During dyno shutdown Heroku sends SIGTERM to all processes on the machine, not just the ones defined in the Procfile.

This has a few consequences, if your processes do not handle SIGTERM as a graceful shutdown you can’t change the configured signal like you can with systemd’s KillSignal and Docker’s STOPSIGNAL.

So if you’re running NGINX which uses SIGQUIT for graceful shutdown, you’ll need to recompile NGINX with a patch to change the shutdown signals. This is true even if you’re using the Heroku maintained NGINX buildpack. Celery has a similar issue with Heroku that can be mitigated with the REMAP_SIGTERM env var.

Additionally, by sending signals to every process on the machine, you can’t have a parent process trap and change the signals that get passed down to the child, which would avoid the NGINX recompile. And since every process gets SIGTERMed, both parent and worker processes need to handle SIGTERM as a graceful shutdown request.

Heroku’s “send signals to all the processes” approach is similar to the default KillMode=crontrol-group setting described in the systemd docs. Which works for some cases, but services like NGINX would be better off with KillMode=mixed and KillSignal=SIGQUIT.


Heroku is easy to get started with but lacks some basic configuration settings that are helpful for robust process management.