FastAPI Patterns

- Mixing sync database drivers (psycopg2, PyMySQL) in async endpoints blocks the event loop — use async drivers (asyncpg, aiomysql) or run sync code in `run_in_executor` - `time.sleep()` in async endpoints blocks everything — use `await asyncio.sleep()` instead - CPU-bound work in async endpoints starves other requests — offload to `ProcessPoolExecutor` or background workers - Async endpoints calling sync functions that do I/O still block — the entire call chain must be async