Run a FastAPI app
Getting an application running on Fly.io is essentially working out how to package it as a deployable image. Once packaged, it can be deployed to the Fly.io platform.
In this guide we’ll learn how to deploy a FastAPI application on Fly.io.
FastAPI is a modern, fast (high-performance), web framework for building APIs with Python based on standard Python type hints.
Deploying a FastAPI app on Fly.io is… well it’s fast! You can be up and running in less than a minute.
Speedrun
First, install flyctl, the Fly.io CLI, and sign up to Fly.io if you haven’t already.
The fastest way to get a basic Fast Api server on Fly.io is to use our FastAPI template:
git clone git@github.com:fly-apps/hello-fastapi.git fastapi-app
cd fastapi-app
fly launch --generate-name
Deploy a FastAPI app from scratch
For managing our project, we use Poetry. For more information on the initial setup with poetry, refer to setting up a python environment.
We can initialize a new project like so:
poetry new fastapi-app
cd fastapi-app
If you’re using Poetry 2.0+ (the default when installed via pipx), the poetry shell command is no longer built in. Use poetry run to run commands (e.g., poetry run python main.py), or activate the environment with poetry env activate. See the initial setup guide for more details.
Then we have to add the FastAPI dependency:
poetry add "fastapi[standard]"
Now, let’s create a simple FastAPI app in main.py:
from fastapi import FastAPI
app = FastAPI()
@app.get("/")
async def hello_fly():
return 'hello from fly.io'
We can then serve the development version of the app using the fastapi cli tool:
fastapi dev
This will display a ‘hello from fly.io!’ message when you visit the root URL. With the development environment we also get cool features like hot-reloading.
To run in production mode, we can amend the command:
fastapi run
The fastapi assumes your app is stored in main.py or app.py - if this is not the case, you must specify that when running the app:
fastapi run hello/entrypoint.py
And with that you can deploy the app!
fly launch
Scanning source code
Detected a FastAPI app
Warning: This organization has no payment method, turning off high availability
Creating app in [redacted]/[app-name]
We're about to launch your app on Fly.io. Here's what you're getting:
Organization: Your Name (fly launch defaults to the personal org)
Name: [app-name] (derived from your directory name)
Region: Amsterdam, Netherlands (this is the fastest region for you)
App Machines: shared-cpu-1x, 1GB RAM (most apps need about 1GB of RAM)
Postgres: <none> (not requested)
Redis: <none> (not requested)
Sentry: false (not requested)
...
==> Building image
...
==> Building image with Docker
...
Watch your deployment at https://fly.io/apps/[app-name]/monitoring
...
Visit your newly deployed app at https://[app-name].fly.dev/
This will generate a fly.toml file with the configuration for your app and a Dockerfile that uses multi-stage builds.
Refer to the fly.toml docs for more configuration options.
To deploy a new version of your app, simply run fly deploy in the project directory.
You can check out the full (yet minimal) example in this GitHub repository for a reference.