API & Backend
FastAPI Async Architecture: Asyncio Event Loop & High-Concurrency Best Practices
Master async def vs sync def in FastAPI, avoid blocking the asyncio event loop, and handle tens of thousands of concurrent requests smoothly.
async def vs def: The Most Common Trap
Writing `async def` on every endpoint without thought is dangerous. If you execute a blocking call (like `time.sleep` or synchronous `requests.get`) inside an `async def` function, the entire event loop freezes.
FastAPI automatically offloads standard synchronous `def` functions to an external AnyIO threadpool, keeping the event loop unblocked.
import httpx
from fastapi import FastAPI
app = FastAPI()
# Non-blocking async I/O endpoint
@app.get("/async-fetch")
async def async_fetch():
async with httpx.AsyncClient() as client:
res = await client.get("https://api.example.com/data")
return res.json()
# Synchronous CPU-bound or blocking I/O (safe via threadpool)
@app.get("/sync-work")
def sync_work():
# FastAPI runs this in a separate thread
return {"status": "completed"}Frequently Asked Questions
What is uvloop and how does it improve FastAPI throughput?
uvloop is a high-performance C-based drop-in replacement for the default asyncio event loop, boosting request throughput by 2x to 4x.
Related Technical Guides
Deepen your understanding with these closely related production architectures and tutorials:
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