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. 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.