Avoid Blocking Event Loop in Python with FastAPI

Most “slow APIs” in Python aren’t CPU-bound. They’re blocking the event loop without realizing it. Classic FastAPI mistake: @app.get("/users") async def get_users(): users = db.fetch_all() # blocking call return users Looks async. Isn’t. Result: * event loop stalls * requests queue up * latency spikes under load Fix → respect async boundaries @app.get("/users") async def get_users(): users = await db.fetch_all() return users Or offload properly: from asyncio import to_thread users = await to_thread(sync_db_call) Advanced production pattern: * separate sync + async layers clearly * use connection pools (asyncpg, aiomysql) * never mix blocking ORM calls inside async routes Hidden issue: One blocking call can freeze thousands of concurrent requests. Build-in-public lesson: Async isn’t about syntax. It’s about protecting the event loop at all costs. AI can convert code to async— but only experience catches where it’s still secretly blocking. #Python #BackendEngineering #FastAPI #Scalability #SystemDesign

To view or add a comment, sign in

Explore content categories