It is sometimes useful to update the running code and / or configuration of a network service, without disrupting existing connections. Usually, this is achieved by starting a new process, somehow transferring clients to it and then exiting the old process.

There are many ways to implement graceful upgrades. They vary wildly in the trade-offs they make, and how much control they afford the user. This library has the following goals:

• No old code keeps running after a successful upgrade
• The new process has a grace period for performing initialisation
• Crashing during initialisation is OK
• Only a single upgrade is ever run in parallel

tableflip works on Linux and macOS.

upg, _ := tableflip.New(tableflip.Options{})
defer upg.Stop()

go func() {
	sig := make(chan os.Signal, 1)
	signal.Notify(sig, syscall.SIGHUP)
	for range sig {
		upg.Upgrade()
	}
}()

// Listen must be called before Ready
ln, _ := upg.Listen("tcp", "localhost:8080")
defer ln.Close()

go http.Serve(ln, nil)

if err := upg.Ready(); err != nil {
	panic(err)
}

<-upg.Exit()

Please see the more elaborate graceful shutdown with net/http example.

[Unit]
Description=Service using tableflip

[Service]
ExecStart=/path/to/binary -some-flag /path/to/pid-file
ExecReload=/bin/kill -HUP $MAINPID
PIDFile=/path/to/pid-file

See the documentation as well.

The logs of a process using tableflip may go missing due to a bug in journald, which has been fixed by systemd v244 release. If you are running an older version of systemd, you can work around this by logging directly to journald, for example by using go-systemd/journal and looking for the $JOURNAL_STREAM environment variable.