respond with 503 on empty backend

middlewares/empty_backend_handler.go

@@ -0,0 +1,31 @@
+package middlewares
+
+import (
+	"net/http"
+
+	"github.com/containous/traefik/healthcheck"
+)
+
+// EmptyBackendHandler is a middlware that checks whether the current Backend
+// has at least one active Server in respect to the healthchecks and if this
+// is not the case, it will stop the middleware chain and respond with 503.
+type EmptyBackendHandler struct {
+	lb   healthcheck.LoadBalancer
+	next http.Handler
+}
+
+// NewEmptyBackendHandler creates a new EmptyBackendHandler instance.
+func NewEmptyBackendHandler(lb healthcheck.LoadBalancer, next http.Handler) *EmptyBackendHandler {
+	return &EmptyBackendHandler{lb: lb, next: next}
+}
+
+// ServeHTTP responds with 503 when there is no active Server and otherwise
+// invokes the next handler in the middleware chain.
+func (h *EmptyBackendHandler) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
+	if len(h.lb.Servers()) == 0 {
+		rw.WriteHeader(http.StatusServiceUnavailable)
+		rw.Write([]byte(http.StatusText(http.StatusServiceUnavailable)))
+	} else {
+		h.next.ServeHTTP(rw, r)
+	}
+}