[snippy]

Sounds interesting. Let's say the software is a web backend. Can you deploy it like this with zero downtime? So that the new version starts, new traffic goes to it, and the old version handles its active requests to completion and then shuts off.

[z_zetetic_z]

I don't think so, by default I think the nixos process will simply stop (probably by sending SIGINT) the service and then start it again.

But if you could have the server into 'lame duck mode' (no new connections accepted, but existing ones can finish) / gracefull shutdown and that's a blocking call (or you could poll if it's still up etc), then you could script that before the 'nixos-rebuild switch' call. Maybe sending SIGINT to the service does that already?