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Introduction

Docker is an open-source application that allows administrators to create, manage, deploy, and replicate applications using containers. Containers can be thought of as a package that houses dependencies that an application requires to run at an operating system level. This means that each application deployed using Docker lives in an environment of its own and its requirements are handled separately.

Flask is a web micro-framework that is built on Python. It is called a micro-framework because it does not require specific tools or plug-ins to run. The Flask framework is lightweight and flexible, yet highly structured, making it especially popular for small web apps written in Python.

Deploying a Flask application with Docker will allow you to replicate the application across different servers with minimal reconfiguration.

In this tutorial, you will create a Flask application and deploy it with Docker. This tutorial will also cover how to update an application after deployment.

Prerequisites

To follow this tutorial, you will need the following:

• A non-root user with sudo privileges configured by following the Initial Server Setup with Ubuntu 20.04 guide.
• One Ubuntu 20.04 server with Docker installed, set up by following this tutorial.
• Nginx installed by following step one of the How To Install Nginx on Ubuntu 20.04 tutorial.

Step 1 — Setting Up the Flask Application

To get started, you will create a directory structure that will hold your Flask application. This tutorial will create a directory called TestApp in /var/www, but you can modify the command to name it whatever you’d like.

sudo mkdir /var/www/TestApp

Move in to the newly created TestApp directory:

cd /var/www/TestApp

Next, create the base folder structure for the Flask application:

sudo mkdir -p app/static app/templates 

The -p flag indicates that mkdir will create a directory and all parent directories that don’t exist. In this case, mkdir will create the app parent directory in the process of making the static and templates directories.

The app directory will contain all files related to the Flask application such as its views and blueprints. Views are the code you write to respond to requests to your application. Blueprints create application components and support common patterns within an application or across multiple applications.

The static directory is where assets such as images, CSS, and JavaScript files live. The templates directory is where you will put the HTML templates for your project.

Now that the base folder structure is complete, you need to create the files needed to run the Flask application. First, create an __init__.py file inside the app directory using nano or a text editor of your choice. This file tells the Python interpreter that the app directory is a package and should be treated as such.

Run the following command to create the file:

sudo nano app/__init__.py

Packages in Python allow you to group modules into logical namespaces or hierarchies. This approach enables the code to be broken down into individual and manageable blocks that perform specific functions.

Next, you will add code to the __init__.py that will create a Flask instance and import the logic from the views.py file, which you will create after saving this file. Add the following code to your new file:

from flask import Flask
app = Flask(__name__)
from app import views

Once you’ve added that code, save and close the file. You can save and close the file by pressing Ctrl+X, then when prompted, Y and Enter.

With the __init__.py file created, you’re ready to create the views.py file in your app directory. This file will contain most of your application logic.

sudo nano app/views.py

Next, add the code to your views.py file. This code will return the hello world! string to users who visit your web page:

from app import app

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

The @app.route line above the function is called a decorator. Decorators are a Python language convention that are widely used by Flask; their purpose is to modify the functions immediately following them. In this case, the decorator tells Flask which URL will trigger the home() function. The hello world text returned by the home function will be displayed to the user on the browser.

With the views.py file in place, you’re ready to create the uwsgi.ini file. This file will contain the uWSGI configurations for our application. uWSGI is a deployment option for Nginx that is both a protocol and an application server; the application server can serve uWSGI, FastCGI, and HTTP protocols.

To create this file, run the following command:

sudo nano uwsgi.ini

Next, add the following content to your file to configure the uWSGI server:

[uwsgi]
module = main
callable = app
master = true

This code defines the module that the Flask application will be served from. In this case, this is the main.py file, referenced here as main. The callable option instructs uWSGI to use the app instance exported by the main application. The master option allows your application to keep running, so there is little downtime even when reloading the entire application.

Next, create the main.py file, which is the entry point to the application. The entry point instructs uWSGI on how to interact with the application.

sudo nano main.py

Next, copy and paste the following into the file. This imports the Flask instance named app from the application package that was previously created.

from app import app

Finally, create a requirements.txt file to specify the dependencies that the pip package manager will install to your Docker deployment:

sudo nano requirements.txt

Add the following line to add Flask as a dependency:

Flask>=2.0.2

This specifies the version of Flask to be installed. At the time of writing this tutorial, 2.0.2 is the latest Flask version, and specifying >=2.0.2 will ensure you get version 2.0.2 or newer. Because you’re making a basic test app in this tutorial, the syntax is unlikely to go out of date due to future updates to Flask, but if you wanted to be safe and still receive minor updates, you could specify that you don’t want to install a future major version by specifying something like Flask>=2.0.2,<3.0. You can check for updates at the official website for Flask, or on the Python Package Index’s landing page for the Flask library.

Save and close the file. You have successfully set up your Flask application and are ready to set up Docker.

Step 2 — Setting Up Docker

In this step you will create two files, Dockerfile and start.sh, to create your Docker deployment. The Dockerfile is a text document that contains the commands used to assemble the image. The start.sh file is a shell script that will build an image and create a container from the Dockerfile.

First, create the Dockerfile.

sudo nano Dockerfile

Next, add your desired configuration to the Dockerfile. These commands specify how the image will be built, and what extra requirements will be included.

FROM tiangolo/uwsgi-nginx-flask:python3.8-alpine
RUN apk --update add bash nano
ENV STATIC_URL /static
ENV STATIC_PATH /var/www/app/static
COPY ./requirements.txt /var/www/requirements.txt
RUN pip install -r /var/www/requirements.txt

In this example, the Docker image will be built off an existing image, tiangolo/uwsgi-nginx-flask, which you can find on DockerHub. This particular Docker image is a good choice over others because it supports a wide range of Python versions and OS images.

The first two lines specify the parent image that you’ll use to run the application and install the bash command processor and the nano text editor. It also installs the git client for pulling and pushing to version control hosting services such as GitHub, GitLab, and Bitbucket. ENV STATIC_URL /static is an environment variable specific to this Docker image. It defines the static folder where all assets such as images, CSS files, and JavaScript files are served from.

The last two lines will copy the requirements.txt file into the container so that it can be executed, and then parses the requirements.txt file to install the specified dependencies.

Save and close the file after adding your configuration.

With your Dockerfile in place, you’re almost ready to write your start.sh script that will build the Docker container. Before writing the start.sh script, first make sure that you have an open port to use in the configuration. To check if a port is free, run the following command:

sudo nc localhost 56733 < /dev/null; echo $?

If the output of the command above is 1, then the port is free and usable. Otherwise, you will need to select a different port to use in your start.sh configuration file.

Once you’ve found an open port to use, create the start.sh script:

sudo nano start.sh

The start.sh script is a shell script that will build an image from the Dockerfile and create a container from the resulting Docker image. Add your configuration to the new file:

#!/bin/bash
app="docker.test"
docker build -t ${app} .
docker run -d -p 56733:80 \
  --name=${app} \
  -v $PWD:/app ${app}

The first line is called a shebang. It specifies that this is a bash file and will be executed as commands. The next line specifies the name you want to give the image and container and saves as a variable named app. The next line instructs Docker to build an image from your Dockerfile located in the current directory. This will create an image called docker.test in this example.

The last three lines create a new container named docker.test that is exposed at port 56733. Finally, it links the present directory to the /var/www directory of the container.

You use the -d flag to start a container in daemon mode, or as a background process. You include the -p flag to bind a port on the server to a particular port on the Docker container. In this case, you are binding port 56733 to port 80 on the Docker container. The -v flag specifies a Docker volume to mount on the container, and in this case, you are mounting the entire project directory to the /var/www folder on the Docker container.

Save and close the file after adding your configuration.

Execute the start.sh script to create the Docker image and build a container from the resulting image:

sudo bash start.sh

Once the script finishes running, use the following command to list all running containers:

sudo docker ps

You will receive output that shows the containers:

Output
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                            NAMES
58b05508f4dd        docker.test         "/entrypoint.sh /sta…"   12 seconds ago      Up 3 seconds       443/tcp, 0.0.0.0:56733->80/tcp   docker.test

You will find that the docker.test container is running. Now that it is running, visit the IP address at the specified port in your browser: http://ip-address:56733

You’ll see a page similar to the following:

In this step you have successfully deployed your Flask application on Docker. Next, you will use templates to display content to users.

Step 3 — Serving Template Files

Templates are files that display static and dynamic content to users who visit your application. In this step, you will create a HTML template to create a homepage for the application.

Start by creating a home.html file in the app/templates directory:

sudo nano app/templates/home.html

Add the code for your template. This code will create an HTML5 page that contains a title and some text.

<!doctype html>

<html lang="en-us">   
  <head>
    <meta charset="utf-8">
    <meta http-equiv="x-ua-compatible" content="ie=edge">
    <title>Welcome home</title>
  </head>
  
  <body>
    <h1>Home Page</h1>
    <p>This is the home page of our application.</p>
  </body> 
</html>

Save and close the file once you’ve added your template.

Next, modify the app/views.py file to serve the newly created file:

sudo nano app/views.py

First, add the following line at the beginning of your file to import the render_template method from Flask. This method parses an HTML file to render a web page to the user.

from flask import render_template
...

At the end of the file, you will also add a new route to render the template file. This code specifies that users are served the contents of the home.html file whenever they visit the /template route on your application.

...

@app.route('/template')
def template():
    return render_template('home.html')

The updated app/views.py file will look like this:

from flask import render_template
from app import app 

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

@app.route('/template')
def template():
    return render_template('home.html')

Save and close the file when done.

In order for these changes to take effect, you will need to stop and restart the Docker containers. Run the following command to rebuild the container:

sudo docker stop docker.test && sudo docker start docker.test

Visit your application at http://your-ip-address:56733/template to see the new template being served.

In this you’ve created a Docker template file to serve visitors on your application. In the next step you will see how the changes you make to your application can take effect without having to restart the Docker container.

Step 4 — Updating the Application

Sometimes you will need to make changes to the application, whether it is installing new requirements, updating the Docker container, or HTML and logic changes. In this section, you will configure touch-reload to make these changes without needing to restart the Docker container.

Python autoreloading watches the entire file system for changes and refreshes the application when it detects a change. Autoreloading is discouraged in production because it can become resource intensive very quickly. In this step, you will use touch-reload to watch for changes to a particular file and reload when the file is updated or replaced.

To implement this, start by opening your uwsgi.ini file:

sudo nano uwsgi.ini

Next, add the highlighted line to the end of the file:

module = main
callable = app
master = true
touch-reload = /app/uwsgi.ini

This specifies a file that will be modified to trigger an entire application reload. Once you’ve made the changes, save and close the file.

To demonstrate this, make a small change to your application. Start by opening your app/views.py file:

sudo nano app/views.py

Replace the string returned by the home function:

from flask import render_template
from app import app

@app.route('/')
def home():
    return "<b>There has been a change</b>"

@app.route('/template')
def template():
    return render_template('home.html')

Save and close the file after you’ve made a change.

Next, if you open your application’s homepage at http://ip-address:56733, you will notice that the changes are not reflected. This is because the condition for reload is a change to the uwsgi.ini file. To reload the application, use touch to activate the condition:

sudo touch uwsgi.ini

Reload the application homepage in your browser again. You will find that the application has incorporated the changes:

In this step, you set up a touch-reload condition to update your application after making changes.

Conclusion

In this tutorial, you created and deployed a Flask application to a Docker container. You also configured touch-reload to refresh your application without needing to restart the container.

With your new application on Docker, you can now scale with ease. To learn more about using Docker, check out their official documentation.

Checking for script in /app/prestart.sh
There is no script /app/prestart.sh
CRIT Supervisor is running as root.  Privileges were not dropped because no user is specified in the config file.  If you intend to run as root, you can set user=root in the config file to avoid this message.
INFO Included extra file "/etc/supervisor.d/supervisord.ini" during parsing
INFO RPC interface 'supervisor' initialized
CRIT Server 'unix_http_server' running without any HTTP authentication checking
INFO supervisord started with pid 1
INFO spawned: 'quit_on_failure' with pid 8
INFO spawned: 'nginx' with pid 9
INFO spawned: 'uwsgi' with pid 10
INFO success: nginx entered RUNNING state, process has stayed up for > than 0 seconds (startsecs)
INFO success: uwsgi entered RUNNING state, process has stayed up for > than 0 seconds (startsecs)
realpath() of /app/uwsgi.ini failed: No such file or directory [core/utils.c line 3662]
INFO success: quit_on_failure entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
INFO exited: uwsgi (exit status 1; not expected)
WARN received SIGTERM indicating exit request
INFO waiting for quit_on_failure, nginx to die
INFO stopped: nginx (exit status 0)
INFO stopped: quit_on_failure (terminated by SIGTERM)
/usr/lib/python3.8/site-packages/supervisor/options.py:473: UserWarning: Supervisord is running as root and it is searching for its configuration file in default locations (including its current working directory); you probably want to specify a "-c" argument specifying an absolute path to a configuration file for improved security.
  self.warnings.warn(