Deploying Flask Applications with Gunicorn as a WSGI Server

Flask is a lightweight and powerful framework for building web applications, but it is not production-ready on its own. For production deployments, using a WSGI server like Gunicorn is an excellent choice. Gunicorn is reliable, scalable, and works seamlessly with Flask.

In this blog post, we’ll walk through setting up Gunicorn to serve your Flask application efficiently.

1. Install Gunicorn

First, ensure Gunicorn is installed in your Python environment. If you don’t have it yet, you can install it using pip:

Copied!pip install gunicorn
pip install gunicorn

2. Flask Application Structure

Let’s assume you have the following Flask app structure:

Copied!my_app/
│
├── app.py
└── requirements.txt
my_app/
│
├── app.py
└── requirements.txt

Here’s a basic example of what app.py might look like:

Copied!from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return "Hello, World!"

if __name__ == "__main__":
    app.run()
from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return "Hello, World!"

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

3. Running the Application with Gunicorn

Gunicorn serves your application using the WSGI interface. To run your Flask app with Gunicorn, navigate to the directory containing app.py and execute:

Copied!gunicorn -w 4 -b 0.0.0.0:8000 app:app
gunicorn -w 4 -b 0.0.0.0:8000 app:app

Here’s what each option does:

• -w 4: Starts 4 worker processes (adjust based on your server’s CPU cores).
• -b 0.0.0.0:8000: Binds the application to all network interfaces on port 8000.
• app:app: Specifies the entry point for Gunicorn. The format is MODULE_NAME:APP_VARIABLE_NAME.

4. Making Gunicorn Production-Ready

Configuring Gunicorn

Instead of passing options via the command line, you can create a Gunicorn configuration file (gunicorn.conf.py) for better manageability. Example:

Copied!# gunicorn.conf.py
bind = "0.0.0.0:8000"
workers = 4
loglevel = "info"
errorlog = "/var/log/gunicorn/error.log"
accesslog = "/var/log/gunicorn/access.log"
# gunicorn.conf.py
bind = "0.0.0.0:8000"
workers = 4
loglevel = "info"
errorlog = "/var/log/gunicorn/error.log"
accesslog = "/var/log/gunicorn/access.log"

Run Gunicorn with the configuration:

Copied!gunicorn -c gunicorn.conf.py app:app
gunicorn -c gunicorn.conf.py app:app

Running Gunicorn as a Service

To ensure your app starts automatically on server boot, set up a systemd service for Gunicorn:

1. Create the Service File
   Create /etc/systemd/system/my_app.service:

Copied![Unit]
Description=Gunicorn instance to serve Flask application
After=network.target

[Service]
User=your_user
Group=your_group
WorkingDirectory=/path/to/your/app
ExecStart=/path/to/venv/bin/gunicorn -c /path/to/gunicorn.conf.py app:app

[Install]
WantedBy=multi-user.target
[Unit]
Description=Gunicorn instance to serve Flask application
After=network.target

[Service]
User=your_user
Group=your_group
WorkingDirectory=/path/to/your/app
ExecStart=/path/to/venv/bin/gunicorn -c /path/to/gunicorn.conf.py app:app

[Install]
WantedBy=multi-user.target

2. Start and Enable the Service

Copied!sudo systemctl start my_app
sudo systemctl enable my_app
sudo systemctl start my_app
sudo systemctl enable my_app

5. Integrating with Nginx

While Gunicorn can serve requests directly, pairing it with a reverse proxy like Nginx improves performance and adds an extra layer of security. A typical Nginx configuration might look like this:

Copied!server {
    listen 80;
    server_name yourdomain.com;

    location / {
        proxy_pass http://127.0.0.1:8000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}
server {
    listen 80;
    server_name yourdomain.com;

    location / {
        proxy_pass http://127.0.0.1:8000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

After editing the configuration, reload Nginx:

Copied!sudo systemctl reload nginx
sudo systemctl reload nginx

6. Testing Your Deployment

Visit your server’s IP address or domain (e.g., http://yourdomain.com). If everything is set up correctly, your Flask application should be live and accessible.

Our recently developed Scalable Data Ingestion is an excellent example to try this yourself.

Conclusion

Using Gunicorn as the WSGI server for your Flask application is a reliable and efficient way to handle production traffic. Pairing it with Nginx ensures better performance and added security. With this setup, you can confidently deploy and scale your Flask applications.