python 55 lines · 8 steps

A background session cleaner in FastAPI

Run a periodic expired-session purge as a background task tied to the FastAPI app lifespan.

Explained by highlit
FastAPI Intermediate background-tasks asyncio lifespan graceful-shutdown sqlalchemy
1import asyncio
2import logging
3from contextlib import asynccontextmanager
4from datetime import datetime, timedelta, timezone
5 
6from fastapi import FastAPI
7 
8from .database import async_session
9from .models import Session as UserSession
10from sqlalchemy import delete
11 
12logger = logging.getLogger(__name__)
13 
14CLEANUP_INTERVAL = 300
15SESSION_TTL = timedelta(hours=24)
16 
17 
18async def purge_expired_sessions() -> int:
19 cutoff = datetime.now(timezone.utc) - SESSION_TTL
20 async with async_session() as db:
21 result = await db.execute(
22 delete(UserSession).where(UserSession.last_seen_at < cutoff)
23 )
24 await db.commit()
25 return result.rowcount
26 
27 
28async def cleanup_loop() -> None:
29 while True:
30 try:
31 await asyncio.sleep(CLEANUP_INTERVAL)
32 removed = await purge_expired_sessions()
33 if removed:
34 logger.info("Purged %d expired sessions", removed)
35 except asyncio.CancelledError:
36 logger.info("Cleanup loop stopping")
37 raise
38 except Exception:
39 logger.exception("Session cleanup failed; retrying next tick")
40 
41 
42@asynccontextmanager
43async def lifespan(app: FastAPI):
44 task = asyncio.create_task(cleanup_loop())
45 try:
46 yield
47 finally:
48 task.cancel()
49 try:
50 await task
51 except asyncio.CancelledError:
52 pass
53 
54 
55app = FastAPI(lifespan=lifespan)
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A lifespan context manager is the clean place to start and stop background work alongside the app.
  2. 2Cancelling a task and awaiting it lets the coroutine unwind its cleanup before shutdown completes.
  3. 3Catching broad exceptions inside the loop keeps one bad tick from killing the whole worker.

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