Uvicorn workers logging Unable to write to their own file

[Jmandarino]

Doesn't seem to be discussed anywhere in the docs but I had a question on the potential log interweaving or corruption when using logfiles instead of stdout/stderr and having mulitple workers (2+)

Is it possible to have an issue with log file corruption while using mulitple workers in uvicorn? I know logging is thread safe in python but I didn't see any queue based handlers in the uvicorn code for logging.

If this is a potential issue or logging is a blocking operation on a given file. I also don't see anywhere in the docs how I can set each child process to have its PID or process name as part of the log file to allow for separate files

Long story short is logging in uvicorn process safe and if its not how can I best make separate logs for each uvicorn worker

Kinda realizing this might not have enough details:
Implementation:
setup specifically Running uvicorn programmically (Docs.

Attempts at logging into mulitple files via uvicorn:
I have also tried to use root loggers, sending "uvicorn" both "uvicorn.access" to a file with a PID suffix. This nets all logs going to one of the files created but not the other.

[Jmandarino]

Played around and disappointed this process seems to contrived. TLDR: inside your application code if you don't set the logger manually uvicorn will have all the loggers log to the same file.

def log_config():
    from uvicorn.config import LOGGING_CONFIG
    from copy import deepcopy
    import os

    config = deepcopy(LOGGING_CONFIG)
    pid = os.getpid()
    log_path = f"logs/app-{pid}.log"
    os.makedirs("logs", exist_ok=True)  # Ensure the logs folder exists

    config["handlers"]["file"] = {
        "level": "INFO",
        "class": "logging.FileHandler",
        "filename": log_path,
        "formatter": "default",
    }

    config["loggers"]["uvicorn"] = {
        "handlers": ["default", "file"],
        "level": "INFO",
        "propagate": False,
    }

    config["loggers"]["uvicorn.access"] = {
        "handlers": ["access", "file"],
        "level": "INFO",
        "propagate": False,
    }

    return config


if __name__ == "__main__":
    import uvicorn

    uvicorn.run("app.main:app", workers=2, log_config=log_config())

this does not work -- if you do this you are left with one log file based off the main process. The only way I got this to work was using a startup event for FastAPI (which seems pretty odd to have to do given Gunicorn has support for each worker to have their own log)

@app.on_event("startup")
def set_handlers():
    import os
    import logging

    pid = os.getpid()
    log_path = f"logs/app-{pid}.log"
    os.makedirs("logs", exist_ok=True)

    # Remove existing handlers (like console)
    for handler in logging.root.handlers[:]:
        logging.root.removeHandler(handler)

    # Add new handler that writes to file
    logging.basicConfig(
        level=logging.INFO,
        format="%(asctime)s - %(levelname)s - %(name)s - %(message)s",
        handlers=[
            logging.FileHandler(log_path),
        ],
    )
    logging.getLogger().info(f"Worker PID {pid} logging to {log_path}")

ADDing this (and the above code) creates a log per file but what a mess.