How to solve the event loop blocking problem in FastAPI when using synchronous code?

In FastAPI, which is built on an asynchronous event loop (usually uvicorn + asyncio), executing long synchronous operations blocks the event loop and slows down the processing of other requests.

To avoid blocking the event loop when using synchronous code, you can:

1. Offload synchronous code to a separate thread or process using run_in_executor:

import asyncio
from fastapi import FastAPI

app = FastAPI()

def blocking_io():
    # Long synchronous operation
    import time
    time.sleep(5)
    return "done"

@app.get("/sync")
async def read_sync():
    loop = asyncio.get_running_loop()
    result = await loop.run_in_executor(None, blocking_io)
    return {"result": result}

2. Use asynchronous libraries instead of synchronous ones, if possible.

3. Rewrite critical parts of the code in an asynchronous style.

Thus, run_in_executor allows running blocking code in a separate thread without blocking the main FastAPI event loop.