@hackage systemd-socket-activation1

Let systemd bind the server's socket for you

"Socket activation" is the a feature of systemd.

https://www.freedesktop.org/software/systemd/man/sd_listen_fds_with_names.html

We use it for web servers, to avoid the momentary downtime that otherwise occurs while restarting processes. Because the socket is manged by systemd, not by our process, the socket remains even while our process is down. Requests to the socket are queued until our process comes back up to respond.