main.go

package main

import (
	"context"
	"flag"
	"html/template"
	"log"
	"net/http"
	"os"
	"os/signal"
	"time"

	"cloud.google.com/go/compute/metadata"
)

const AppName = "k8s-demo-app"

var (
	listenAddr string
	healthy    bool
	hostname   string
	zone       string
	node       string
	cluster    string
	message    string
)

func main() {
	flag.StringVar(&listenAddr, "listen-addr", lookupEnvOrString("K8S_DEMO_APP_LISTEN_ADDR", ":8080"), "server listen address")
	flag.Parse()

	logger := log.New(os.Stdout, AppName+":", log.LstdFlags)

	logger.Println("Starting")

	hostname, _ = os.Hostname()
	zone, _ = metadata.Zone()
	node, _ = metadata.Hostname()
	cluster, _ = metadata.InstanceAttributeValue("cluster-name")
	message = lookupEnvOrString("K8S_DEMO_APP_MESSAGE", "Hello K8s World!")

	// HTTP Server
	router := http.NewServeMux()
	fs := http.FileServer(http.Dir("static"))

	router.Handle("/", index())

	router.Handle("/healthz", healthz())
	router.Handle("/static/", http.StripPrefix("/static/", fs))

	server := &http.Server{
		Addr:         listenAddr,
		Handler:      logging(logger)(defaultHeaders(router)),
		ErrorLog:     logger,
		ReadTimeout:  5 * time.Second,
		WriteTimeout: 5 * time.Second,
		IdleTimeout:  10 * time.Second,
	}

	done := make(chan bool)
	quit := make(chan os.Signal, 1)
	signal.Notify(quit, os.Interrupt)

	go func() {
		<-quit
		logger.Println("Shutting down")
		healthy = false

		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
		defer cancel()

		logger.Println("Waiting for graceful period of 10s")
		time.Sleep(10 * time.Second)

		if err := server.Shutdown(ctx); err != nil {
			logger.Fatalf("Failed to shudown: %v\n", err)
		}
		close(done)
	}()

	healthy = true
	logger.Println("Listening at ", listenAddr)

	if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
		logger.Fatalf("Failed to listen on %s: %v\n", listenAddr, err)
	}

	<-done
	logger.Println("Stopped")
}

func lookupEnvOrString(key string, defaultVal string) string {
	if val, ok := os.LookupEnv(key); ok {
		return val
	}
	return defaultVal
}

func defaultHeaders(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Server", AppName)
		h.ServeHTTP(w, r)
	})
}

func index() http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		tmpl := template.Must(template.ParseFiles("template.html"))

		tempData := map[string]interface{}{
			"Zone":     zone,
			"Hostname": hostname,
			"Node":     node,
			"Cluster":  cluster,
			"Message":  message,
			"Path":     r.URL.Path,
		}

		tmpl.Execute(w, tempData)
	})
}

func healthz() http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if healthy {
			w.WriteHeader(http.StatusOK)
			return
		}
		w.WriteHeader(http.StatusServiceUnavailable)
	})
}

func logging(logger *log.Logger) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			defer func() {
				logger.Println(r.Method, r.URL.Path, r.RemoteAddr, r.UserAgent())
			}()
			next.ServeHTTP(w, r)
		})
	}
}