How to migrate (or create) and deploy a Flask project

This guide will take you through the steps to deploy a portable, vendor-neutral Flask project, either by building it from scratch or migrating an existing application, using Docker. The project architecture is in line with Twelve-factor design principles.

This guide assumes that you are familiar with the basics of the Divio platform and have Docker and the Divio CLI installed.

Edit (or create) the project files

Start in an existing Flask project, or if necessary, create a new directory.

Create a minimal application if required

You may already have a Flask application of your own to migrate, but if not, the example below provides a minimal one. The example is taken from the Flask tutorial’s own flaskr example. Create a flaskr directory, containing an file:

import os

from flask import Flask

def create_app(test_config=None):
    # create and configure the app
    app = Flask(__name__, instance_relative_config=True)
        SECRET_KEY = os.environ.get('SECRET_KEY', 'dev'),
        DATABASE = os.environ.get('DATABASE_URL', os.path.join(app.instance_path, 'flaskr.sqlite'),
        STORAGE = os.environ.get('DEFAULT_STORAGE_DSN')

    if test_config is None:
        # load the instance config, if it exists, when not testing
        app.config.from_pyfile('', silent=True)
        # load the test config if passed in

    # ensure the instance folder exists
    except OSError:

    # a simple page that says hello
    def hello():
        return 'Hello, World!'

    return app

Note the highlighted sections above, in which the application obtains configuration values from its environment. If you are working on your own application that has database or other configuration of this kind, you should adapt it so that it is similarly able to obtain these values.

The next step is to Dockerise the application.

The Dockerfile

Create a file named Dockerfile, adding:

FROM python:3.8
COPY . /app
RUN pip install -r requirements.txt

# Select one of the following application gateway server commands
CMD uwsgi --http= --module="flaskr:create_app()"
CMD gunicorn --bind= --forwarded-allow-ips="*" "flaskr:create_app()"

You may need to change the version of Python, and should also select the CMD that will start your preferred gateway server for production - if you’re not using the flaskr example, you’ll need to amend the name in the command too.

Python requirements in requirements.txt

The Dockerfile expects to find a requirements.txt file, so add one if required. Where indicated below, choose the appropriate options to install the components for Postgres/MySQL if you plan to use them, and uWSGI/Gunicorn, for example:


# Select one of the following for the database as required

# Select one of the following for the gateway server

Check that the version of Flask is correct, and include any other Python components required by your project.

Local container orchestration with docker-compose.yml

Create a docker-compose.yml file, for local development purposes. This will replicate the web image used in cloud deployments, allowing you to run the application in an environment as close to that of the cloud servers as possible. Amongst other things, it will allow the project to use a Postgres or MySQL database running in a local container, and provides convenient access to files inside the containerised application.

You will need to include/delete the highlighted sections below appropriately:

version: "2.4"
    # the application's web service (container) will use an image based on our Dockerfile
    build: "."
    # map the internal port 80 to port 8000 on the host
      - "8000:80"
    # map the host directory to app (which allows us to see and edit files inside the container)
      - ".:/app:rw"
      - "./data:/data:rw"
    # the default command to run whenever the container is launched
    command: flask run --host= --port=80
    # the URL 'postgres' or 'mysql' will point to the application's db service
      - "database_default"
    env_file: .env-local

    # Select one of the following db configurations for the database
    image: postgres:9.6-alpine
      POSTGRES_DB: "db"
      SERVICE_MANAGER: "fsm-postgres"
      - ".:/app:rw"

    image: mysql:5.7
      MYSQL_DATABASE: "db"
      SERVICE_MANAGER: "fsm-mysql"
      - ".:/app:rw"
      - "./data/db:/var/lib/mysql"
        test: "/usr/bin/mysql --user=root -h --execute \"SHOW DATABASES;\""
        interval: 2s
        timeout: 20s
        retries: 10

Local configuration using .env-local

As you will see above, the web service refers to an env_file containing the environment variables that will be used in the local development environment. Create a .env-local file. As with the docker-compose.yml, select the DATABASE_URL as required.

The FLASK_APP variable is used by the flask run command. It assumes that your application can be found at flaskr; amend this appropriately if required.

# Select one of the following for the database



With this, you have the basics for a Dockerised application that can equally effectively be deployed in a production environment or run locally, using environment variables for configuration in ether case.

Build with Docker

Now you can build the application containers locally:

docker-compose build

Application configuration

It’s beyond the scope of this guide to cover configuration in detail, as that will depend to a great extent on the application you have or are planning to build. However the basic principle for all configuration is similar: exactly the same application code should run without modification whether locally or in one of the multiple cloud environments, and all configuration should be provided by environment variables.

For example:


In the flaskr example above, the database configuration is read from the DATABASE_URL environment variable, and falls back to use SQLite if not provided.

Each Divio cloud environment with a database attached to it will be provided automatically with a DATABASE_URL environment variable. In the .env-local and docker-compose.yml files above, example configuration is provided so that when running locally, the application can use the same database type as it does in production. (This is a much more satisfactory approach than using say Postgres in production and SQLite for development.)

Media storage

If your application needs to handle media, it should parse the DEFAULT_STORAGE_DSN to configure an appropriate storage interface. Each Divio cloud environment with media object storage provisioned will be provided with a DEFAULT_STORAGE_DSN variable.

Use DEFAULT_STORAGE_DSN in .env-local to configure storage for local development. This can be one of the cloud storage instances, but it’s often convenient to use local file storage rather than a cloud media store (as in the example given, file:///data/media/?url=%2Fmedia%2F) if your Flask code can handle both kinds of storage backend.

Serving static files

For handling static files, various suitable options are available, including WhiteNoise - see Using WhiteNoise with Flask.

Other configuration

Divio cloud projects include a number of environment variables as standard. In addition, user-supplied variables may be applied per-environment.

Check the local site

To start up the site locally to test it:

docker-compose up

and access it at (if using the flaskr example).

Test using the production gateway server

In cloud environments: the Dockerfile contains a CMD that starts up Flask using the uWSGI/Gunicorn or other application gateway server.

In the local environment: the command line in docker-compose.yml starts up Flask using the flask run command, overriding the CMD in the Dockerfile. If the command line is commented out, docker-compose up will use the application gateway server locally instead.

Deployment and further development

Create a new project on Divio

In the Divio Control Panel add a new project, selecting the Build your own option.

Add database and media services

The new project does not include any additional services; they must be added manually using the Divio Control Panel if required. Use the Services menu to add a Postgres or MySQL database to match your choice earlier, and an S3 object storage instance for media.

Connect the local project to the cloud project

Your Divio project has a slug, based on the name you gave it when you created it. Run divio project list -g to get your project’s slug; you can also read the slug from the Control Panel.


divio project configure

and provide the slug. (This creates a new file in the project at .divio/config.json.)

If you have done this correctly, divio project dashboard will open the project in the Control Panel.

Configure the Git repository

Initialise the project as a Git repository if it’s not Git-enabled already:

git init .

A .gitignore file is needed to exclude unwanted files from the repository. Add:

# Python

# Divio

# OS-specific patterns - add your own here

Add the project’s Git repository as a remote, using the slug value in the remote address:

git remote add origin<slug>.git

(Use e.g. divio instead if you already have a remote named origin.)

Commit your work

git add .                                                 # add all the newly-created files
git commit -m "Created new project"                       # commit
git push --set-upstream --force origin [or divio] master  # push, overwriting any unneeded commits made by the Control Panel at creation time

You’ll now see “1 undeployed commit” listed for the project in the Control Panel.

Deploy the Test server

Deploy with:

divio project deploy

(or use the Deploy button in the Control Panel).

Once deployed, your project will be accessible via the Test server URL shown in the Control Panel.