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
It's easy to get started:
upg, err := tableflip.New(tableflip.Options{})
if err != nil {
panic(err)
}
defer upg.Stop()
go func() {
sig := make(chan os.Signal, 1)
signal.Notify(sig, syscall.SIGHUP)
for range sig {
err := upg.Upgrade()
if err != nil {
log.Println("Upgrade failed:", err)
continue
}
log.Println("Upgrade succeeded")
}
}()
ln, err := upg.Fds.Listen("tcp", "localhost:8080")
if err != nil {
log.Fatalln("Can't listen:", err)
}
var server http.Server
go server.Serve(ln)
if err := upg.Ready(); err != nil {
panic(err)
}
<-upg.Exit()
time.AfterFunc(30*time.Second, func() {
os.Exit(1)
})
_ = server.Shutdown(context.Background())
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