服務端代碼經常需要升級，對于線上系統的升級常用的做法是，通過前端的負載均衡（如nginx）來保證升級時至少有一個服務可用，依次（灰度）升級。
而另一種更方便的方法是在應用上做熱重啟，直接升級應用而不停服務。
原理
熱重啟的原理非常簡單，但是涉及到一些系統調用以及父子進程之間文件句柄的傳遞等等細節比較多。
處理過程分為以下幾個步驟：
細節
代碼
package mainimport ( "context" "errors" "flag" "log" "net" "net/http" "os" "os/exec" "os/signal" "syscall" "time") var ( server *http.Server listener net.Listener graceful = flag.Bool("graceful", false, "listen on fd open 3 (internal use only)")) func handler(w http.ResponseWriter, r *http.Request) { time.Sleep(20 * time.Second) w.Write([]byte("hello world233333!!!!"))} func main() { flag.Parse() http.HandleFunc("/hello", handler) server = &http.Server{Addr: ":9999"} var err error if *graceful { log.Print("main: Listening to existing file descriptor 3.") // cmd.ExtraFiles: If non-nil, entry i becomes file descriptor 3+i. // when we put socket FD at the first entry, it will always be 3(0+3) f := os.NewFile(3, "") listener, err = net.FileListener(f) } else { log.Print("main: Listening on a new file descriptor.") listener, err = net.Listen("tcp", server.Addr) } if err != nil { log.Fatalf("listener error: %v", err) } go func() { // server.Shutdown() stops Serve() immediately, thus server.Serve() should not be in main goroutine err = server.Serve(listener) log.Printf("server.Serve err: %v/n", err) }() signalHandler() log.Printf("signal end")} func reload() error { tl, ok := listener.(*net.TCPListener) if !ok { return errors.New("listener is not tcp listener") } f, err := tl.File() if err != nil { return err } args := []string{"-graceful"} cmd := exec.Command(os.Args[0], args...) cmd.Stdout = os.Stdout cmd.Stderr = os.Stderr // put socket FD at the first entry cmd.ExtraFiles = []*os.File{f} return cmd.Start()} func signalHandler() { ch := make(chan os.Signal, 1) signal.Notify(ch, syscall.SIGINT, syscall.SIGTERM, syscall.SIGUSR2) for { sig := <-ch log.Printf("signal: %v", sig) // timeout context for shutdown ctx, _ := context.WithTimeout(context.Background(), 20*time.Second) switch sig { case syscall.SIGINT, syscall.SIGTERM: // stop log.Printf("stop") signal.Stop(ch) server.Shutdown(ctx) log.Printf("graceful shutdown") return case syscall.SIGUSR2: // reload log.Printf("reload") err := reload() if err != nil { log.Fatalf("graceful restart error: %v", err) } server.Shutdown(ctx) log.Printf("graceful reload") return } }}references
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