Most idiomatic way to run a task only once at startup?

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn’t find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google “How to X in FastAPI” and didn’t find any information.
• I already read and followed all the tutorial in the docs and didn’t find an answer.
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Example Code

from fastapi import FastAPI

app = FastAPI()

async def download_data_files():
    # download some files from s3 using aiohttp
    # post process the files
    pass

@app.get("/data")
def data():
    # return some data that relies on the downloaded files
    return "something"

Description

I think my question is simple, but having done a lot of searching GitHub and google I’m starting to wonder if I’m trying to bend FastAPI in a way it doesn’t like!

What is the most idiomatic way of running a one-off task, on startup?

In my simple example above, I want to run download_data_files once and only once, before serving any requests.

I’ve tried using the startup event but this executes in every worker which is wasteful (I end up downloading the same files n times).

I’ve tried to use the create_app pattern along with gunicorn --preload, but I couldn’t get it to work, and from what I’ve read it doesn’t sound like a recommended approach anyway.

Operating System

Linux

Operating System Details

N/A

FastAPI Version

0.68.0

Python Version

Python 3.8.7

Additional Context

No response

[tiangolo]

The best way to for it would be to run the script before starting the workers. How you run it depends on how you deploy.

Then you could reuse that script/function inside the path operation to update the files.

[bram2000]

Something like this you mean? https://fastapi.tiangolo.com/advanced/events/

Thanks, but I mentioned in the question that this is not suitable because it runs for each worker.