Reference: A FastAPI SaaS Backend

acme-api/
  src/acme_api/
    main.py              # FastAPI app factory
    api/routes/          # HTTP routers
    domain/              # business logic (framework-free)
    infra/db.py          # SQLAlchemy session
    workers/tasks.py     # Celery tasks
  alembic/               # migrations
  tests/
  Dockerfile             # Python 3.14 slim + uv sync

"""src/acme_api/main.py - minimal SaaS app factory."""
from __future__ import annotations
 
from contextlib import asynccontextmanager
 
from fastapi import FastAPI
from pydantic import BaseModel
 
from acme_api.api.routes import tenants, webhooks
from acme_api.infra.db import init_db
 
 
class Health(BaseModel):
    status: str
 
 
@asynccontextmanager
async def lifespan(app: FastAPI):
    init_db()
    yield
 
 
def create_app() -> FastAPI:
    app = FastAPI(title="Acme API", version="1.0.0", lifespan=lifespan)
    app.include_router(tenants.router, prefix="/v1/tenants")
    app.include_router(webhooks.router, prefix="/v1/webhooks")
 
    @app.get("/health/ready", response_model=Health)
    def ready() -> Health:
        return Health(status="ok")
 
    return app
 
 
app = create_app()

"""src/acme_api/domain/billing.py - framework-free logic."""
from __future__ import annotations
 
from decimal import Decimal
 
 
def compute_total(subtotal: Decimal, tax_rate: Decimal) -> Decimal:
    return (subtotal * (Decimal("1") + tax_rate)).quantize(Decimal("0.01"))

"""src/acme_api/workers/tasks.py"""
import os
 
from celery import Celery
 
celery_app = Celery("acme", broker=os.getenv("REDIS_URL", "redis://localhost:6379/0"))
 
 
@celery_app.task(bind=True, max_retries=5)
def send_invoice_email(self, invoice_id: str) -> None:
    ...

Client → Ingress → FastAPI (Gunicorn/Uvicorn workers)
                    ↓ SQLAlchemy
                 Postgres
                    ↓ enqueue
                 Redis → Celery workers → S3 / email

FROM python:3.14.0-slim
COPY uv.lock pyproject.toml ./
RUN pip install uv && uv sync --frozen --no-dev
COPY src/ ./src/
CMD ["gunicorn", "acme_api.main:app", "-k", "uvicorn.workers.UvicornWorker"]