FastAPI deployment concepts related to ASGI servers and process managers

06-05-2025

Uvicorn version 0.30.0:

• https://www.uvicorn.org/release-notes/#0300-may-28-2024
  • adds a new multiprocess manager and deprecates the uvivorn.workers module
  • this removes the need for gunicorn for process worker management - https://github.com/tiangolo/uvicorn-gunicorn-starlette-docker/blob/master/README.md

ASGI servers and worker managers:

• installing FastAPI comes with the recommended production ASGI server (uvicorn) by default
  • this comes from fastapi-cli - https://github.com/fastapi/fastapi-cli
  • when you install FastAPI with something like pip install "fastapi[standard]" you already get uvicorn[standard] as well

Starting multiple workers:

• https://fastapi.tiangolo.com/deployment/server-workers/#multiple-workers
• you can use fastapi directly, or one of the ASGI servers
• note that not all commands and config are supported by fastapi-cli
  • add ability to pass in more arguments to FastAPI CLI to match Uvicorn options - https://github.com/fastapi/fastapi-cli/discussions/13

Passing in config to uvicorn:

• https://www.uvicorn.org/#usage

How many workers should you run?

• https://docs.gunicorn.org/en/stable/design.html#how-many-workers
• (2 x $num_cores) + 1

Uvicorn vs gunicorn:

• https://www.uvicorn.org/deployment/?h=workers#built-in
• uvicorn
  • uses spawn worker model - https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods
• gunicorn
  • uses pre-fork worker model - https://docs.gunicorn.org/en/latest/design.html
  • uvicorn inlcudes a gunicorn worker class
    • uvicorn.workers.UvicornWorker
    • this will be deprecated in favour of the uvicorn-worker package -

# using uvloop and httptools
gunicorn -w 4 -k uvicorn.workers.UvicornWorker
 
# pure python implementation
gunicorn -w 4 -k uvicorn.workers.UvicornH11Worker
 
# uvicorn_worker module (from the uvicorn-worker package)
gunicorn example:app -w 4 -k uvicorn_worker.UvicornWorker

Local development with locally-trusted SSL certificates:

• https://github.com/FiloSottile/mkcert