Does Flask Work With Fly.io?
Flask deploys seamlessly on Fly.io with minimal configuration, making it an excellent choice for Python web applications at the edge.
Quick Facts
How Flask Works With Fly.io
Flask is a stateless WSGI framework that aligns perfectly with Fly.io's containerized deployment model. You package your Flask app in a Docker container (which Fly.io can generate automatically), and it handles routing traffic to instances across multiple regions worldwide. Fly.io's architecture supports Flask's request-response cycle without friction—no vendor lock-in or framework-specific requirements. The developer experience is straightforward: define your app, create a Dockerfile or let `fly launch` generate one, and deploy with `fly deploy`. Fly.io automatically scales instances and manages health checks. The main architectural consideration is ensuring your Flask app is stateless; if you need persistent data, use Fly.io's PostgreSQL or Redis add-ons. WebSocket support works via Flask-SocketIO if needed, though you'll want to use sticky sessions or a shared message broker for multi-instance deployments.
Best Use Cases
Quick Setup
pip install flask gunicorn && fly launch# app.py
from flask import Flask
app = Flask(__name__)
@app.route('/')
def hello():
return {'message': 'Hello from Fly.io', 'status': 'ok'}
@app.route('/api/data')
def get_data():
return {'data': [1, 2, 3], 'region': 'edge'}
if __name__ == '__main__':
app.run(host='0.0.0.0', port=8080)
# Dockerfile
FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
EXPOSE 8080
CMD ["gunicorn", "--bind", "0.0.0.0:8080", "app:app"]Known Issues & Gotchas
Flask's development server (flask run) won't work in production; you need a production WSGI server like Gunicorn
Fix: Use Gunicorn in your Dockerfile: install it and run `gunicorn app:app` instead of Flask's development server
File uploads and temp files are lost when instances restart since Fly.io uses ephemeral storage by default
Fix: Store uploads in Fly.io's persistent volumes or use S3-compatible object storage (Tigris or similar)
Session data stored in memory is lost across deployments or instance restarts
Fix: Use Flask-Session with Redis or PostgreSQL as the backend, configured via Fly.io's add-ons
Build times can be slow if your requirements file is large and Docker rebuilds from scratch
Fix: Use Docker layer caching effectively by separating requirements installation from code copying in your Dockerfile
Alternatives
- •FastAPI + Fly.io: Modern async Python framework with better performance for high-concurrency workloads
- •Django + Fly.io: Full-featured framework with more built-in tools, but heavier than Flask
- •Node.js (Express) + Fly.io: JavaScript alternative with strong async patterns and larger ecosystem
Resources
Related Compatibility Guides
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