How to do zero-downtime deploys using systemd.
systemd has a socket-activation feature that's associated with starting services on demand, often paired with shutting them down they are idle.
But today I confirmed they have a great value for always-on services-- zero-downtime deploys for web services!
What happens is that systemd owns the listening socket and never stops listening during the restart. Connections are queued and succeed when the service up.