Does FastAPI Work With Fly.io?
FastAPI works excellently with Fly.io—deploy containerized Python APIs globally with minimal configuration.
Quick Facts
How FastAPI Works With Fly.io
FastAPI and Fly.io are a natural pairing. FastAPI applications run in Docker containers, which Fly.io deploys across its global infrastructure. You create a standard Dockerfile, configure a fly.toml file, and push to Fly's infrastructure—your API automatically scales and runs close to users worldwide. The developer experience is seamless: FastAPI's ASGI server (uvicorn) starts instantly in Fly containers, health checks work out of the box, and environment variables integrate cleanly. Fly handles load balancing, SSL termination, and geographic distribution automatically. The main architectural consideration is that Fly's shared-cpu instances are cost-effective for moderate traffic, but you can scale to dedicated VMs. Database connections and persistent state require external services (Fly Postgres, Redis, or third-party databases) since Fly containers are ephemeral. FastAPI's async/await design pairs well with Fly's connection pooling and request routing.
Best Use Cases
Quick Setup: FastAPI + Fly.io
pip install fastapi uvicorn# main.py
from fastapi import FastAPI
from fastapi.responses import JSONResponse
import os
app = FastAPI()
@app.get("/")
async def root():
return {"message": "Hello from Fly.io", "env": os.getenv("FLY_REGION", "local")}
@app.get("/health")
async def health():
return JSONResponse({"status": "ok"})
if __name__ == "__main__":
import uvicorn
uvicorn.run(app, host="0.0.0.0", port=8080)
# Dockerfile
FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8080"]
# fly.toml (auto-generated with 'fly launch')
app = "my-fastapi-app"
primary_region = "ord"
[build]
image = "my-fastapi-app:latest"
[[services]]
internal_port = 8080
processes = ["app"]
protocol = "tcp"
[services.concurrency]
hard_limit = 25
soft_limit = 20Known Issues & Gotchas
Fly's free tier has limited shared-cpu resources; production workloads may incur unexpected costs
Fix: Monitor resource usage via fly status and scale to dedicated-cpu machines for consistent performance. Set up billing alerts.
Persistent file uploads to /tmp disappear when containers restart; no persistent filesystem by default
Fix: Use Fly Volumes for persistent storage, or upload files to S3/external blob storage instead of local disk
Cold starts on the free tier can add 5-15 seconds latency when instances scale down
Fix: Use min_machines = 1 in fly.toml or upgrade to paid tiers for guaranteed warm instances
Database connections from multiple instances can exhaust connection pools in small Postgres instances
Fix: Use PgBouncer (Fly's connection pooler) or implement connection pooling in your FastAPI app with sqlalchemy.pool
Alternatives
- •Django + Fly.io: More batteries-included web framework with built-in ORM and admin panel, but heavier than FastAPI
- •Starlette + Heroku: Lighter alternative to FastAPI but Heroku's pricing model is less cost-effective than Fly
- •Node.js/Express + Vercel: Better for serverless deployment if you need zero cold starts, but requires JavaScript ecosystem
Resources
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