Deploy Flask on Nodion

Deploy Flask step by step

1. Add Gunicorn to your dependencies Nodion starts Gunicorn but does not install it, so list it next to Flask. Pin the Python version in .python-version, because a range in pyproject.toml picks the newest release. requirements.txt

   flask
   gunicorn

2. Keep the app object in app.py Nodion takes the first of app.py, application.py and wsgi.py that contains Flask and runs gunicorn <file>:app with 3 workers on the PORT it sets. The object has to be called app. app.py

   from flask import Flask
   
   app = Flask(__name__)
   
   
   @app.get("/")
   def index():
       return "Hello from Frankfurt"

3. Using an app factory? Add a web line With create_app() there is no module-level app to find, so tell Gunicorn to call the factory in a Procfile. The line replaces the default start command. Procfile

   web: gunicorn "myapp:create_app()" --bind 0.0.0.0:$PORT --workers 3

4. Run migrations on every deploy With Flask-Migrate, a release: line runs flask db upgrade once per build, after the build and before the new version starts. If it fails, the deploy stops and the running version stays live. Procfile

   release: flask --app myapp db upgrade

5. Create the app and the database Create an application from your repository in the region EU Central, then a PostgreSQL database in the same region, and attach it on the Environment tab: it arrives as DATABASE_URL on the next deploy. Set your SECRET_KEY as a secret variable, read it with os.environ, then push.

Good to know

• DATABASE_URL and SQLAlchemyNodion's DATABASE_URL starts with postgres://, which SQLAlchemy 1.4 and newer do not accept. Replace the scheme before you pass it on: postgresql:// for psycopg2, postgresql+psycopg:// for psycopg 3.
• Health checkTraffic switches once / answers with 2xx or 3xx, so a redirect to a login page is fine. If / returns 401 or 404, add a route such as /healthz and set it as the health check path in the app settings.
• Celery and RQ workersAdd worker: celery -A myapp worker to the Procfile together with an explicit web: gunicorn app:app --bind 0.0.0.0:$PORT --workers 3; without a web: line the first Procfile entry other than release: becomes the default command. Then add a process named worker without a port on the Processes tab. Valkey in the same region gives you REDIS_URL.

What Nodion runs for you

• Deploys without downtime Push to your branch and Nodion builds a new release. Traffic switches once the new instances answer their health check; a failed release leaves the running one in place. Apps with a local volume restart instead, because the disk belongs to one instance.
• Databases next to the app Managed PostgreSQL, pgvector, MySQL, Valkey and ClickHouse in the same region. Attached databases arrive as DATABASE_URL, REDIS_URL or CLICKHOUSE_URL.
• Previews per pull request Turn on previews and pull requests from your repository get their own app with its own URL and fresh, empty databases, created automatically or by hand. Forks and bot pull requests are skipped.
• Workers, cron jobs and scaling Run workers as extra processes of the same app, schedule jobs on the Cron tab, and scale every process to up to 16 instances.
• Release step With buildpacks, a Procfile release: line runs with every new build. Dockerfile apps, and redeploys or rollbacks to an earlier image, use a release command set on the app itself, which also takes precedence over the Procfile line. On static sites a release step runs in an image that holds only the built files.
• Logs and metrics Build, release and runtime logs plus CPU, memory, disk and network metrics for every instance in the dashboard.
• Domains and HTTPS A default hostname with HTTPS from the first deploy; your own domains get certificates automatically.

Further reading

• Heroku Alternatives with EU Hosting: 5 Options Compared A German platform and US clouds with EU regions, compared by headquarters, regions and managed databases.
• Preview Environments Explained: An App per Pull Request A running copy of your app for every pull request: what preview environments are good for, where they get tricky and how Nodion handles them.

Flask on Nodion: questions and answers