How do you integrate FastAPI with Celery for reliable background task processing?

FastAPI's built-in BackgroundTasks is suitable for lightweight, lossy tasks. For tasks that must be reliable, retried, scheduled, or distributed across workers, use Celery with a broker (Redis or RabbitMQ).

# pip install celery redis
# celery_app.py
from celery import Celery

celery_app = Celery(
    "worker",
    broker="redis://localhost:6379/0",
    backend="redis://localhost:6379/1",
)
celery_app.conf.task_serializer = "json"

@celery_app.task(bind=True, max_retries=3, default_retry_delay=60)
def send_email(self, email: str, subject: str, body: str):
    try:
        # call email service
        print(f"Sending {subject} to {email}")
    except Exception as exc:
        raise self.retry(exc=exc)  # retry up to 3 times

# main.py — FastAPI triggers Celery tasks
from fastapi import FastAPI
from pydantic import BaseModel
from celery_app import send_email

app = FastAPI()

class EmailRequest(BaseModel):
    to:      str
    subject: str
    body:    str

@app.post("/send-email", status_code=202)
def trigger_email(req: EmailRequest):
    # .delay() sends the task to the broker queue
    task = send_email.delay(req.to, req.subject, req.body)
    return {"task_id": task.id, "status": "queued"}

@app.get("/tasks/{task_id}")
def task_status(task_id: str):
    from celery.result import AsyncResult
    result = AsyncResult(task_id)
    return {"task_id": task_id, "status": result.status}

# Run the Celery worker (separate process)
# celery -A celery_app worker --loglevel=info

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