nginx 502 when working with gunicorn + uvicorn worker + fastAPI

[zffocussss]

Hi team
I am facing an issue where when a user requests a fast API endpoint through Nginx, the Nginx server log emits 502 errors,which highlight "upstream prematurely closed connection while reading response header from upstream " in the Nginx log.
but I do not find what happened at that time.,as I can not find the corresponding logs in backend application log.Can someone help me how to investigate it?

[tnusser]

We face a similar issue that gunicorn rarely (~every 50.000th request) returns a 502-Error without ever showing up in the application logs. Since we stoped using the max-request parameter #3038 it happens less frequently but it is still around.