fastapi and proxies

README.md

Quickstart

pdm install

These are equivalent (given the main.py code)

UVICORN_ROOT_PATH=/api/ pdm run uvicorn main:app
FASTAPI_ROOT_PATH=/api/ pdm run uvicorn main:app
pdm run uvicorn main:app --root-path /api

Expected response from all 3 variants:

$ curl http://127.0.0.1:8000/
{"message":"Hello World","root_path":"/api/"}

But

AWS ALB doesn't do url path rewriting, so when we have a Listener Rule that forward all requests that match an /api prefix to our FastAPI backend, it will pass back the whole url, including the prefix. So what we really need to work is this:

$ curl http://127.0.0.1:8000/api/
{"message":"Hello World","root_path":"/api/"}

without changing the whole application.

References

• https://fastapi.tiangolo.com/advanced/behind-a-proxy/

main.py

from fastapi import FastAPI, Request
from proxy_middleware import ProxyPrefixMiddleware
import os

root_path = os.environ.get("FASTAPI_ROOT_PATH")

app = FastAPI(root_path=root_path)
if root_path:
    app.add_middleware(ProxyPrefixMiddleware)

@app.get("/")
async def root(request: Request):
    return {"message": "Hello World", "root_path": request.scope.get("root_path")}

proxy_middleware.py

'''
Usage:
if root_path:
    app.add_middleware(ProxyPrefixMiddleware)
'''
class ProxyPrefixMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] in {"http", "websocket"}:
            root_path = scope["root_path"].rstrip('/')
            scope["path"] = scope["path"].removeprefix(root_path)
        await self.app(scope, receive, send)

pyproject.toml

[project]
name = "fastapi-proxies"
version = "0.0.1"
description = ""
authors = []
dependencies = [
    "fastapi>=0.99.1",
    "uvicorn>=0.22.0",
]
requires-python = ">=3.9"
license = {text = "MIT"}

also expected (probably)

$ curl -s http://127.0.0.1:8000/openapi.json | jq '.servers[0].url'
"/api"

ps.
neither UVICORN_ROOT_PATH nor --root-path work when using the gunicorn app server:

pdm run gunicorn --worker-class uvicorn.workers.UvicornWorker main:app

so it seems, my FASTAPI_ROOT_PATH custom implementation is a better fit.