How to deploy a Flask Application on Ubuntu

Updated , by Austin Balarin

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Flask is a light-weight web framework for Python that includes several utilities and libraries you can use to create a web application. After you have developed a Flask application in a local environment, you need to prepare the application’s production environment in order to run the application and serve it to the users of the application through the internet.

This guide walks you through the steps to deploy a Flask application to a production environment running on a Linode. The production environment uses NGINX as the web server and reverse proxy, Gunicorn as the web server gateway interface (WSGI) application server, and Supervisor for monitoring and auto-reloading Gunicorn should it go down. This guide does not cover creating a Flask application or related Python concepts.

In this guide you complete the following:

Before You Begin

  1. Create a Flask Application or use this Example Blog Application. Clone and run it on the local machine using GitHub.

    git clone https://github.com/abalarin/Flask-on-Linode.git flask_app_project
    
    Note
    The Example Flask Blog Application is used throughout this guide. The root directory of the application is flask_app_project.
  2. If you are not using the example application, host the Flask application code on a remote version control system, such as GitHub. This guide uses GitHub for all examples.

  3. Create a 1GB Linode to host the Flask web application. Depending on the size of the application and the amount of users you expect to visit it, you may consider a large Linode plan.

  4. Familiarize yourself with the Getting Started guide and complete the steps for updating the system’s software, setting the Linode’s hostname and timezone.

  5. This guide uses sudo wherever possible. Complete the sections of Securing the Server to create a limited user account, harden SSH access and remove unnecessary network services.

Copy the Flask App to Linode

After creating the Flask application in the local development environment, you are now ready to deploy it to a production environment. You need to copy the local Flask application code to the Linode. You can accomplish this by either cloning the GitHub project to the Linode using Git or by using the secure copy method to directly transfer the application files to the Linode. This section provides steps for both options.

Note
This guide’s examples transfer the Flask application files to the Linode’s /home directory. If you prefer, you can store the application files in a different directory, however, ensure you run the examples in the directory of the application.

Clone the App From Source Control

  1. Ensure that the latest Flask application code is available in the project’s remote repository.

  2. SSH into the Linode. Replace the example IP address with the IP address of the Linode:

    ssh [email protected]
    
  3. Navigate to the home directory of Linode:

     cd ~
    
  4. Clone the project from the remote version control system. If you are not using the example repository, Example Flask Blog Application, replace the example repo with the name of the repo that you want to use:

     git clone https://github.com/abalarin/Flask-on-Linode.git flask_app_project
    
      [email protected]:~# git clone https://github.com/abalarin/Flask-on-Linode.git
    Cloning into 'Flask-on-Linode'...
    remote: Enumerating objects: 79, done.
    remote: Counting objects: 100% (79/79), done.
    remote: Compressing objects: 100% (57/57), done.
    remote: Total 79 (delta 26), reused 66 (delta 16), pack-reused 0
    Unpacking objects: 100% (79/79), done.
    Checking connectivity... done.
    

Secure Copy the App From a Local Machine

  1. From the local machine, secure copy (SCP) the project into the /home directory of the Linode. Replace the example IP address with the IP address of the Linode and flask_app with the name of the root directory of the project:

    scp -r flask_app_project/ [email protected]:/~
    
  2. After the application is copied, navigate to the /home directory of the Linode and view the contents of the directory that you copied:

     ls flask_app_project
    

    An output similar to the following appears:

      flask_app  FlaskDeployment.md  README.md
    

    Now that the Linode contains the application files, you can move on to prepare the production environment.

Prepare the Production Environment

Install and Configure NGINX

NGINX is open-source software that can be used as a high-performance web server, reverse proxy, load-balancer, and more. In this section you configure NGINX as a web server and reverse proxy for the Flask application. This means that NGINX sits between the Flask application and external clients and forwards all client requests to the running Flask application.

  1. Install NGINX:

     sudo apt install nginx
    
  2. Using an editor of choice, create an NGINX configuration file for the app with the example content and save it. This example uses the nano text editor. Replace flask_app with the name of the application and 192.0.2.0 with the IP address of the Linode or the fully qualified domain name (FQDN):

     sudo nano /etc/nginx/sites-enabled/flask_app
    
    File: /etc/nginx/sites-enabled/flask_app
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    server {
        listen 80;
        server_name 192.0.2.0;
    
        location / {
            proxy_pass http://127.0.0.1:8000;
            proxy_set_header Host $host;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        }
    }
  3. Disable the NGINX’s default configuration file by removing its symlink:

     sudo unlink /etc/nginx/sites-enabled/default
    
  4. Reload the NGINX configuration file:

     sudo nginx -s reload
    
  5. Navigate to the IP address of the Linode in a web browser. You should see a similar NGINX Gateway error. This error appears because you have not set up the WSGI application server yet. You set up the application server in the Install and Configure Gunicorn section of the guide.

    502Gateway

Install Python and Packages

To run the Flask application, you need to install Python, Flask, pip3 and any other required package dependencies on the Linode.

Note
This guide was created using Python 3.6.8
  1. In the Linode’s /home directory, install Python 3:

     sudo apt install python3
    
  2. Install pip3, the standard package manager for Python:

     sudo apt install python3-pip
    
  3. Navigate to the project’s root directory:

     cd ~/flask_app_project
    
  4. Install Flask packages and libraries using pip3. If you are using the Example Flask Blog Application, then the packages that the application need are listed in the /home/flask_app_project/flask_app/requirements.txt file. You can use pip to install all listed packages.

     pip3 install -r flask_app/requirements.txt
    

    While the packages and libraries are being installed, an output similar to the following appears:

      [email protected]:~/flask_app_project# pip3 install -r flask_app/requirements.txt
    Collecting flask-sqlalchemy (from -r flask_app/requirements.txt (line 1))
      Downloading https://files.pythonhosted.org/packages/08/ca/582442cad71504a1514a2f053006c8bb128844133d6076a4df17117545fa/Flask_SQLAlchemy-2.4.0-py2.py3-none-any.whl
    Collecting sqlalchemy (from -r flask_app/requirements.txt (line 2))
      Downloading https://files.pythonhosted.org/packages/55/98/56b7155bab287cd0c78dee26258835db36e91f2efef41f125ed6f6f1f334/SQLAlchemy-1.3.6.tar.gz (5.9MB)
        100% |████████████████████████████████| 5.9MB 218kB/s
    Collecting flask-security (from -r flask_app/requirements.txt (line 3))
      Downloading https://files.pythonhosted.org/packages/88/47/4908a5040120768ff4fb2465c7eeafeb9239c27d2919bd67c4ccc1b43e14/Flask_Security-3.0.0-py2.py3-none-any.whl (68kB)
        100% |████████████████████████████████| 71kB 8.7MB/s
    Collecting flask-wtf (from -r flask_app/requirements.txt (line 4))
      Downloading https://files.pythonhosted.org/packages/60/3a/58c629472d10539ae5167dc7c1fecfa95dd7d0b7864623931e3776438a24/Flask_WTF-0.14.2-py2.py3-none-any.whl
    ...
    

    Now that the Linode has all required dependencies, you configure Flask in the next section.

Configure Flask

Depending on the environment of the Flask application, there are different settings you may need to configure, like toggling the debug mode, setting the secret key, setting the database URI, etc. For more information on Flask’s available configuration options see the configuration docs.

In this section, you create a JSON file to store the configuration of the environment and then load that configuration into the Flask app. The configuration created in this section is a basic example of some Flask environment variables you might include in the application.

Caution
You should keep sensitive configuration files outside of source control. If you source control the configuration file, which contains sensitive values, in a remote repository, then someone could access it and use that information to compromise security of the Linode server or the application. To keep the configuration file out of the Git repository, add it to the .gitignore file.
  1. Create a JSON configuration file with a text editor:

    sudo nano /etc/config.json
    
  2. Store the environment variables of the application in the JSON configuration file with the example content:

    File: /etc/config.json
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    {
      "SECRET_KEY": "1A37BbcCJh67",
      "SQLALCHEMY_DATABASE_URI": "sqlite:///site.db"
    }
    • The SECRET_KEY is used to keep client-side sessions secure using a session cookie that can only be modified if the secret key is known and used for signing. Replace the value included in the example with a randomly generated value.

    • The SQLALCHEMY_DATABASE_URI defines the database URI that should be used for the application’s connection to SQLite.

  3. Modify the __init__.py file to import the newly created JSON configuration:

    File: ~/flask_app_project/flask_app/__init__.py
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    from flask import Flask
    from flask_sqlalchemy import SQLAlchemy
    from flask_login import LoginManager
    import json
    import urllib3
    
    app = Flask(__name__)
    
    with open('/etc/config.json') as config_file:
      config = json.load(config_file)
    
    app.config['SECRET_KEY'] = config.get('SECRET_KEY')
    app.config['SQLALCHEMY_DATABASE_URI'] = config.get('SQLALCHEMY_DATABASE_URI')
    db = SQLAlchemy(app)
    
    login_manager = LoginManager()
    login_manager.init_app(app)
    
    from flask_app import routes
        

Install and Configure Gunicorn

Gunicorn, Green Unicorn, is a Python web server gateway interface (WSGI) HTTP Server for UNIX. It is used to forward requests from the NGINX web server to the Flask application.

  1. Install Gunicorn on Linode:

     sudo apt-get install gunicorn
    
  2. Run Gunicorn from the root directory of the application, flask_app_project. The command tells Gunicorn to look for the WSGI instance named app in the flask_app directory. In the example project, the WSGI instance named app is located in /home/Flask-on-Linode/flask_app/__init__.py.

     gunicorn -w 3 flask_app:app
    
      [email protected]:~/Flask-on-Linode# gunicorn -w 3 flask_app:app
    [2019-07-25 15:09:04 +0000] [32421] [INFO] Starting gunicorn 19.9.0
    [2019-07-25 15:09:04 +0000] [32421] [INFO] Listening at: http://127.0.0.1:8000 (32421)
    [2019-07-25 15:09:04 +0000] [32421] [INFO] Using worker: sync
    [2019-07-25 15:09:04 +0000] [32424] [INFO] Booting worker with pid: 32424
    [2019-07-25 15:09:04 +0000] [32425] [INFO] Booting worker with pid: 32425
    [2019-07-25 15:09:04 +0000] [32426] [INFO] Booting worker with pid: 32426
    
    Note
    You can specify the number of workers you want Gunicorn to use with the --workers flag. A good rule of thumb to determine worker count is to double the system’s CPU cores and add 1. For a 1GB Linode (Nanode) with 1 CPU core you should use 3 workers.
  3. After running Gunicorn, the Flask application should be live and available over the internet. Open a web browser and enter the IP address of the Linode to access the application. If you used the example Flask blog application, you should see the following:

    Your App

    Continue on to the next section to configure Supervisor to monitor and control the Flask app.

Install and Configure Supervisor

Supervisor is a client/server system that allows its users to monitor and control a number of processes on UNIX-like operating systems. Supervisor can handle auto-reloading Gunicorn if it crashes or if the Linode is rebooted unexpectedly. In this section, you install and configure Supervisor.

  1. Open a new shell session and SSH into the Linode:

     ssh [email protected]
    
  2. Install Supervisor:

     sudo apt install supervisor
    
  3. Create a Supervisor script. Replace any instances of flask_app with the name of the application:

     sudo nano /etc/supervisor/conf.d/flask_app.conf
    
    File: /etc/supervisor/conf.d/flask_app.conf
    [program:flask_app]
    directory=/home/flask_app_project
    command=gunicorn3 --workers=3 flask_app:app
    autostart=true
    autorestart=true
    stopasgroup=true
    killasgroup=true
    stderr_logfile=/var/log/flask_app/flask_app.err.log
    stdout_logfile=/var/log/flask_app/flask_app.out.log
  4. Create the log directories and files listed in the flask_app.conf file. Make sure to replace flask_app if it was modified in the Supervisor script above:

     sudo mkdir /var/log/flask_app
     sudo touch /var/log/flask_app/flask_app.out.log
     sudo touch /var/log/flask_app/flask_app.err.log
    
  5. Reload Supervisor to apply the changes:

     sudo supervisorctl reload
    

    An output similar to the following appears:

      Restarted supervisord
    

    Note

    The application should now be accessible again through the IP address of the Linode. If you are unable to access the application or receive a bad gateway error, Gunicorn is likely not running. Check the log files to further investigate the issue.

    cat /var/log/flask_app/flask_app.err.log
    cat /var/log/flask_app/flask_app.out.log
    

    The Flask application is now deployed to the production environment and available to anyone for viewing. You can follow a similar workflow to deploy any Flask application to a Linode.

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