frontend: add simple /healthz endpoint

Decouple frontend health checks from GET /, which relies on other services to
be reachable. See #34.

Closes #34.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

kubernetes-manifests/frontend.yaml

@@ -30,7 +30,7 @@ spec:
           readinessProbe:
             initialDelaySeconds: 10
             httpGet:
-              path: "/"
+              path: "/_healthz"
               port: 8080
               httpHeaders:
               - name: "Cookie"

src/frontend/main.go

@@ -121,7 +121,8 @@ func main() {
 	r.HandleFunc("/logout", svc.logoutHandler).Methods(http.MethodGet)
 	r.HandleFunc("/cart/checkout", svc.placeOrderHandler).Methods(http.MethodPost)
 	r.PathPrefix("/static/").Handler(http.StripPrefix("/static/", http.FileServer(http.Dir("./static/"))))
-	r.HandleFunc("/robots.txt", func(w http.ResponseWriter, r *http.Request) { fmt.Fprint(w, "User-agent: *\nDisallow: /") })
+	r.HandleFunc("/robots.txt", func(w http.ResponseWriter, _ *http.Request) { fmt.Fprint(w, "User-agent: *\nDisallow: /") })
+	r.HandleFunc("/_healthz", func(w http.ResponseWriter, _ *http.Request) { fmt.Fprint(w, "ok") })
 
 	var handler http.Handler = r
 	handler = &logHandler{log: log, next: handler} // add logging