FastAPI Async Operations

Published on September 10, 2025

FastAPI has made asynchronous programming in Python accessible and powerful. By leveraging async/await, you can build high‑throughput APIs that scale with I/O‑bound workloads. This post explores how async works in FastAPI, shows practical examples, and highlights common pitfalls.

Why Use Async with FastAPI?

Non‑blocking I/O: Handle many concurrent requests without spawning new threads.
Better resource utilization: Keep the event loop busy while waiting for external services.
Improved latency: Reduce response times when dealing with databases, HTTP calls, or file I/O.

Basic Async Endpoint

from fastapi import FastAPI
import httpx

app = FastAPI()

@app.get("/external")
async def fetch_data():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://api.example.com/data")
    return response.json()

Running Blocking Code in an Async Context

If you need to call a CPU‑bound function, run it in a thread pool to avoid blocking the event loop.

import time
from fastapi import FastAPI
from concurrent.futures import ThreadPoolExecutor

executor = ThreadPoolExecutor(max_workers=10)
app = FastAPI()

def heavy_computation(x: int) -> int:
    time.sleep(2)  # Simulate CPU‑bound work
    return x * x

@app.get("/compute/{number}")
async def compute(number: int):
    loop = asyncio.get_event_loop()
    result = await loop.run_in_executor(executor, heavy_computation, number)
    return {"result": result}

Best Practices

Use async throughout the stack: database drivers, HTTP clients, and file operations should all be async.
Avoid mixing sync and async calls in the same request cycle.
Leverage background tasks for fire‑and‑forget operations.
Keep the number of concurrent connections reasonable; configure limits on the ASGI server.

Conclusion

Async programming with FastAPI unlocks performance gains for I/O‑heavy workloads. By writing truly asynchronous endpoints and respecting the event loop, you can serve more requests with fewer resources.