> I couldn't have the old and the new version of my service running on the same port...
You can, actually! You just can’t open the port twice by default. So one or both of the processes needs to inherit the port from a parent process, get passed the port over a socket (Unix sockets can transmit file descriptors), or use SO_REUSEADDR.
There are some libraries that abstract this, and some of this is provided by tools like systemd.
Some of this is probably going to have to be done in your application—like, once your new version starts, the old version should stop accepting new connections and finish the requests it has already started.
The master/worker thing that nginx / gunicorn et al do is pretty neat, but relies on signals; so seems pretty messy and error prone to write yourself.
People have a healthy skepticism of signals from the C days, but if we’re talking about Go, you’d just call signal.Notify. Any way of signaling your app to shut down works, though.
FWICT tableflip does exactly this: https://github.com/cloudflare/tableflip
In either case, you will need a reverse proxy like Traefik/Nginx in front to smartly "balance" incoming requests to the two instances of the service.
But the one way to do no downtime deployments is to have more than one server.
The post says the process starts up quick enough that the process being temporarily unavailable isn't noticeable - but what if the process _doesn't come back_? It's also impossible to do blue/green deployments this way.
It's clearly not a solution suitable to large-scale deployments. The simplicity has its trade-offs.
One good option would be to spin up a second server/instance/container, run binary on new system, ensure it's good, once comfortable then swap DNS entry to the new system.
func reloadConfig(config) {
if newServer, err := startServer(config); err != nil {
// gracefully shutdown previous server
// no new connections will go to old server
oldServer.Shutdown(...)
oldServer = newServer
}
}Just make sure you’re not slowly recreating bad, homebrew versions of all of the nice things that Kubernetes does in an attempt to turn a simple deployment into a production ready deployment.
But TECHNICALLY to do that in one without external proxy you'd need to figure out how to set SO_REUSEPORT for the web socket handler, then start the second one before the first.
Haven't actually tried it but someone apparently did: https://iximiuz.com/en/posts/go-net-http-setsockopt-example/
You'd still have any ongoing connections cut unless you unbind socket and then finish any existing connection, which would be pretty hard with default http server.
I just put HAProxy instance on my VPS that does all of that, including only allowing traffic once app says "yes I am ok" in healthcheck. Then the app can have "shutting down" phase, where it reports "I am down" on healthcheck but still finished any active connections to the client.
If you want stateful zero downtime deployments, use Elixir or Erlang that has the ability to live migrate data from one version of code to the next.