion

In this guide, you created and secured a simple Flask application within a Python virtual environment. You created a WSGI entry point so that any WSGI-capable application server can interface with it, and then configured the Gunicorn app server to provide this function. Afterwards, you created a systemd service file to automatically launch the application server on boot. You also created an Nginx server block that passes web client traffic to the application server, relaying external requests, and secured traffic to your server with Let’s Encrypt.

Flask is a very simple, but extremely flexible framework meant to provide your applications with functionality without being too restrictive about structure and design. You can use the general stack described in this guide to serve the flask applications that you design.