How will a synchronous handle call affect the behavior of the entire FastAPI server?

FastAPI is built on an asynchronous framework that allows handling multiple requests simultaneously thanks to async/await.

If a synchronous (blocking) function is called in a FastAPI request handler, this call will block the current execution thread, leading to reduced performance and fewer requests being processed concurrently.

For example, if the server uses a single worker process, other requests will wait until the synchronous function completes.

To avoid this, it is recommended to:

• Use asynchronous functions and libraries
• If it is necessary to call synchronous code, execute it in a separate thread using run_in_executor

Example of calling a synchronous function in FastAPI using run_in_executor:

import asyncio
from fastapi import FastAPI
import time

app = FastAPI()

def blocking_io():
    time.sleep(5)  # blocking operation
    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}

Thus, calling a synchronous function directly in a FastAPI handler can negatively impact server performance.