Why does the FastAPI startup event run twice in HEROKU?

Question

When I deployed a FastAPI app to heroku the startup event is fired twice. Is it my problem?

The code below is not my original app, but a temporary app I made for testing. And the Heroku was also made fresh for testing.

# Heroku logs

2022-10-18T08:37:32 app[api]: Build started by user junah.dev+1@gmail.com
2022-10-18T08:37:49 app[api]: Deploy 298098b0 by user junah.dev+1@gmail.com
2022-10-18T08:37:49 app[api]: Release v6 created by user junah.dev+1@gmail.com
2022-10-18T08:37:52 heroku[web.1]: State changed from down to starting
2022-10-18T08:37:54 heroku[web.1]: Starting process with command `uvicorn app.main:app --host=0.0.0.0 --port=${PORT:-5000}`
2022-10-18T08:37:55 app[api]: Build succeeded
2022-10-18T08:37:55.490285+00:00 app[web.1]: INFO:     Uvicorn running on http://0.0.0.0:51121 (Press CTRL+C to quit)
2022-10-18T08:37:55 app[web.1]: INFO:     Started parent process [4]
2022-10-18T08:37:55 app[web.1]: INFO:     Started server process [11]
2022-10-18T08:37:55 app[web.1]: INFO:     Waiting for application startup.
2022-10-18T08:37:55 app[web.1]: INFO:     Application startup complete.
2022-10-18T08:37:55 app[web.1]: INFO:     Started server process [10]
2022-10-18T08:37:55 app[web.1]: INFO:     Waiting for application startup.
2022-10-18T08:37:55 app[web.1]: INFO:     Application startup complete.
2022-10-18T08:37:56 heroku[web.1]: State changed from starting to up

# main.py

from fastapi import Depends, FastAPI
import uvicorn

app = FastAPI()

@app.get("/")
async def root():
    return {"message": "Hello World"}

# Procfile

web: uvicorn app.main:app --host=0.0.0.0 --port=${PORT:-5000}

score 2 · Accepted Answer · answered Oct 18 '22 at 11:06

Uvicorn operates by running worker processes.

If you don't give it a number of processes to use via the --workers argument, it defaults to the value of the WEB_CONCURRENCY environment variable, falling back to 1 if neither is set:

--workers INTEGER               Number of worker processes. Defaults to the
                                $WEB_CONCURRENCY environment variable if
                                available, or 1. Not valid with --reload.

Heroku sets a default value for the WEB_CONCURRENCY environment variable:

The WEB_CONCURRENCY environment variable is automatically set by Heroku, based on the processes’ Dyno size. This feature is intended to be a sane starting point for your application. We recommend knowing the memory requirements of your processes and setting this configuration variable accordingly.

Heroku has probably set this value to 2 based on the size of the dynos you are running. This would cause two workers to run in parallel, which is what your logs are showing:

A parent process: Started parent process [4]
And two worker processes:
- Started server process [11]
- Started server process [10]

You can change the value of WEB_CONCURRENCY if you wish.

If your code is malfunctioning with multiple workers that indicates a problem with your code. In that case, instead of setting this to 1, I suggest you identify why it is misbehaving and fix the problem. One common cause of this is use of global variables.

Why does the FastAPI startup event run twice in HEROKU?

1 Answers1