Sentry Answers>FastAPI>

Make long-running tasks time out in FastAPI

Make long-running tasks time out in FastAPI

David Y.

—

The ProblemJump To Solution

One of the API endpoints in my FastAPI application executes a data retrieval task that sometimes takes a very long time to complete, making my application feel sluggish. How can I cancel the execution of this task if it takes more than a set amount of time and serve cached data instead?

The Solution

We can do this using the asyncio.wait_for function that takes an awaitable and a timeout value in seconds as parameters. The awaitable is then run as a task. If the task is completed before the timeout is reached, wait_for will return the value the task returns. If the timeout is reached before the task returns, the task will be canceled, and wait_for will raise a TimeoutError. In Python versions before 3.11, it raised an asyncio.TimeoutError instead.

The following example code demonstrates using a simulated data retrieval task that sleeps for 10 seconds before returning a random number and a timeout of 5 seconds. For Python versions below 3.11 replace except TimeoutError with asyncio.exceptions.TimeoutError.

Click to Copy
from fastapi import FastAPI
import asyncio, random

app = FastAPI()

cache = {"result": random.random()}  # seed the cache


async def long_running_task():
    await asyncio.sleep(10)  # sleep for 10 seconds
    return random.random()  # return a random number


@app.get("/retrieve-data")
async def retrieve_data():
    try:
        result = await asyncio.wait_for(long_running_task(), timeout=5)  # timeout after five seconds of waiting
        cache["result"] = result  # cache the result
        return {"message": result, "cache": False}
    except TimeoutError:
        return {"message": cache["result"], "cache": True}


# Run FastAPI app
if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=8000)

A request to this application’s /retrieve-data endpoint will invoke long_running_task, cancel it after five seconds, and then return the cached random number. If we swap the task’s sleep time and the timeout value, a request to /retrieve-data will wait for five seconds before returning a new random number. In both cases, the API indicates whether the value returned was sourced from the cache.

  • Community SeriesIdentify, Trace, and Fix Endpoint Regression Issues
  • ResourcesBackend Error Monitoring 101
  • Syntax.fm logo
    Listen to the Syntax Podcast

    Tasty treats for web developers brought to you by Sentry. Get tips and tricks from Wes Bos and Scott Tolinski.

    SEE EPISODES

Considered “not bad” by 4 million developers and more than 100,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

learn more about sentryjoin our discord
© 2024 • Sentry is a registered Trademark
of Functional Software, Inc.