# INFO: 127.0.0.1:54321 - 'GET /sync-endpoint HTTP/1.1' 200 OK（耗时 5.02 秒，检测到阻塞事件循环）

- **ID:** `python/fastapi-event-loop-blocked`
- **领域:** python
- **类别:** runtime_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

在 async def 端点中直接执行了同步阻塞调用（如 time.sleep、requests、重 CPU 计算），阻塞了事件循环，导致所有并发请求停滞。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| 3.8 | active | — | — |
| 3.9 | active | — | — |
| 3.10 | active | — | — |
| 3.11 | active | — | — |
| 3.12 | active | — | — |

## 解决方案

1. **** (95% 成功率)
   ```
   Offload blocking work to a thread:

import anyio
@app.get('/sync-endpoint')
async def handler():
    result = await anyio.to_thread.run_sync(blocking_fn)
   ```
2. **** (92% 成功率)
   ```
   Define the route as def (not async def) so FastAPI runs it in a threadpool:

@app.get('/sync-endpoint')
def handler():
    return blocking_fn()
   ```
3. **** (90% 成功率)
   ```
   Replace requests with httpx.AsyncClient and time.sleep with asyncio.sleep.
   ```

## 无效尝试

- **** — Each worker is still blocked; throughput per worker stays at one request. (70% 失败率)
- **** — Does not fix the blocking call inside the existing one. (90% 失败率)
- **** — Unrelated to loop blocking. (95% 失败率)