About 99%. We need to restart servers maybe once a year. Maybe next year we will finally migrate from erlang 21 to latest. Most "stopping everything" deployments take max 1s of downtime, like this month when we needed to upgrade postgresql database by switching it over to new machine, having zero downtime here would take a little longer to get the consistency, but we could spare a second to make it much simpler task on database side (it was a restart of database module, rest of erlang server was unaffected and clients were not disconnected). Otherwise, most deployments are not visible as disconnections, we have a lot long-running connections.

> At the very least you will have a different procedure to upgrade the erlang itself, right?

Yes.

> I think on a spectrum of difficult things there is an area between hosting static file on rpi at home and running massivele distributed system full of long running stateful processes.

Is PHP good for both? I think PHP is NOT good for long running stateful processes, but I didn't use it in 10years. And it probably is not needed for static files.