服务端代码经常需要升级,对于线上系统的升级常用的做法是,通过前端的负载均衡(如nginx)来保证升级时至少有一个服务可用,依次(灰度)升级。
而另一种更方便的方法是在应用上做热重启,直接升级应用而不停服务。
原理
热重启的原理非常简单,但是涉及到一些系统调用以及父子进程之间文件句柄的传递等等细节比较多。
处理过程分为以下几个步骤:
细节
代码
package main import ( "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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